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mdbx: refine README.
Change-Id: Ib548c994753ab619ea8c813531b81a562f9d21fd
This commit is contained in:
758
README.md
758
README.md
@@ -9,9 +9,21 @@ libmdbx
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## Project Status for now
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- The stable versions ([_stable/0.0_](https://github.com/leo-yuriev/libmdbx/tree/stable/0.0) and [_stable/0.1_](https://github.com/leo-yuriev/libmdbx/tree/stable/0.1) branches) of _MDBX_ are frozen, i.e. no new features or API changes, but only bug fixes.
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- The next version ([_devel_](https://github.com/leo-yuriev/libmdbx/tree/devel) branch) **is under active non-public development**, i.e. current API and set of features are extreme volatile.
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- The immediate goal of development is formation of the stable API and the stable internal database format, which allows realise all PLANNED FEATURES:
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- The stable versions
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([_stable/0.0_](https://github.com/leo-yuriev/libmdbx/tree/stable/0.0)
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and
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[_stable/0.1_](https://github.com/leo-yuriev/libmdbx/tree/stable/0.1)
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branches) of _MDBX_ are frozen, i.e. no new features or API changes, but
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only bug fixes.
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- The next version
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([_devel_](https://github.com/leo-yuriev/libmdbx/tree/devel) branch)
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**is under active non-public development**, i.e. current API and set of
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features are extreme volatile.
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- The immediate goal of development is formation of the stable API and
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the stable internal database format, which allows realise all PLANNED
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FEATURES:
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1. Integrity check by [Merkle tree](https://en.wikipedia.org/wiki/Merkle_tree);
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2. Support for [raw block devices](https://en.wikipedia.org/wiki/Raw_device);
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3. Separate place (HDD) for large data items;
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@@ -24,19 +36,21 @@ Don't miss [Java Native Interface](https://github.com/castortech/mdbxjni) by [Ca
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-----
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Nowadays MDBX intended for Linux, and support Windows (since
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Windows Server 2008) as a complementary platform. Support for
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other OS could be implemented on commercial basis. However such
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enhancements (i.e. pull requests) could be accepted in
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mainstream only when corresponding public and free Continuous
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Integration service will be available.
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Nowadays MDBX intended for Linux, and support Windows (since Windows
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Server 2008) as a complementary platform. Support for other OS could be
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implemented on commercial basis. However such enhancements (i.e. pull
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requests) could be accepted in mainstream only when corresponding public
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and free Continuous Integration service will be available.
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## Contents
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- [Overview](#overview)
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- [Comparison with other DBs](#comparison-with-other-dbs)
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- [History & Acknowledgments](#history)
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- [Main features](#main-features)
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- [Improvements over LMDB](#improvements-over-lmdb)
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- [Gotchas](#gotchas)
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- [Long-time read transactions problem](#long-time-read-transactions-problem)
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- [Data safety in async-write-mode](#data-safety-in-async-write-mode)
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- [Performance comparison](#performance-comparison)
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- [Integral performance](#integral-performance)
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- [Read scalability](#read-scalability)
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@@ -44,52 +58,58 @@ Integration service will be available.
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- [Lazy-write mode](#lazy-write-mode)
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- [Async-write mode](#async-write-mode)
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- [Cost comparison](#cost-comparison)
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- [Gotchas](#gotchas)
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- [Long-time read transactions problem](#long-time-read-transactions-problem)
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- [Data safety in async-write-mode](#data-safety-in-async-write-mode)
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- [Improvements over LMDB](#improvements-over-lmdb)
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## Overview
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_libmdbx_ is an embedded lightweight key-value database engine oriented
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for performance under Linux and Windows.
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_libmdbx_ is an embedded lightweight key-value database engine oriented for performance under Linux and Windows.
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_libmdbx_ allows multiple processes to read and update several key-value tables concurrently,
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while being [ACID](https://en.wikipedia.org/wiki/ACID)-compliant, with minimal overhead and operation cost of Olog(N).
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_libmdbx_ allows multiple processes to read and update several key-value
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tables concurrently, while being
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[ACID](https://en.wikipedia.org/wiki/ACID)-compliant, with minimal
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overhead and operation cost of Olog(N).
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_libmdbx_ provides
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[serializability](https://en.wikipedia.org/wiki/Serializability) and consistency of data after crash.
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Read-write transactions don't block read-only transactions and are
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[serialized](https://en.wikipedia.org/wiki/Serializability) by [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion).
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[serializability](https://en.wikipedia.org/wiki/Serializability) and
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consistency of data after crash. Read-write transactions don't block
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read-only transactions and are
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[serialized](https://en.wikipedia.org/wiki/Serializability) by
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[mutex](https://en.wikipedia.org/wiki/Mutual_exclusion).
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_libmdbx_ [wait-free](https://en.wikipedia.org/wiki/Non-blocking_algorithm#Wait-freedom) provides parallel read transactions
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without atomic operations or synchronization primitives.
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_libmdbx_
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[wait-free](https://en.wikipedia.org/wiki/Non-blocking_algorithm#Wait-freedom)
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provides parallel read transactions without atomic operations or
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synchronization primitives.
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_libmdbx_ uses [B+Trees](https://en.wikipedia.org/wiki/B%2B_tree) and [mmap](https://en.wikipedia.org/wiki/Memory-mapped_file),
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doesn't use [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging). This might have caveats for some workloads.
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_libmdbx_ uses [B+Trees](https://en.wikipedia.org/wiki/B%2B_tree) and
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[mmap](https://en.wikipedia.org/wiki/Memory-mapped_file), doesn't use
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[WAL](https://en.wikipedia.org/wiki/Write-ahead_logging). This might
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have caveats for some workloads.
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### Comparison with other DBs
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Because _libmdbx_ is currently overhauled, I think it's better to just link
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[chapter of Comparison with other databases](https://github.com/coreos/bbolt#comparison-with-other-databases) here.
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Because _libmdbx_ is currently overhauled, I think it's better to just
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link [chapter of Comparison with other
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databases](https://github.com/coreos/bbolt#comparison-with-other-databases)
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here.
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### History
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The _libmdbx_ design is based on [Lightning Memory-Mapped
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Database](https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database).
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Initial development was going in
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[ReOpenLDAP](https://github.com/leo-yuriev/ReOpenLDAP) project, about a
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year later it received separate development effort and in autumn 2015
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was isolated to separate project, which was [presented at Highload++
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2015 conference](http://www.highload.ru/2015/abstracts/1831.html).
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The _libmdbx_ design is based on [Lightning Memory-Mapped Database](https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database).
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Initial development was going in [ReOpenLDAP](https://github.com/leo-yuriev/ReOpenLDAP) project, about a year later it
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received separate development effort and in autumn 2015 was isolated to separate project, which was
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[presented at Highload++ 2015 conference](http://www.highload.ru/2015/abstracts/1831.html).
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Since early 2017 _libmdbx_ is used in [Fast Positive Tables](https://github.com/leo-yuriev/libfpta),
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Since early 2017 _libmdbx_ is used in [Fast PositiveTables](https://github.com/leo-yuriev/libfpta),
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by [Positive Technologies](https://www.ptsecurity.com).
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#### Acknowledgments
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Howard Chu (Symas Corporation) - the author of LMDB, from which
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originated the MDBX in 2015.
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Howard Chu (Symas Corporation) - the author of LMDB,
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from which originated the MDBX in 2015.
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Martin Hedenfalk <martin@bzero.se> - the author of `btree.c` code,
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which was used for begin development of LMDB.
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Martin Hedenfalk <martin@bzero.se> - the author of `btree.c` code, which
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was used for begin development of LMDB.
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Main features
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@@ -98,39 +118,331 @@ Main features
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_libmdbx_ inherits all keys features and characteristics from
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[LMDB](https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database):
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1. Data is stored in ordered map, keys are always sorted, range lookups are supported.
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1. Data is stored in ordered map, keys are always sorted, range lookups
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are supported.
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2. Data is [mmaped](https://en.wikipedia.org/wiki/Memory-mapped_file) to memory of each worker DB process, read transactions are zero-copy.
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2. Data is [mmaped](https://en.wikipedia.org/wiki/Memory-mapped_file) to
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memory of each worker DB process, read transactions are zero-copy.
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3. Transactions are [ACID](https://en.wikipedia.org/wiki/ACID)-compliant, thanks to
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[MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control) and [CoW](https://en.wikipedia.org/wiki/Copy-on-write).
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Writes are strongly serialized and aren't blocked by reads, transactions can't conflict with each other.
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Reads are guaranteed to get only commited data
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([relaxing serializability](https://en.wikipedia.org/wiki/Serializability#Relaxing_serializability)).
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3. Transactions are
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[ACID](https://en.wikipedia.org/wiki/ACID)-compliant, thanks to
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[MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control)
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and [CoW](https://en.wikipedia.org/wiki/Copy-on-write). Writes are
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strongly serialized and aren't blocked by reads, transactions can't
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conflict with each other. Reads are guaranteed to get only commited data
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([relaxing serializability](https://en.wikipedia.org/wiki/Serializability#Relaxing_serializability)).
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4. Reads and queries are [non-blocking](https://en.wikipedia.org/wiki/Non-blocking_algorithm),
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don't use [atomic operations](https://en.wikipedia.org/wiki/Linearizability#High-level_atomic_operations).
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Readers don't block each other and aren't blocked by writers. Read performance scales linearly with CPU core count.
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> Though "connect to DB" (start of first read transaction in thread) and "disconnect from DB" (shutdown or thread
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> termination) requires to acquire a lock to register/unregister current thread from "readers table"
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4. Reads and queries are
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[non-blocking](https://en.wikipedia.org/wiki/Non-blocking_algorithm),
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don't use [atomic
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operations](https://en.wikipedia.org/wiki/Linearizability#High-level_atomic_operations).
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Readers don't block each other and aren't blocked by writers. Read
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performance scales linearly with CPU core count.
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> Though "connect to DB" (start of first read transaction in thread) and
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> "disconnect from DB" (shutdown or thread termination) requires to
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> acquire a lock to register/unregister current thread from "readers
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> table"
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5. Keys with multiple values are stored efficiently without key duplication, sorted by value, including integers
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(reasonable for secondary indexes).
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5. Keys with multiple values are stored efficiently without key
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duplication, sorted by value, including integers (reasonable for
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secondary indexes).
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6. Efficient operation on short fixed length keys, including integer ones.
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6. Efficient operation on short fixed length keys, including integer
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ones.
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7. [WAF](https://en.wikipedia.org/wiki/Write_amplification) (Write Amplification Factor) и RAF (Read Amplification Factor)
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are Olog(N).
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7. [WAF](https://en.wikipedia.org/wiki/Write_amplification) (Write
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Amplification Factor) и RAF (Read Amplification Factor) are Olog(N).
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8. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) and transaction journal.
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In case of a crash no recovery needed. No need for regular maintenance. Backups can be made on the fly on working DB
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without freezing writers.
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8. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) and
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transaction journal. In case of a crash no recovery needed. No need for
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regular maintenance. Backups can be made on the fly on working DB
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without freezing writers.
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9. No custom memory management, all done with standard OS syscalls.
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--------------------------------------------------------------------------------
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Improvements over LMDB
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======================
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1. `mdbx_chk` tool for DB integrity check.
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2. Automatic dynamic DB size management according to the parameters
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specified by `mdbx_env_set_geometry()` function. Including including
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growth step and truncation threshold, as well as the choice of page
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size.
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3. Automatic returning of freed pages into unallocated space at the end
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of database file with optionally automatic shrinking it. This reduces
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amount of pages resides in RAM and circulated in disk I/O. In fact
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_libmdbx_ constantly performs DB compactification, without spending
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additional resources for that.
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4. Support for keys and values of zero length, including sorted
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duplicates.
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5. Ability to assign up to 3 markers to commiting transaction with
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`mdbx_canary_put()` and then get them in read transaction by
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`mdbx_canary_get()`.
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6. Ability to update or delete record and get previous value via
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`mdbx_replace()` Also can update specific multi-value.
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7. `LIFO RECLAIM` mode:
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The newest pages are picked for reuse instead of the oldest. This allows
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to minimize reclaim loop and make it execution time independent of total
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page count.
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This results in OS kernel cache mechanisms working with maximum
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efficiency. In case of using disk controllers or storages with
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[BBWC](https://en.wikipedia.org/wiki/Disk_buffer#Write_acceleration)
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this may greatly improve write performance.
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8. Sequence generation via `mdbx_dbi_sequence()`.
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9. `OOM-KICK` callback.
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`mdbx_env_set_oomfunc()` allows to set a callback, which will be called
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in the event of DB space exhausting during long-time read transaction in
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parallel with extensive updating. Callback will be invoked with PID and
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pthread_id of offending thread as parameters. Callback can do any of
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these things to remedy the problem:
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* wait for read transaction to finish normally;
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* kill the offending process (signal 9), if separate process is doing
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long-time read;
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* abort or restart offending read transaction if it's running in sibling
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thread;
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* abort current write transaction with returning error code.
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10. Ability to open DB in exclusive mode with `MDBX_EXCLUSIVE` flag.
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11. Ability to get how far current read-only snapshot is from latest
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version of the DB by `mdbx_txn_straggler()`.
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12. Ability to explicitly request update of present record without
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creating new record. Implemented as `MDBX_CURRENT` flag for
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`mdbx_put()`.
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13. Fixed `mdbx_cursor_count()`, which returns correct count of
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duplicated for all table types and any cursor position.
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14. `mdbx_env_info()` to getting additional info, including number of
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the oldest snapshot of DB, which is used by one of the readers.
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15. `mdbx_del()` doesn't ignore additional argument (specifier) `data`
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for tables without duplicates (without flag `MDBX_DUPSORT`), if `data`
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is not null then always uses it to verify record, which is being
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deleted.
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16. Ability to open dbi-table with simultaneous setup of comparators for
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keys and values, via `mdbx_dbi_open_ex()`.
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17. `mdbx_is_dirty()`to find out if key or value is on dirty page, that
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useful to avoid copy-out before updates.
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18. Correct update of current record in `MDBX_CURRENT` mode of
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`mdbx_cursor_put()`, including sorted duplicated.
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19. Check if there is a row with data after current cursor position via
|
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`mdbx_cursor_eof()`.
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20. Additional error code `MDBX_EMULTIVAL`, which is returned by
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`mdbx_put()` and `mdbx_replace()` in case is ambiguous update or delete.
|
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21. Ability to get value by key and duplicates count by `mdbx_get_ex()`.
|
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22. Functions `mdbx_cursor_on_first()` and `mdbx_cursor_on_last()`,
|
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which allows to know if cursor is currently on first or last position
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respectively.
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23. Automatic creation of synchronization points (flush changes to
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persistent storage) when changes reach set threshold (threshold can be
|
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set by `mdbx_env_set_syncbytes()`).
|
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24. Control over debugging and receiving of debugging messages via
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`mdbx_setup_debug()`.
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|
||||
25. Function `mdbx_env_pgwalk()` for page-walking all pages in DB.
|
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26. Three meta-pages instead of two, this allows to guarantee
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consistently update weak sync-points without risking to corrupt last
|
||||
steady sync-point.
|
||||
|
||||
27. Guarantee of DB integrity in `WRITEMAP+MAPSYNC` mode:
|
||||
> Current _libmdbx_ gives a choice of safe async-write mode (default)
|
||||
> and `UTTERLY_NOSYNC` mode which may result in full
|
||||
> DB corruption during system crash as with LMDB. For details see
|
||||
> [Data safety in async-write mode](#data-safety-in-async-write-mode).
|
||||
|
||||
28. Ability to close DB in "dirty" state (without data flush and
|
||||
creation of steady synchronization point) via `mdbx_env_close_ex()`.
|
||||
|
||||
29. If read transaction is aborted via `mdbx_txn_abort()` or
|
||||
`mdbx_txn_reset()` then DBI-handles, which were opened in it, aren't
|
||||
closed or deleted. This allows to avoid several types of hard-to-debug
|
||||
errors.
|
||||
|
||||
30. All cursors in all read and write transactions can be reused by
|
||||
`mdbx_cursor_renew()` and MUST be freed explicitly.
|
||||
> ## Caution, please pay attention!
|
||||
>
|
||||
> This is the only change of API, which changes semantics of cursor management
|
||||
> and can lead to memory leaks on misuse. This is a needed change as it eliminates ambiguity
|
||||
> which helps to avoid such errors as:
|
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> - use-after-free;
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||||
> - double-free;
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||||
> - memory corruption and segfaults.
|
||||
|
||||
--------------------------------------------------------------------------------
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||||
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||||
## Gotchas
|
||||
|
||||
1. At one moment there can be only one writer. But this allows to
|
||||
serialize writes and eliminate any possibility of conflict or logical
|
||||
errors during transaction rollback.
|
||||
|
||||
2. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) means
|
||||
relatively big [WAF](https://en.wikipedia.org/wiki/Write_amplification)
|
||||
(Write Amplification Factor). Because of this syncing data to disk might
|
||||
be quite resource intensive and be main performance bottleneck during
|
||||
intensive write workload.
|
||||
> As compromise _libmdbx_ allows several modes of lazy and/or periodic
|
||||
> syncing, including `MAPASYNC` mode, which modificate data in memory and
|
||||
> asynchronously syncs data to disk, moment to sync is picked by OS.
|
||||
>
|
||||
> Although this should be used with care, synchronous transactions in a DB
|
||||
> with transaction journal will require 2 IOPS minimum (probably 3-4 in
|
||||
> practice) because of filesystem overhead, overhead depends on
|
||||
> filesystem, not on record count or record size. In _libmdbx_ IOPS count
|
||||
> will grow logarithmically depending on record count in DB (height of B+
|
||||
> tree) and will require at least 2 IOPS per transaction too.
|
||||
|
||||
3. [CoW](https://en.wikipedia.org/wiki/Copy-on-write) for
|
||||
[MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control)
|
||||
is done on memory page level with
|
||||
[B+trees](https://ru.wikipedia.org/wiki/B-%D0%B4%D0%B5%D1%80%D0%B5%D0%B2%D0%BE).
|
||||
Therefore altering data requires to copy about Olog(N) memory pages,
|
||||
which uses [memory bandwidth](https://en.wikipedia.org/wiki/Memory_bandwidth) and is main
|
||||
performance bottleneck in `MAPASYNC` mode.
|
||||
> This is unavoidable, but isn't that bad. Syncing data to disk requires
|
||||
> much more similar operations which will be done by OS, therefore this is
|
||||
> noticeable only if data sync to persistent storage is fully disabled.
|
||||
> _libmdbx_ allows to safely save data to persistent storage with minimal
|
||||
> performance overhead. If there is no need to save data to persistent
|
||||
> storage then it's much more preferable to use `std::map`.
|
||||
|
||||
|
||||
4. LMDB has a problem of long-time readers which degrades performance
|
||||
and bloats DB.
|
||||
> _libmdbx_ addresses that, details below.
|
||||
|
||||
5. _LMDB_ is susceptible to DB corruption in `WRITEMAP+MAPASYNC` mode.
|
||||
_libmdbx_ in `WRITEMAP+MAPASYNC` guarantees DB integrity and consistency
|
||||
of data.
|
||||
> Additionally there is an alternative: `UTTERLY_NOSYNC` mode.
|
||||
> Details below.
|
||||
|
||||
|
||||
#### Long-time read transactions problem
|
||||
Garbage collection problem exists in all databases one way or another
|
||||
(e.g. VACUUM in PostgreSQL). But in _libmdbx_ and LMDB it's even more
|
||||
important because of high performance and deliberate simplification of
|
||||
internals with emphasis on performance.
|
||||
|
||||
* Altering data during long read operation may exhaust available space
|
||||
on persistent storage.
|
||||
|
||||
* If available space is exhausted then any attempt to update data
|
||||
results in `MAP_FULL` error until long read operation ends.
|
||||
|
||||
* Main examples of long readers is hot backup and debugging of client
|
||||
application which actively uses read transactions.
|
||||
|
||||
* In _LMDB_ this results in degraded performance of all operations of
|
||||
syncing data to persistent storage.
|
||||
|
||||
* _libmdbx_ has a mechanism which aborts such operations and `LIFO RECLAIM`
|
||||
mode which addresses performance degradation.
|
||||
|
||||
Read operations operate only over snapshot of DB which is consistent on
|
||||
the moment when read transaction started. This snapshot doesn't change
|
||||
throughout the transaction but this leads to inability to reclaim the
|
||||
pages until read transaction ends.
|
||||
|
||||
In _LMDB_ this leads to a problem that memory pages, allocated for
|
||||
operations during long read, will be used for operations and won't be
|
||||
reclaimed until DB process terminates. In _LMDB_ they are used in
|
||||
[FIFO](https://en.wikipedia.org/wiki/FIFO_(computing_and_electronics))
|
||||
manner, which causes increased page count and less chance of cache hit
|
||||
during I/O. In other words: one long-time reader can impact performance
|
||||
of all database until it'll be reopened.
|
||||
|
||||
_libmdbx_ addresses the problem, details below. Illustrations to this
|
||||
problem can be found in the
|
||||
[presentation](http://www.slideshare.net/leoyuriev/lmdb). There is also
|
||||
example of performance increase thanks to
|
||||
[BBWC](https://en.wikipedia.org/wiki/Disk_buffer#Write_acceleration)
|
||||
when `LIFO RECLAIM` enabled in _libmdbx_.
|
||||
|
||||
#### Data safety in async-write mode
|
||||
In `WRITEMAP+MAPSYNC` mode dirty pages are written to persistent storage
|
||||
by kernel. This means that in case of application crash OS kernel will
|
||||
write all dirty data to disk and nothing will be lost. But in case of
|
||||
hardware malfunction or OS kernel fatal error only some dirty data might
|
||||
be synced to disk, and there is high probability that pages with
|
||||
metadata saved, will point to non-saved, hence non-existent, data pages.
|
||||
In such situation, DB is completely corrupted and can't be repaired even
|
||||
if there was full sync before the crash via `mdbx_env_sync().
|
||||
|
||||
_libmdbx_ addresses this by fully reimplementing write path of data:
|
||||
|
||||
* In `WRITEMAP+MAPSYNC` mode meta-data pages aren't updated in place,
|
||||
instead their shadow copies are used and their updates are synced after
|
||||
data is flushed to disk.
|
||||
|
||||
* During transaction commit _libmdbx_ marks synchronization points as
|
||||
steady or weak depending on how much synchronization needed between RAM
|
||||
and persistent storage, e.g. in `WRITEMAP+MAPSYNC` commited transactions
|
||||
are marked as weak, but during explicit data synchronization - as
|
||||
steady.
|
||||
|
||||
* _libmdbx_ maintains three separate meta-pages instead of two. This
|
||||
allows to commit transaction with steady or weak synchronization point
|
||||
without losing two previous synchronization points (one of them can be
|
||||
steady, and second - weak). This allows to order weak and steady
|
||||
synchronization points in any order without losing consistency in case
|
||||
of system crash.
|
||||
|
||||
* During DB open _libmdbx_ rollbacks to the last steady synchronization
|
||||
point, this guarantees database integrity.
|
||||
|
||||
For data safety pages which form database snapshot with steady
|
||||
synchronization point must not be updated until next steady
|
||||
synchronization point. So last steady synchronization point creates
|
||||
"long-time read" effect. The only difference that in case of memory
|
||||
exhaustion the problem will be immediately addressed by flushing changes
|
||||
to persistent storage and forming new steady synchronization point.
|
||||
|
||||
So in async-write mode _libmdbx_ will always use new pages until memory
|
||||
is exhausted or `mdbx_env_sync()` is invoked. Total disk usage will be
|
||||
almost the same as in sync-write mode.
|
||||
|
||||
Current _libmdbx_ gives a choice of safe async-write mode (default) and
|
||||
`UTTERLY_NOSYNC` mode which may result in full DB corruption during
|
||||
system crash as with LMDB.
|
||||
|
||||
Next version of _libmdbx_ will create steady synchronization points
|
||||
automatically in async-write mode.
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Performance comparison
|
||||
=====================
|
||||
======================
|
||||
|
||||
All benchmarks were done by [IOArena](https://github.com/pmwkaa/ioarena)
|
||||
and multiple [scripts](https://github.com/pmwkaa/ioarena/tree/HL%2B%2B2015)
|
||||
@@ -143,18 +455,21 @@ SSD SAMSUNG MZNTD512HAGL-000L1 (DXT23L0Q) 512 Gb.
|
||||
|
||||
Here showed sum of performance metrics in 3 benchmarks:
|
||||
|
||||
- Read/Search on 4 CPU cores machine;
|
||||
- Read/Search on 4 CPU cores machine;
|
||||
|
||||
- Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD) operations
|
||||
in sync-write mode (fdatasync is called after each transaction);
|
||||
- Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD)
|
||||
operations in sync-write mode (fdatasync is called after each
|
||||
transaction);
|
||||
|
||||
- Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD) operations
|
||||
in lazy-write mode (moment to sync data to persistent storage is decided by OS).
|
||||
- Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD)
|
||||
operations in lazy-write mode (moment to sync data to persistent storage
|
||||
is decided by OS).
|
||||
|
||||
*Reasons why asynchronous mode isn't benchmarked here:*
|
||||
|
||||
1. It doesn't make sense as it has to be done with DB engines, oriented for keeping data in memory e.g.
|
||||
[Tarantool](https://tarantool.io/), [Redis](https://redis.io/)), etc.
|
||||
1. It doesn't make sense as it has to be done with DB engines, oriented
|
||||
for keeping data in memory e.g. [Tarantool](https://tarantool.io/),
|
||||
[Redis](https://redis.io/)), etc.
|
||||
|
||||
2. Performance gap is too high to compare in any meaningful way.
|
||||
|
||||
@@ -164,7 +479,8 @@ Here showed sum of performance metrics in 3 benchmarks:
|
||||
|
||||
### Read Scalability
|
||||
|
||||
Summary performance with concurrent read/search queries in 1-2-4-8 threads on 4 CPU cores machine.
|
||||
Summary performance with concurrent read/search queries in 1-2-4-8
|
||||
threads on 4 CPU cores machine.
|
||||
|
||||

|
||||
|
||||
@@ -172,15 +488,21 @@ Summary performance with concurrent read/search queries in 1-2-4-8 threads on 4
|
||||
|
||||
### Sync-write mode
|
||||
|
||||
- Linear scale on left and dark rectangles mean arithmetic mean transactions per second;
|
||||
- Linear scale on left and dark rectangles mean arithmetic mean
|
||||
transactions per second;
|
||||
|
||||
- Logarithmic scale on right is in seconds and yellow intervals mean execution time of transactions.
|
||||
Each interval shows minimal and maximum execution time, cross marks standard deviation.
|
||||
- Logarithmic scale on right is in seconds and yellow intervals mean
|
||||
execution time of transactions. Each interval shows minimal and maximum
|
||||
execution time, cross marks standard deviation.
|
||||
|
||||
**10,000 transactions in sync-write mode**. In case of a crash all data is consistent and state is right after last successful transaction. [fdatasync](https://linux.die.net/man/2/fdatasync) syscall is used after each write transaction in this mode.
|
||||
**10,000 transactions in sync-write mode**. In case of a crash all data
|
||||
is consistent and state is right after last successful transaction.
|
||||
[fdatasync](https://linux.die.net/man/2/fdatasync) syscall is used after
|
||||
each write transaction in this mode.
|
||||
|
||||
In the benchmark each transaction contains combined CRUD operations (2 inserts, 1 read, 1 update, 1 delete).
|
||||
Benchmark starts on empty database and after full run the database contains 10,000 small key-value records.
|
||||
In the benchmark each transaction contains combined CRUD operations (2
|
||||
inserts, 1 read, 1 update, 1 delete). Benchmark starts on empty database
|
||||
and after full run the database contains 10,000 small key-value records.
|
||||
|
||||

|
||||
|
||||
@@ -188,18 +510,25 @@ Benchmark starts on empty database and after full run the database contains 10,0
|
||||
|
||||
### Lazy-write mode
|
||||
|
||||
- Linear scale on left and dark rectangles mean arithmetic mean of thousands transactions per second;
|
||||
- Linear scale on left and dark rectangles mean arithmetic mean of
|
||||
thousands transactions per second;
|
||||
|
||||
- Logarithmic scale on right in seconds and yellow intervals mean execution time of transactions. Each interval shows minimal and maximum execution time, cross marks standard deviation.
|
||||
- Logarithmic scale on right in seconds and yellow intervals mean
|
||||
execution time of transactions. Each interval shows minimal and maximum
|
||||
execution time, cross marks standard deviation.
|
||||
|
||||
**100,000 transactions in lazy-write mode**.
|
||||
In case of a crash all data is consistent and state is right after one of last transactions, but transactions after it
|
||||
will be lost. Other DB engines use [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) or transaction journal for that,
|
||||
which in turn depends on order of operations in journaled filesystem. _libmdbx_ doesn't use WAL and hands I/O operations
|
||||
**100,000 transactions in lazy-write mode**. In case of a crash all data
|
||||
is consistent and state is right after one of last transactions, but
|
||||
transactions after it will be lost. Other DB engines use
|
||||
[WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) or transaction
|
||||
journal for that, which in turn depends on order of operations in
|
||||
journaled filesystem. _libmdbx_ doesn't use WAL and hands I/O operations
|
||||
to filesystem and OS kernel (mmap).
|
||||
|
||||
In the benchmark each transaction contains combined CRUD operations (2 inserts, 1 read, 1 update, 1 delete).
|
||||
Benchmark starts on empty database and after full run the database contains 100,000 small key-value records.
|
||||
In the benchmark each transaction contains combined CRUD operations (2
|
||||
inserts, 1 read, 1 update, 1 delete). Benchmark starts on empty database
|
||||
and after full run the database contains 100,000 small key-value
|
||||
records.
|
||||
|
||||
|
||||

|
||||
@@ -208,14 +537,23 @@ Benchmark starts on empty database and after full run the database contains 100,
|
||||
|
||||
### Async-write mode
|
||||
|
||||
- Linear scale on left and dark rectangles mean arithmetic mean of thousands transactions per second;
|
||||
- Linear scale on left and dark rectangles mean arithmetic mean of
|
||||
thousands transactions per second;
|
||||
|
||||
- Logarithmic scale on right in seconds and yellow intervals mean execution time of transactions. Each interval shows minimal and maximum execution time, cross marks standard deviation.
|
||||
- Logarithmic scale on right in seconds and yellow intervals mean
|
||||
execution time of transactions. Each interval shows minimal and maximum
|
||||
execution time, cross marks standard deviation.
|
||||
|
||||
**1,000,000 transactions in async-write mode**. In case of a crash all data will be consistent and state will be right after one of last transactions, but lost transaction count is much higher than in lazy-write mode. All DB engines in this mode do as little writes as possible on persistent storage. _libmdbx_ uses [msync(MS_ASYNC)](https://linux.die.net/man/2/msync) in this mode.
|
||||
**1,000,000 transactions in async-write mode**. In case of a crash all
|
||||
data will be consistent and state will be right after one of last
|
||||
transactions, but lost transaction count is much higher than in
|
||||
lazy-write mode. All DB engines in this mode do as little writes as
|
||||
possible on persistent storage. _libmdbx_ uses
|
||||
[msync(MS_ASYNC)](https://linux.die.net/man/2/msync) in this mode.
|
||||
|
||||
In the benchmark each transaction contains combined CRUD operations (2 inserts, 1 read, 1 update, 1 delete).
|
||||
Benchmark starts on empty database and after full run the database contains 10,000 small key-value records.
|
||||
In the benchmark each transaction contains combined CRUD operations (2
|
||||
inserts, 1 read, 1 update, 1 delete). Benchmark starts on empty database
|
||||
and after full run the database contains 10,000 small key-value records.
|
||||
|
||||

|
||||
|
||||
@@ -229,237 +567,22 @@ Summary of used resources during lazy-write mode benchmarks:
|
||||
|
||||
- Sum of user CPU time and sys CPU time;
|
||||
|
||||
- Used space on persistent storage after the test and closed DB, but not waiting for the end of all internal
|
||||
housekeeping operations (LSM compactification, etc).
|
||||
- Used space on persistent storage after the test and closed DB, but not
|
||||
waiting for the end of all internal housekeeping operations (LSM
|
||||
compactification, etc).
|
||||
|
||||
_ForestDB_ is excluded because benchmark showed it's resource consumption for each resource (CPU, IOPS) much higher than other engines which prevents to meaningfully compare it with them.
|
||||
_ForestDB_ is excluded because benchmark showed it's resource
|
||||
consumption for each resource (CPU, IOPS) much higher than other engines
|
||||
which prevents to meaningfully compare it with them.
|
||||
|
||||
All benchmark data is gathered by [getrusage()](http://man7.org/linux/man-pages/man2/getrusage.2.html) syscall and by
|
||||
scanning data directory.
|
||||
All benchmark data is gathered by
|
||||
[getrusage()](http://man7.org/linux/man-pages/man2/getrusage.2.html)
|
||||
syscall and by scanning data directory.
|
||||
|
||||

|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
## Gotchas
|
||||
|
||||
1. At one moment there can be only one writer. But this allows to serialize writes and eliminate any possibility
|
||||
of conflict or logical errors during transaction rollback.
|
||||
|
||||
2. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) means relatively
|
||||
big [WAF](https://en.wikipedia.org/wiki/Write_amplification) (Write Amplification Factor).
|
||||
Because of this syncing data to disk might be quite resource intensive and be main performance bottleneck
|
||||
during intensive write workload.
|
||||
> As compromise _libmdbx_ allows several modes of lazy and/or periodic syncing, including `MAPASYNC` mode, which modificate
|
||||
> data in memory and asynchronously syncs data to disk, moment to sync is picked by OS.
|
||||
>
|
||||
> Although this should be used with care, synchronous transactions in a DB with transaction journal will require 2 IOPS
|
||||
> minimum (probably 3-4 in practice) because of filesystem overhead, overhead depends on filesystem, not on record
|
||||
> count or record size. In _libmdbx_ IOPS count will grow logarithmically depending on record count in DB (height of B+ tree)
|
||||
> and will require at least 2 IOPS per transaction too.
|
||||
|
||||
3. [CoW](https://en.wikipedia.org/wiki/Copy-on-write)
|
||||
for [MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control) is done on memory page level with [B+
|
||||
trees](https://ru.wikipedia.org/wiki/B-%D0%B4%D0%B5%D1%80%D0%B5%D0%B2%D0%BE).
|
||||
Therefore altering data requires to copy about Olog(N) memory pages, which uses [memory bandwidth](https://en.wikipedia.org/wiki/Memory_bandwidth) and is main performance bottleneck in `MAPASYNC` mode.
|
||||
> This is unavoidable, but isn't that bad. Syncing data to disk requires much more similar operations which will
|
||||
> be done by OS, therefore this is noticeable only if data sync to persistent storage is fully disabled.
|
||||
> _libmdbx_ allows to safely save data to persistent storage with minimal performance overhead. If there is no need
|
||||
> to save data to persistent storage then it's much more preferable to use `std::map`.
|
||||
|
||||
|
||||
4. LMDB has a problem of long-time readers which degrades performance and bloats DB
|
||||
> _libmdbx_ addresses that, details below.
|
||||
|
||||
5. _LMDB_ is susceptible to DB corruption in `WRITEMAP+MAPASYNC` mode.
|
||||
_libmdbx_ in `WRITEMAP+MAPASYNC` guarantees DB integrity and consistency of data.
|
||||
> Additionally there is an alternative: `UTTERLY_NOSYNC` mode. Details below.
|
||||
|
||||
|
||||
#### Long-time read transactions problem
|
||||
|
||||
Garbage collection problem exists in all databases one way or another (e.g. VACUUM in PostgreSQL).
|
||||
But in _libmdbx_ and LMDB it's even more important because of high performance and deliberate
|
||||
simplification of internals with emphasis on performance.
|
||||
|
||||
* Altering data during long read operation may exhaust available space on persistent storage.
|
||||
|
||||
* If available space is exhausted then any attempt to update data
|
||||
results in `MAP_FULL` error until long read operation ends.
|
||||
|
||||
* Main examples of long readers is hot backup
|
||||
and debugging of client application which actively uses read transactions.
|
||||
|
||||
* In _LMDB_ this results in degraded performance of all operations
|
||||
of syncing data to persistent storage.
|
||||
|
||||
* _libmdbx_ has a mechanism which aborts such operations and `LIFO RECLAIM`
|
||||
mode which addresses performance degradation.
|
||||
|
||||
Read operations operate only over snapshot of DB which is consistent on the moment when read transaction started.
|
||||
This snapshot doesn't change throughout the transaction but this leads to inability to reclaim the pages until
|
||||
read transaction ends.
|
||||
|
||||
In _LMDB_ this leads to a problem that memory pages, allocated for operations during long read, will be used for operations
|
||||
and won't be reclaimed until DB process terminates. In _LMDB_ they are used in
|
||||
[FIFO](https://en.wikipedia.org/wiki/FIFO_(computing_and_electronics)) manner, which causes increased page count
|
||||
and less chance of cache hit during I/O. In other words: one long-time reader can impact performance of all database
|
||||
until it'll be reopened.
|
||||
|
||||
_libmdbx_ addresses the problem, details below. Illustrations to this problem can be found in the
|
||||
[presentation](http://www.slideshare.net/leoyuriev/lmdb). There is also example of performance increase thanks to
|
||||
[BBWC](https://en.wikipedia.org/wiki/Disk_buffer#Write_acceleration) when `LIFO RECLAIM` enabled in _libmdbx_.
|
||||
|
||||
#### Data safety in async-write mode
|
||||
|
||||
In `WRITEMAP+MAPSYNC` mode dirty pages are written to persistent storage by kernel. This means that in case of application
|
||||
crash OS kernel will write all dirty data to disk and nothing will be lost. But in case of hardware malfunction or OS kernel
|
||||
fatal error only some dirty data might be synced to disk, and there is high probability that pages with metadata saved,
|
||||
will point to non-saved, hence non-existent, data pages. In such situation, DB is completely corrupted and can't be
|
||||
repaired even if there was full sync before the crash via `mdbx_env_sync().
|
||||
|
||||
_libmdbx_ addresses this by fully reimplementing write path of data:
|
||||
|
||||
* In `WRITEMAP+MAPSYNC` mode meta-data pages aren't updated in place, instead their shadow copies are used and their updates
|
||||
are synced after data is flushed to disk.
|
||||
|
||||
* During transaction commit _libmdbx_ marks synchronization points as steady or weak depending on how much synchronization
|
||||
needed between RAM and persistent storage, e.g. in `WRITEMAP+MAPSYNC` commited transactions are marked as weak,
|
||||
but during explicit data synchronization - as steady.
|
||||
|
||||
* _libmdbx_ maintains three separate meta-pages instead of two. This allows to commit transaction with steady or
|
||||
weak synchronization point without losing two previous synchronization points (one of them can be steady, and second - weak).
|
||||
This allows to order weak and steady synchronization points in any order without losing consistency in case of system crash.
|
||||
|
||||
* During DB open _libmdbx_ rollbacks to the last steady synchronization point, this guarantees database integrity.
|
||||
|
||||
For data safety pages which form database snapshot with steady synchronization point must not be updated until next steady
|
||||
synchronization point. So last steady synchronization point creates "long-time read" effect. The only difference that in case
|
||||
of memory exhaustion the problem will be immediately addressed by flushing changes to persistent storage and forming new steady
|
||||
synchronization point.
|
||||
|
||||
So in async-write mode _libmdbx_ will always use new pages until memory is exhausted or `mdbx_env_sync()` is invoked. Total
|
||||
disk usage will be almost the same as in sync-write mode.
|
||||
|
||||
Current _libmdbx_ gives a choice of safe async-write mode (default) and `UTTERLY_NOSYNC` mode which may result in full DB
|
||||
corruption during system crash as with LMDB.
|
||||
|
||||
Next version of _libmdbx_ will create steady synchronization points automatically in async-write mode.
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
Improvements over LMDB
|
||||
================================================
|
||||
|
||||
1. `LIFO RECLAIM` mode:
|
||||
|
||||
The newest pages are picked for reuse instead of the oldest.
|
||||
This allows to minimize reclaim loop and make it execution time independent of total page count.
|
||||
|
||||
This results in OS kernel cache mechanisms working with maximum efficiency.
|
||||
In case of using disk controllers or storages with
|
||||
[BBWC](https://en.wikipedia.org/wiki/Disk_buffer#Write_acceleration) this may greatly improve
|
||||
write performance.
|
||||
|
||||
2. `OOM-KICK` callback.
|
||||
|
||||
`mdbx_env_set_oomfunc()` allows to set a callback, which will be called
|
||||
in the event of memory exhausting during long-time read transaction.
|
||||
Callback will be invoked with PID and pthread_id of offending thread as parameters.
|
||||
Callback can do any of these things to remedy the problem:
|
||||
|
||||
* wait for read transaction to finish normally;
|
||||
|
||||
* kill the offending process (signal 9), if separate process is doing long-time read;
|
||||
|
||||
* abort or restart offending read transaction if it's running in sibling thread;
|
||||
|
||||
* abort current write transaction with returning error code.
|
||||
|
||||
3. Guarantee of DB integrity in `WRITEMAP+MAPSYNC` mode:
|
||||
> Current _libmdbx_ gives a choice of safe async-write mode (default)
|
||||
> and `UTTERLY_NOSYNC` mode which may result in full
|
||||
> DB corruption during system crash as with LMDB. For details see
|
||||
> [Data safety in async-write mode](#data-safety-in-async-write-mode).
|
||||
|
||||
4. Automatic creation of synchronization points (flush changes to persistent storage)
|
||||
when changes reach set threshold (threshold can be set by `mdbx_env_set_syncbytes()`).
|
||||
|
||||
5. Ability to get how far current read-only snapshot is from latest version of the DB by `mdbx_txn_straggler()`.
|
||||
|
||||
6. `mdbx_chk` tool for DB checking and `mdbx_env_pgwalk()` for page-walking all pages in DB.
|
||||
|
||||
7. Control over debugging and receiving of debugging messages via `mdbx_setup_debug()`.
|
||||
|
||||
8. Ability to assign up to 3 markers to commiting transaction with `mdbx_canary_put()` and then get them in read transaction
|
||||
by `mdbx_canary_get()`.
|
||||
|
||||
9. Check if there is a row with data after current cursor position via `mdbx_cursor_eof()`.
|
||||
|
||||
10. Ability to explicitly request update of present record without creating new record. Implemented as `MDBX_CURRENT` flag
|
||||
for `mdbx_put()`.
|
||||
|
||||
11. Ability to update or delete record and get previous value via `mdbx_replace()` Also can update specific multi-value.
|
||||
|
||||
12. Support for keys and values of zero length, including sorted duplicates.
|
||||
|
||||
13. Fixed `mdbx_cursor_count()`, which returns correct count of duplicated for all table types and any cursor position.
|
||||
|
||||
14. Ability to open DB in exclusive mode with `MDBX_EXCLUSIVE` flag, e.g. for integrity check.
|
||||
|
||||
15. Ability to close DB in "dirty" state (without data flush and creation of steady synchronization point)
|
||||
via `mdbx_env_close_ex()`.
|
||||
|
||||
16. Ability to get additional info, including number of the oldest snapshot of DB, which is used by one of the readers.
|
||||
Implemented via `mdbx_env_info()`.
|
||||
|
||||
17. `mdbx_del()` doesn't ignore additional argument (specifier) `data`
|
||||
for tables without duplicates (without flag `MDBX_DUPSORT`), if `data` is not zero then always uses it to verify
|
||||
record, which is being deleted.
|
||||
|
||||
18. Ability to open dbi-table with simultaneous setup of comparators for keys and values, via `mdbx_dbi_open_ex()`.
|
||||
|
||||
19. Ability to find out if key or value is in dirty page. This may be useful to make a decision to avoid
|
||||
excessive CoW before updates. Implemented via `mdbx_is_dirty()`.
|
||||
|
||||
20. Correct update of current record in `MDBX_CURRENT` mode of `mdbx_cursor_put()`, including sorted duplicated.
|
||||
|
||||
21. All cursors in all read and write transactions can be reused by `mdbx_cursor_renew()` and MUST be freed explicitly.
|
||||
> ## Caution, please pay attention!
|
||||
>
|
||||
> This is the only change of API, which changes semantics of cursor management
|
||||
> and can lead to memory leaks on misuse. This is a needed change as it eliminates ambiguity
|
||||
> which helps to avoid such errors as:
|
||||
> - use-after-free;
|
||||
> - double-free;
|
||||
> - memory corruption and segfaults.
|
||||
|
||||
22. Additional error code `MDBX_EMULTIVAL`, which is returned by `mdbx_put()` and
|
||||
`mdbx_replace()` in case is ambiguous update or delete.
|
||||
|
||||
23. Ability to get value by key and duplicates count by `mdbx_get_ex()`.
|
||||
|
||||
24. Functions `mdbx_cursor_on_first() and mdbx_cursor_on_last(), which allows to know if cursor is currently on first or
|
||||
last position respectively.
|
||||
|
||||
25. If read transaction is aborted via `mdbx_txn_abort()` or `mdbx_txn_reset()` then DBI-handles, which were opened in it,
|
||||
aren't closed or deleted. This allows to avoid several types of hard-to-debug errors.
|
||||
|
||||
26. Sequence generation via `mdbx_dbi_sequence()`.
|
||||
|
||||
27. Advanced dynamic control over DB size, including ability to choose page size via `mdbx_env_set_geometry()`,
|
||||
including on Windows.
|
||||
|
||||
28. Three meta-pages instead of two, this allows to guarantee consistently update weak sync-points without risking to
|
||||
corrupt last steady sync-point.
|
||||
|
||||
29. Automatic reclaim of freed pages to specific reserved space at the end of database file. This lowers amount of pages,
|
||||
loaded to memory, used in update/flush loop. In fact _libmdbx_ constantly performs compactification of data,
|
||||
but doesn't use additional resources for that. Space reclaim of DB and setup of database geometry parameters also decreases
|
||||
size of the database on disk, including on Windows.
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
```
|
||||
$ objdump -f -h -j .text libmdbx.so
|
||||
|
||||
@@ -474,16 +597,3 @@ Idx Name Size VMA LMA File off Algn
|
||||
CONTENTS, ALLOC, LOAD, READONLY, CODE
|
||||
|
||||
```
|
||||
|
||||
```
|
||||
$ gcc -v
|
||||
Using built-in specs.
|
||||
COLLECT_GCC=gcc
|
||||
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
|
||||
OFFLOAD_TARGET_NAMES=nvptx-none
|
||||
OFFLOAD_TARGET_DEFAULT=1
|
||||
Target: x86_64-linux-gnu
|
||||
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.2.0-8ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
|
||||
Thread model: posix
|
||||
gcc version 7.2.0 (Ubuntu 7.2.0-8ubuntu3)
|
||||
```
|
||||
|
Reference in New Issue
Block a user