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490 lines
26 KiB
Markdown
490 lines
26 KiB
Markdown
libmdbx
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======================================
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**Revised and extended descendant of [Symas LMDB](https://symas.com/lmdb/).**
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*The Future will be positive.*
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[](https://travis-ci.org/leo-yuriev/libmdbx)
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[](https://ci.appveyor.com/project/leo-yuriev/libmdbx/branch/master)
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[](https://scan.coverity.com/projects/reopen-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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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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4. Using "[Roaring bitmaps](http://roaringbitmap.org/about/)" inside garbage collector;
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5. Non-sequential reclaiming, like PostgreSQL's [Vacuum](https://www.postgresql.org/docs/9.1/static/sql-vacuum.html);
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6. [Asynchronous lazy data flushing](https://sites.fas.harvard.edu/~cs265/papers/kathuria-2008.pdf) to disk(s);
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7. etc...
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Don't miss [Java Native Interface](https://github.com/castortech/mdbxjni) by [Castor Technologies](https://castortech.com/).
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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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## 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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- [Performance comparison](#performance-comparison)
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- [Integral performance](#integral-performance)
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- [Read scalability](#read-scalability)
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- [Sync-write mode](#sync-write-mode)
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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 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_ 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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_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_ 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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### 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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### History
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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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by [Positive Technologies](https://www.ptsecurity.com).
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#### Acknowledgments
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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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Main features
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=============
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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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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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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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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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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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6. Efficient operation on short fixed length keys, including integer 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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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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9. No custom memory management, all done with standard OS syscalls.
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Performance comparison
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=====================
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All benchmarks were done by [IOArena](https://github.com/pmwkaa/ioarena)
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and multiple [scripts](https://github.com/pmwkaa/ioarena/tree/HL%2B%2B2015)
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runs on Lenovo Carbon-2 laptop, i7-4600U 2.1 GHz, 8 Gb RAM,
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SSD SAMSUNG MZNTD512HAGL-000L1 (DXT23L0Q) 512 Gb.
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--------------------------------------------------------------------------------
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### Integral performance
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Here showed sum of performance metrics in 3 benchmarks:
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- Read/Search on 4 CPU cores machine;
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- Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD) operations
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in sync-write mode (fdatasync is called after each transaction);
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- Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD) operations
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in lazy-write mode (moment to sync data to persistent storage is decided by OS).
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*Reasons why asynchronous mode isn't benchmarked here:*
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1. It doesn't make sense as it has to be done with DB engines, oriented for keeping data in memory e.g.
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[Tarantool](https://tarantool.io/), [Redis](https://redis.io/)), etc.
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2. Performance gap is too high to compare in any meaningful way.
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--------------------------------------------------------------------------------
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### Read Scalability
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Summary performance with concurrent read/search queries in 1-2-4-8 threads on 4 CPU cores machine.
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--------------------------------------------------------------------------------
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### Sync-write mode
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- Linear scale on left and dark rectangles mean arithmetic mean transactions per second;
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- Logarithmic scale on right is in seconds and yellow intervals mean execution time of transactions.
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Each interval shows minimal and maximum execution time, cross marks standard deviation.
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**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.
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In the benchmark each transaction contains combined CRUD operations (2 inserts, 1 read, 1 update, 1 delete).
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Benchmark starts on empty database and after full run the database contains 10,000 small key-value records.
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--------------------------------------------------------------------------------
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### Lazy-write mode
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- Linear scale on left and dark rectangles mean arithmetic mean of thousands transactions per second;
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- 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.
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**100,000 transactions in lazy-write mode**.
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In case of a crash all data is consistent and state is right after one of last transactions, but transactions after it
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will be lost. Other DB engines use [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) or transaction journal for that,
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which in turn depends on order of operations in journaled filesystem. _libmdbx_ doesn't use WAL and hands I/O operations
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to filesystem and OS kernel (mmap).
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In the benchmark each transaction contains combined CRUD operations (2 inserts, 1 read, 1 update, 1 delete).
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Benchmark starts on empty database and after full run the database contains 100,000 small key-value records.
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--------------------------------------------------------------------------------
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### Async-write mode
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- Linear scale on left and dark rectangles mean arithmetic mean of thousands transactions per second;
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- 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.
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**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.
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In the benchmark each transaction contains combined CRUD operations (2 inserts, 1 read, 1 update, 1 delete).
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Benchmark starts on empty database and after full run the database contains 10,000 small key-value records.
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--------------------------------------------------------------------------------
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### Cost comparison
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Summary of used resources during lazy-write mode benchmarks:
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- Read and write IOPS;
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- Sum of user CPU time and sys CPU time;
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- Used space on persistent storage after the test and closed DB, but not waiting for the end of all internal
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housekeeping operations (LSM compactification, etc).
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_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.
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All benchmark data is gathered by [getrusage()](http://man7.org/linux/man-pages/man2/getrusage.2.html) syscall and by
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scanning data directory.
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--------------------------------------------------------------------------------
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## Gotchas
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1. At one moment there can be only one writer. But this allows to serialize writes and eliminate any possibility
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of conflict or logical errors during transaction rollback.
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2. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) means relatively
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big [WAF](https://en.wikipedia.org/wiki/Write_amplification) (Write Amplification Factor).
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Because of this syncing data to disk might be quite resource intensive and be main performance bottleneck
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during intensive write workload.
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> As compromise _libmdbx_ allows several modes of lazy and/or periodic syncing, including `MAPASYNC` mode, which modificate
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> data in memory and asynchronously syncs data to disk, moment to sync is picked by OS.
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>
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> Although this should be used with care, synchronous transactions in a DB with transaction journal will require 2 IOPS
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> minimum (probably 3-4 in practice) because of filesystem overhead, overhead depends on filesystem, not on record
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> count or record size. In _libmdbx_ IOPS count will grow logarithmically depending on record count in DB (height of B+ tree)
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> and will require at least 2 IOPS per transaction too.
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3. [CoW](https://en.wikipedia.org/wiki/Copy-on-write)
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for [MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control) is done on memory page level with [B+
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trees](https://ru.wikipedia.org/wiki/B-%D0%B4%D0%B5%D1%80%D0%B5%D0%B2%D0%BE).
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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.
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> This is unavoidable, but isn't that bad. Syncing data to disk requires much more similar operations which will
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> be done by OS, therefore this is noticeable only if data sync to persistent storage is fully disabled.
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> _libmdbx_ allows to safely save data to persistent storage with minimal performance overhead. If there is no need
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> to save data to persistent storage then it's much more preferable to use `std::map`.
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4. LMDB has a problem of long-time readers which degrades performance and bloats DB
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> _libmdbx_ addresses that, details below.
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5. _LMDB_ is susceptible to DB corruption in `WRITEMAP+MAPASYNC` mode.
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_libmdbx_ in `WRITEMAP+MAPASYNC` guarantees DB integrity and consistency of data.
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> Additionally there is an alternative: `UTTERLY_NOSYNC` mode. Details below.
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#### Long-time read transactions problem
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Garbage collection problem exists in all databases one way or another (e.g. VACUUM in PostgreSQL).
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But in _libmdbx_ and LMDB it's even more important because of high performance and deliberate
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simplification of internals with emphasis on performance.
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* Altering data during long read operation may exhaust available space on persistent storage.
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* If available space is exhausted then any attempt to update data
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results in `MAP_FULL` error until long read operation ends.
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* Main examples of long readers is hot backup
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and debugging of client application which actively uses read transactions.
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* In _LMDB_ this results in degraded performance of all operations
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of syncing data to persistent storage.
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* _libmdbx_ has a mechanism which aborts such operations and `LIFO RECLAIM`
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mode which addresses performance degradation.
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Read operations operate only over snapshot of DB which is consistent on the moment when read transaction started.
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This snapshot doesn't change throughout the transaction but this leads to inability to reclaim the pages until
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read transaction ends.
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In _LMDB_ this leads to a problem that memory pages, allocated for operations during long read, will be used for operations
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and won't be reclaimed until DB process terminates. In _LMDB_ they are used in
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[FIFO](https://en.wikipedia.org/wiki/FIFO_(computing_and_electronics)) manner, which causes increased page count
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and less chance of cache hit during I/O. In other words: one long-time reader can impact performance of all database
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until it'll be reopened.
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_libmdbx_ addresses the problem, details below. Illustrations to this problem can be found in the
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[presentation](http://www.slideshare.net/leoyuriev/lmdb). There is also example of performance increase thanks to
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[BBWC](https://en.wikipedia.org/wiki/Disk_buffer#Write_acceleration) when `LIFO RECLAIM` enabled in _libmdbx_.
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#### Data safety in async-write mode
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In `WRITEMAP+MAPSYNC` mode dirty pages are written to persistent storage by kernel. This means that in case of application
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crash OS kernel will write all dirty data to disk and nothing will be lost. But in case of hardware malfunction or OS kernel
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fatal error only some dirty data might be synced to disk, and there is high probability that pages with metadata saved,
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will point to non-saved, hence non-existent, data pages. In such situation, DB is completely corrupted and can't be
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repaired even if there was full sync before the crash via `mdbx_env_sync().
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_libmdbx_ addresses this by fully reimplementing write path of data:
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* In `WRITEMAP+MAPSYNC` mode meta-data pages aren't updated in place, instead their shadow copies are used and their updates
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are synced after data is flushed to disk.
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* During transaction commit _libmdbx_ marks synchronization points as steady or weak depending on how much synchronization
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needed between RAM and persistent storage, e.g. in `WRITEMAP+MAPSYNC` commited transactions are marked as weak,
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but during explicit data synchronization - as steady.
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* _libmdbx_ maintains three separate meta-pages instead of two. This allows to commit transaction with steady or
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weak synchronization point without losing two previous synchronization points (one of them can be steady, and second - weak).
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This allows to order weak and steady synchronization points in any order without losing consistency in case of system crash.
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* During DB open _libmdbx_ rollbacks to the last steady synchronization point, this guarantees database integrity.
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For data safety pages which form database snapshot with steady synchronization point must not be updated until next steady
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synchronization point. So last steady synchronization point creates "long-time read" effect. The only difference that in case
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of memory exhaustion the problem will be immediately addressed by flushing changes to persistent storage and forming new steady
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synchronization point.
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So in async-write mode _libmdbx_ will always use new pages until memory is exhausted or `mdbx_env_sync()` is invoked. Total
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disk usage will be almost the same as in sync-write mode.
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Current _libmdbx_ gives a choice of safe async-write mode (default) and `UTTERLY_NOSYNC` mode which may result in full DB
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corruption during system crash as with LMDB.
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Next version of _libmdbx_ will create steady synchronization points automatically in async-write mode.
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--------------------------------------------------------------------------------
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Improvements over LMDB
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================================================
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1. `LIFO RECLAIM` mode:
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The newest pages are picked for reuse instead of the oldest.
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This allows to minimize reclaim loop and make it execution time independent of total page count.
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This results in OS kernel cache mechanisms working with maximum efficiency.
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In case of using disk controllers or storages with
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[BBWC](https://en.wikipedia.org/wiki/Disk_buffer#Write_acceleration) this may greatly improve
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write performance.
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2. `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 memory exhausting during long-time read transaction.
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Callback will be invoked with PID and pthread_id of offending thread as parameters.
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Callback can do any of 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 long-time read;
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* abort or restart offending read transaction if it's running in sibling thread;
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* abort current write transaction with returning error code.
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3. Guarantee of DB integrity in `WRITEMAP+MAPSYNC` mode:
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> Current _libmdbx_ gives a choice of safe async-write mode (default)
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> and `UTTERLY_NOSYNC` mode which may result in full
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> DB corruption during system crash as with LMDB. For details see
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> [Data safety in async-write mode](#data-safety-in-async-write-mode).
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4. Automatic creation of synchronization points (flush changes to persistent storage)
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when changes reach set threshold (threshold can be set by `mdbx_env_set_syncbytes()`).
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5. Ability to get how far current read-only snapshot is from latest version of the DB by `mdbx_txn_straggler()`.
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6. `mdbx_chk` tool for DB checking and `mdbx_env_pgwalk()` for page-walking all pages in DB.
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7. Control over debugging and receiving of debugging messages via `mdbx_setup_debug()`.
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8. Ability to assign up to 3 markers to commiting transaction with `mdbx_canary_put()` and then get them in read transaction
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by `mdbx_canary_get()`.
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9. Check if there is a row with data after current cursor position via `mdbx_cursor_eof()`.
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10. Ability to explicitly request update of present record without creating new record. Implemented as `MDBX_CURRENT` flag
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for `mdbx_put()`.
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11. Ability to update or delete record and get previous value via `mdbx_replace()` Also can update specific multi-value.
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12. Support for keys and values of zero length, including sorted duplicates.
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13. Fixed `mdbx_cursor_count()`, which returns correct count of duplicated for all table types and any cursor position.
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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
|
|
|
|
libmdbx.so: file format elf64-x86-64
|
|
architecture: i386:x86-64, flags 0x00000150:
|
|
HAS_SYMS, DYNAMIC, D_PAGED
|
|
start address 0x000030e0
|
|
|
|
Sections:
|
|
Idx Name Size VMA LMA File off Algn
|
|
11 .text 00014d84 00000000000030e0 00000000000030e0 000030e0 2**4
|
|
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)
|
|
```
|