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The [repository was moved](https://abf.io/erthink/libmdbx) due to illegal discriminatory blocking of access from the territory of the Russian Crimea and for sovereign crimeans. Crimea is Russia. Change-Id: I5a4eb6b50be2e88f4dc6658d00331954e373603a
581 lines
25 KiB
Markdown
581 lines
25 KiB
Markdown
## The [repository was moved](https://abf.io/erthink/libmdbx) due to illegal discriminatory blocking of access from the territory of the Russian Crimea and for sovereign crimeans.
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---
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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
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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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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 libmdbx for other runtimes.
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| Runtime | GitHub | Author |
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| ------------- | ------------- | ------------- |
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| JVM | [mdbxjni](https://github.com/castortech/mdbxjni) | [Castor Technologies](https://castortech.com/) |
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| .NET | [mdbx.NET](https://github.com/wangjia184/mdbx.NET) | [Jerry Wang](https://github.com/wangjia184) |
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-----
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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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- [Problem of long-time reading](#problem-of-long-time-reading)
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- [Durability in asynchronous writing mode](#durability-in-asynchronous-writing-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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- [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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## 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_ 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 Olog(N) operation cost.
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_libmdbx_ enforce [serializability](https://en.wikipedia.org/wiki/Serializability) for writers by single [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion) and affords [wait-free](https://en.wikipedia.org/wiki/Non-blocking_algorithm#Wait-freedom) for parallel readers without atomic/interlocked operations, while writing and reading transactions do not block each other.
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_libmdbx_ can guarantee consistency after crash depending of operation mode.
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_libmdbx_ uses [B+Trees](https://en.wikipedia.org/wiki/B%2B_tree) and
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[Memory-Mapping](https://en.wikipedia.org/wiki/Memory-mapped_file), doesn't use
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[WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) which
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might be a caveat for some workloads.
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### Comparison with other DBs
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For now please refer to [chapter of "BoltDB comparison with other
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databases"](https://github.com/coreos/bbolt#comparison-with-other-databases)
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which is also (mostly) applicable to MDBX.
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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 [ReOpenLDAP](https://github.com/leo-yuriev/ReOpenLDAP) project.
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About a year later libmdbx 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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Since early 2017 _libmdbx_ is used in [Fast Positive Tables](https://github.com/leo-yuriev/libfpta),
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and development is funded 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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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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=============
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_libmdbx_ inherits all features and characteristics from
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[LMDB](https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database):
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1. Key-value pairs are stored in ordered map(s), keys are always sorted,
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range lookups are supported.
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2. Data is [memory-mapped](https://en.wikipedia.org/wiki/Memory-mapped_file)
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into each worker DB process, and could be accessed zero-copy from transactions.
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3. Transactions are
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[ACID](https://en.wikipedia.org/wiki/ACID)-compliant, through 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. Read transactions are
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[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
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performance scales linearly with CPU core count.
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> Nonetheless, "connect to DB" (starting the first read transaction in a thread) and
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> "disconnect from DB" (closing DB or thread termination) requires a lock
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> acquisition to register/unregister at the "readers table".
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5. Keys with multiple values are stored efficiently without key
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duplication, sorted by value, including integers (valuable for
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secondary indexes).
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6. Efficient operation on short fixed length keys,
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including 32/64-bit integer types.
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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
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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 additional memory management, all done by basic OS services.
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--------------------------------------------------------------------------------
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Improvements over LMDB
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======================
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1. Automatic dynamic DB size management according to the parameters
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specified by `mdbx_env_set_geometry()` function. 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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2. 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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3. `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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4. Fast estimation of range query result size via functions `mdbx_estimate_range()`,
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`mdbx_estimate_move()` and `mdbx_estimate_distance()`. E.g. for selection the
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optimal query execution plan.
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5. `mdbx_chk` tool for DB integrity check.
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6. Support for keys and values of zero length, including multi-values (aka sorted duplicates).
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7. Ability to assign up to 3 persistent 64-bit 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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8. Ability to update or delete record and get previous value via `mdbx_replace()`.
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Also allows update the specific item from multi-value with the same key.
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9. Sequence generation via `mdbx_dbi_sequence()`.
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10. `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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11. Ability to open DB in exclusive mode by `MDBX_EXCLUSIVE` flag.
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12. Ability to get how far current read-transaction snapshot lags
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from the latest version of the DB by `mdbx_txn_straggler()`.
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13. Ability to explicitly update the existing record, not insertion
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a new one. Implemented as `MDBX_CURRENT` flag for `mdbx_put()`.
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14. Fixed `mdbx_cursor_count()`, which returns correct count of
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duplicated (aka multi-value) for all cases and any cursor position.
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15. `mdbx_env_info()` to getting additional info, including number of
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the oldest snapshot of DB, which is used by someone of the readers.
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16. `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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17. Ability to open dbi-table with simultaneous with race-free setup
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of comparators for keys and values, via `mdbx_dbi_open_ex()`.
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18. `mdbx_is_dirty()`to find out if given key or value is on dirty page, that
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useful to avoid copy-out before updates.
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19. Correct update of current record in `MDBX_CURRENT` mode of
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`mdbx_cursor_put()`, including sorted duplicated.
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20. Check if there is a row with data after current cursor position via
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`mdbx_cursor_eof()`.
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21. 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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22. Ability to get value by key and duplicates count by `mdbx_get_ex()`.
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23. Functions `mdbx_cursor_on_first()` and `mdbx_cursor_on_last()`,
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which allows to check cursor is currently on first or last position
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respectively.
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24. Automatic creation of steady commit-points (flushing data to the
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disk) when the volume of changes reaches a threshold, which can be
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set by `mdbx_env_set_syncbytes()`.
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25. Control over debugging and receiving of debugging messages via
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`mdbx_setup_debug()`.
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26. Function `mdbx_env_pgwalk()` for page-walking the DB.
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27. Three meta-pages instead of two, that allows to guarantee
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consistency of data when updating weak commit-points without the
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risk of damaging the last steady commit-point.
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28. 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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29. Ability to close DB in "dirty" state (without data flush and
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creation of steady synchronization point) via `mdbx_env_close_ex()`.
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30. If read transaction is aborted via `mdbx_txn_abort()` or
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`mdbx_txn_reset()` then DBI-handles, which were opened during it,
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will not be closed or deleted. In several cases this allows
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to avoid hard-to-debug errors.
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31. All cursors in all read and write transactions can be reused by
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`mdbx_cursor_renew()` and MUST be freed explicitly.
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> ## Caution, please pay attention!
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>
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> This is the only change of API, which changes semantics of cursor management
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> and can lead to memory leaks on misuse. This is a needed change as it eliminates ambiguity
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> 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
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1. There cannot be more than one writer at a time. This allows serialize an
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updates and eliminate any possibility of conflicts, deadlocks or logical errors.
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2. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) means
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relatively big [WAF](https://en.wikipedia.org/wiki/Write_amplification)
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(Write Amplification Factor). Because of this syncing data to disk might
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be quite resource intensive and be main performance bottleneck during
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intensive write workload.
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> As compromise _libmdbx_ allows several modes of lazy and/or periodic
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> syncing, including `MAPASYNC` mode, which modificate data in memory and
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> 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
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> with transaction journal will require 2 IOPS minimum (probably 3-4 in
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> practice) because of filesystem overhead, overhead depends on
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> filesystem, not on record count or record size. In _libmdbx_ IOPS count
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> will grow logarithmically depending on record count in DB (height of B+
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> tree) and will require at least 2 IOPS per transaction too.
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3. [CoW](https://en.wikipedia.org/wiki/Copy-on-write) for
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[MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control)
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is done on memory page level with
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[B+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,
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which uses [memory bandwidth](https://en.wikipedia.org/wiki/Memory_bandwidth) and is main
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performance bottleneck in `MAPASYNC` mode.
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> This is unavoidable, but isn't that bad. Syncing data to disk requires
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> much more similar operations which will be done by OS, therefore this is
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> 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
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> performance overhead. If there is no need to save data to persistent
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> 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
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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
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of data.
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> Additionally there is an alternative: `UTTERLY_NOSYNC` mode.
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> Details below.
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#### Problem of long-time reading
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Garbage collection problem exists in all databases one way or another
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(e.g. VACUUM in PostgreSQL). But in _libmdbx_ and LMDB it's even more
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discernible because of high transaction rate and intentional internals
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simplification in favor of performance.
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Understanding the problem requires some explanation, but can be
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difficult for quick perception. So is is reasonable
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to simplify this as follows:
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* Massive altering of data during a parallel long read operation may
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exhaust the free DB space.
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* If the available space is exhausted, any attempt to update the data
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will cause a "MAP_FULL" error until a long read transaction is completed.
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* A good example of long readers is a hot backup or debugging of
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a client application while retaining an active read transaction.
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* In _LMDB_ this results in degraded performance of all operations of
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writing data to persistent storage.
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* _libmdbx_ has the `OOM-KICK` mechanism which allow to abort such
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operations and the `LIFO RECLAIM` mode which addresses performance
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degradation.
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#### Durability in asynchronous writing mode
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In `WRITEMAP+MAPSYNC` mode updated (aka dirty) pages are written
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to persistent storage by the OS kernel. This means that if the
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application fails, the OS kernel will finish writing all updated
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data to disk and nothing will be lost.
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However, in the case of hardware malfunction or OS kernel fatal error,
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only some updated data can be written to disk and the database structure
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is likely to be destroyed.
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In such situation, DB is completely corrupted and can't be repaired.
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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,
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instead their shadow copies are used and their updates are synced after
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data is flushed to disk.
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* During transaction commit _libmdbx_ marks it as a steady or weak
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depending on synchronization status between RAM and persistent storage.
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For instance, in the `WRITEMAP+MAPSYNC` mode committed transactions
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are marked as weak by default, but as steady after explicit data flushes.
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* _libmdbx_ maintains three separate meta-pages instead of two. This
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allows to commit transaction as steady or weak without losing two
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previous commit points (one of them can be steady, and another
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weak). Thus, after a fatal system failure, it will be possible to
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rollback to the last steady commit point.
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* During DB open _libmdbx_ rollbacks to the last steady commit point,
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this guarantees database integrity after a crash. However, if the
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database opening in read-only mode, such rollback cannot be performed
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which will cause returning the MDBX_WANNA_RECOVERY error.
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For data integrity a pages which form database snapshot with steady
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commit point, must not be updated until next steady commit point.
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Therefore the last steady commit point creates an effect analogues to "long-time read".
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The only difference that now in case of space exhaustion the problem
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will be immediately addressed by writing changes to disk and forming
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the new steady commit point.
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So in async-write mode _libmdbx_ will always use new pages until the
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free DB space will be exhausted or `mdbx_env_sync()` will be invoked,
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and the total write traffic to the disk will be the same as in sync-write mode.
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Currently libmdbx gives a choice between a safe async-write mode (default) and
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`UTTERLY_NOSYNC` mode which may lead to DB corruption after a system crash, i.e. like the LMDB.
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Next version of _libmdbx_ will be automatically create steady commit
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points in async-write mode upon completion transfer data to the disk.
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--------------------------------------------------------------------------------
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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)
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operations in sync-write mode (fdatasync is called after each
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transaction);
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- Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD)
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operations in lazy-write mode (moment to sync data to persistent storage
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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
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for keeping data in memory e.g. [Tarantool](https://tarantool.io/),
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[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
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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
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transactions per second;
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- Logarithmic scale on right is in seconds and yellow intervals mean
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execution time of transactions. Each interval shows minimal and maximum
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execution time, cross marks standard deviation.
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**10,000 transactions in sync-write mode**. In case of a crash all data
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is consistent and state is right after last successful transaction.
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[fdatasync](https://linux.die.net/man/2/fdatasync) syscall is used after
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each write transaction in this mode.
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In the benchmark each transaction contains combined CRUD operations (2
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inserts, 1 read, 1 update, 1 delete). Benchmark starts on empty database
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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
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thousands transactions per second;
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- Logarithmic scale on right in seconds and yellow intervals mean
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execution time of transactions. Each interval shows minimal and maximum
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execution time, cross marks standard deviation.
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**100,000 transactions in lazy-write mode**. In case of a crash all data
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is consistent and state is right after one of last transactions, but
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transactions after it will be lost. Other DB engines use
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[WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) or transaction
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journal for that, which in turn depends on order of operations in
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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
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inserts, 1 read, 1 update, 1 delete). Benchmark starts on empty database
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and after full run the database contains 100,000 small key-value
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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
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thousands transactions per second;
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- Logarithmic scale on right in seconds and yellow intervals mean
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execution time of transactions. Each interval shows minimal and maximum
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execution time, cross marks standard deviation.
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**1,000,000 transactions in async-write mode**. In case of a crash all
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data will be consistent and state will be right after one of last
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transactions, but lost transaction count is much higher than in
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lazy-write mode. All DB engines in this mode do as little writes as
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possible on persistent storage. _libmdbx_ uses
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[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
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inserts, 1 read, 1 update, 1 delete). Benchmark starts on empty database
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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
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waiting for the end of all internal housekeeping operations (LSM
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compactification, etc).
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_ForestDB_ is excluded because benchmark showed it's resource
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consumption for each resource (CPU, IOPS) much higher than other engines
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which prevents to meaningfully compare it with them.
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All benchmark data is gathered by
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[getrusage()](http://man7.org/linux/man-pages/man2/getrusage.2.html)
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syscall and by scanning data directory.
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--------------------------------------------------------------------------------
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```
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$ objdump -f -h -j .text libmdbx.so
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libmdbx.so: file format elf64-x86-64
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architecture: i386:x86-64, flags 0x00000150:
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HAS_SYMS, DYNAMIC, D_PAGED
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start address 0x0000000000003870
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Sections:
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Idx Name Size VMA LMA File off Algn
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11 .text 000173d4 0000000000003870 0000000000003870 00003870 2**4
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CONTENTS, ALLOC, LOAD, READONLY, CODE
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```
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