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diff --git a/libs/libmdbx/src/README.md b/libs/libmdbx/src/README.md index 92b6542fa7..7c07de316e 100644 --- a/libs/libmdbx/src/README.md +++ b/libs/libmdbx/src/README.md @@ -9,9 +9,21 @@ libmdbx ## Project Status for now - - 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. - - 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. - - The immediate goal of development is formation of the stable API and the stable internal database format, which allows realise all PLANNED FEATURES: + - 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. + + - 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. + + - The immediate goal of development is formation of the stable API and + the stable internal database format, which allows realise all PLANNED + FEATURES: 1. Integrity check by [Merkle tree](https://en.wikipedia.org/wiki/Merkle_tree); 2. Support for [raw block devices](https://en.wikipedia.org/wiki/Raw_device); 3. Separate place (HDD) for large data items; @@ -24,19 +36,21 @@ Don't miss [Java Native Interface](https://github.com/castortech/mdbxjni) by [Ca ----- -Nowadays MDBX intended for Linux, and support Windows (since -Windows Server 2008) as a complementary platform. Support for -other OS could be implemented on commercial basis. However such -enhancements (i.e. pull requests) could be accepted in -mainstream only when corresponding public and free Continuous -Integration service will be available. +Nowadays MDBX intended for Linux, and support Windows (since Windows +Server 2008) as a complementary platform. Support for other OS could be +implemented on commercial basis. However such enhancements (i.e. pull +requests) could be accepted in mainstream only when corresponding public +and free Continuous Integration service will be available. ## Contents - - [Overview](#overview) - [Comparison with other DBs](#comparison-with-other-dbs) - [History & Acknowledgments](#history) - [Main features](#main-features) +- [Improvements over LMDB](#improvements-over-lmdb) +- [Gotchas](#gotchas) + - [Long-time read transactions problem](#long-time-read-transactions-problem) + - [Data safety in async-write-mode](#data-safety-in-async-write-mode) - [Performance comparison](#performance-comparison) - [Integral performance](#integral-performance) - [Read scalability](#read-scalability) @@ -44,52 +58,58 @@ Integration service will be available. - [Lazy-write mode](#lazy-write-mode) - [Async-write mode](#async-write-mode) - [Cost comparison](#cost-comparison) -- [Gotchas](#gotchas) - - [Long-time read transactions problem](#long-time-read-transactions-problem) - - [Data safety in async-write-mode](#data-safety-in-async-write-mode) -- [Improvements over LMDB](#improvements-over-lmdb) ## Overview +_libmdbx_ is an embedded lightweight key-value database engine oriented +for performance under Linux and Windows. -_libmdbx_ is an embedded lightweight key-value database engine oriented for performance under Linux and Windows. - -_libmdbx_ allows multiple processes to read and update several key-value tables concurrently, -while being [ACID](https://en.wikipedia.org/wiki/ACID)-compliant, with minimal overhead and operation cost of Olog(N). +_libmdbx_ allows multiple processes to read and update several key-value +tables concurrently, while being +[ACID](https://en.wikipedia.org/wiki/ACID)-compliant, with minimal +overhead and operation cost of Olog(N). _libmdbx_ provides -[serializability](https://en.wikipedia.org/wiki/Serializability) and consistency of data after crash. -Read-write transactions don't block read-only transactions and are -[serialized](https://en.wikipedia.org/wiki/Serializability) by [mutex](https://en.wikipedia.org/wiki/Mutual_exclusion). - -_libmdbx_ [wait-free](https://en.wikipedia.org/wiki/Non-blocking_algorithm#Wait-freedom) provides parallel read transactions -without atomic operations or synchronization primitives. - -_libmdbx_ uses [B+Trees](https://en.wikipedia.org/wiki/B%2B_tree) and [mmap](https://en.wikipedia.org/wiki/Memory-mapped_file), -doesn't use [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging). This might have caveats for some workloads. +[serializability](https://en.wikipedia.org/wiki/Serializability) and +consistency of data after crash. Read-write transactions don't block +read-only transactions and are +[serialized](https://en.wikipedia.org/wiki/Serializability) by +[mutex](https://en.wikipedia.org/wiki/Mutual_exclusion). + +_libmdbx_ +[wait-free](https://en.wikipedia.org/wiki/Non-blocking_algorithm#Wait-freedom) +provides parallel read transactions without atomic operations or +synchronization primitives. + +_libmdbx_ uses [B+Trees](https://en.wikipedia.org/wiki/B%2B_tree) and +[mmap](https://en.wikipedia.org/wiki/Memory-mapped_file), doesn't use +[WAL](https://en.wikipedia.org/wiki/Write-ahead_logging). This might +have caveats for some workloads. ### Comparison with other DBs - -Because _libmdbx_ is currently overhauled, I think it's better to just link -[chapter of Comparison with other databases](https://github.com/coreos/bbolt#comparison-with-other-databases) here. +Because _libmdbx_ is currently overhauled, I think it's better to just +link [chapter of Comparison with other +databases](https://github.com/coreos/bbolt#comparison-with-other-databases) +here. ### History - -The _libmdbx_ design is based on [Lightning Memory-Mapped Database](https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database). -Initial development was going in [ReOpenLDAP](https://github.com/leo-yuriev/ReOpenLDAP) project, about a year later it -received separate development effort and in autumn 2015 was isolated to separate project, which was -[presented at Highload++ 2015 conference](http://www.highload.ru/2015/abstracts/1831.html). - -Since early 2017 _libmdbx_ is used in [Fast Positive Tables](https://github.com/leo-yuriev/libfpta), +The _libmdbx_ design is based on [Lightning Memory-Mapped +Database](https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database). +Initial development was going in +[ReOpenLDAP](https://github.com/leo-yuriev/ReOpenLDAP) project, about a +year later it received separate development effort and in autumn 2015 +was isolated to separate project, which was [presented at Highload++ +2015 conference](http://www.highload.ru/2015/abstracts/1831.html). + +Since early 2017 _libmdbx_ is used in [Fast PositiveTables](https://github.com/leo-yuriev/libfpta), by [Positive Technologies](https://www.ptsecurity.com). #### Acknowledgments +Howard Chu (Symas Corporation) - the author of LMDB, from which +originated the MDBX in 2015. -Howard Chu (Symas Corporation) - the author of LMDB, -from which originated the MDBX in 2015. - -Martin Hedenfalk <martin@bzero.se> - the author of `btree.c` code, -which was used for begin development of LMDB. +Martin Hedenfalk <martin@bzero.se> - the author of `btree.c` code, which +was used for begin development of LMDB. Main features @@ -98,365 +118,468 @@ Main features _libmdbx_ inherits all keys features and characteristics from [LMDB](https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database): -1. Data is stored in ordered map, keys are always sorted, range lookups are supported. - -2. Data is [mmaped](https://en.wikipedia.org/wiki/Memory-mapped_file) to memory of each worker DB process, read transactions are zero-copy. - -3. Transactions are [ACID](https://en.wikipedia.org/wiki/ACID)-compliant, thanks to - [MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control) and [CoW](https://en.wikipedia.org/wiki/Copy-on-write). - Writes are strongly serialized and aren't blocked by reads, transactions can't conflict with each other. - Reads are guaranteed to get only commited data - ([relaxing serializability](https://en.wikipedia.org/wiki/Serializability#Relaxing_serializability)). - -4. Reads and queries are [non-blocking](https://en.wikipedia.org/wiki/Non-blocking_algorithm), - don't use [atomic operations](https://en.wikipedia.org/wiki/Linearizability#High-level_atomic_operations). - Readers don't block each other and aren't blocked by writers. Read performance scales linearly with CPU core count. - > Though "connect to DB" (start of first read transaction in thread) and "disconnect from DB" (shutdown or thread - > termination) requires to acquire a lock to register/unregister current thread from "readers table" - -5. Keys with multiple values are stored efficiently without key duplication, sorted by value, including integers - (reasonable for secondary indexes). - -6. Efficient operation on short fixed length keys, including integer ones. - -7. [WAF](https://en.wikipedia.org/wiki/Write_amplification) (Write Amplification Factor) и RAF (Read Amplification Factor) - are Olog(N). - -8. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) and transaction journal. - In case of a crash no recovery needed. No need for regular maintenance. Backups can be made on the fly on working DB - without freezing writers. +1. Data is stored in ordered map, keys are always sorted, range lookups +are supported. + +2. Data is [mmaped](https://en.wikipedia.org/wiki/Memory-mapped_file) to +memory of each worker DB process, read transactions are zero-copy. + +3. Transactions are +[ACID](https://en.wikipedia.org/wiki/ACID)-compliant, thanks to +[MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control) +and [CoW](https://en.wikipedia.org/wiki/Copy-on-write). Writes are +strongly serialized and aren't blocked by reads, transactions can't +conflict with each other. Reads are guaranteed to get only commited data +([relaxing serializability](https://en.wikipedia.org/wiki/Serializability#Relaxing_serializability)). + +4. Reads and queries are +[non-blocking](https://en.wikipedia.org/wiki/Non-blocking_algorithm), +don't use [atomic +operations](https://en.wikipedia.org/wiki/Linearizability#High-level_atomic_operations). +Readers don't block each other and aren't blocked by writers. Read +performance scales linearly with CPU core count. + > Though "connect to DB" (start of first read transaction in thread) and + > "disconnect from DB" (shutdown or thread termination) requires to + > acquire a lock to register/unregister current thread from "readers + > table" + +5. Keys with multiple values are stored efficiently without key +duplication, sorted by value, including integers (reasonable for +secondary indexes). + +6. Efficient operation on short fixed length keys, including integer +ones. + +7. [WAF](https://en.wikipedia.org/wiki/Write_amplification) (Write +Amplification Factor) и RAF (Read Amplification Factor) are Olog(N). + +8. No [WAL](https://en.wikipedia.org/wiki/Write-ahead_logging) and +transaction journal. In case of a crash no recovery needed. No need for +regular maintenance. Backups can be made on the fly on working DB + without freezing writers. 9. No custom memory management, all done with standard OS syscalls. +-------------------------------------------------------------------------------- -Performance comparison -===================== +Improvements over LMDB +====================== -All benchmarks were done by [IOArena](https://github.com/pmwkaa/ioarena) -and multiple [scripts](https://github.com/pmwkaa/ioarena/tree/HL%2B%2B2015) -runs on Lenovo Carbon-2 laptop, i7-4600U 2.1 GHz, 8 Gb RAM, -SSD SAMSUNG MZNTD512HAGL-000L1 (DXT23L0Q) 512 Gb. +1. `mdbx_chk` tool for DB integrity check. --------------------------------------------------------------------------------- +2. Automatic dynamic DB size management according to the parameters +specified by `mdbx_env_set_geometry()` function. Including including +growth step and truncation threshold, as well as the choice of page +size. -### Integral performance +3. Automatic returning of freed pages into unallocated space at the end +of database file with optionally automatic shrinking it. This reduces +amount of pages resides in RAM and circulated in disk I/O. In fact +_libmdbx_ constantly performs DB compactification, without spending +additional resources for that. -Here showed sum of performance metrics in 3 benchmarks: +4. Support for keys and values of zero length, including sorted +duplicates. - - Read/Search on 4 CPU cores machine; +5. 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()`. - - Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD) operations - in sync-write mode (fdatasync is called after each transaction); +6. Ability to update or delete record and get previous value via +`mdbx_replace()` Also can update specific multi-value. - - 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). +7. `LIFO RECLAIM` mode: -*Reasons why asynchronous mode isn't benchmarked here:* + 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. - 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. + 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. Performance gap is too high to compare in any meaningful way. +8. Sequence generation via `mdbx_dbi_sequence()`. - +9. `OOM-KICK` callback. --------------------------------------------------------------------------------- + `mdbx_env_set_oomfunc()` allows to set a callback, which will be called + in the event of DB space exhausting during long-time read transaction in + parallel with extensive updating. 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: -### Read Scalability + * wait for read transaction to finish normally; -Summary performance with concurrent read/search queries in 1-2-4-8 threads on 4 CPU cores machine. + * 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. -### Sync-write mode +10. Ability to open DB in exclusive mode with `MDBX_EXCLUSIVE` flag. - - Linear scale on left and dark rectangles mean arithmetic mean transactions per second; +11. Ability to get how far current read-only snapshot is from latest +version of the DB by `mdbx_txn_straggler()`. - - 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. +12. Ability to explicitly request update of present record without +creating new record. Implemented as `MDBX_CURRENT` flag for +`mdbx_put()`. -**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. +13. Fixed `mdbx_cursor_count()`, which returns correct count of +duplicated for all table types and any cursor position. -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. +14. `mdbx_env_info()` to getting additional info, including number of +the oldest snapshot of DB, which is used by one of the readers. - +15. `mdbx_del()` doesn't ignore additional argument (specifier) `data` +for tables without duplicates (without flag `MDBX_DUPSORT`), if `data` +is not null then always uses it to verify record, which is being +deleted. --------------------------------------------------------------------------------- +16. Ability to open dbi-table with simultaneous setup of comparators for +keys and values, via `mdbx_dbi_open_ex()`. -### Lazy-write mode +17. `mdbx_is_dirty()`to find out if key or value is on dirty page, that +useful to avoid copy-out before updates. - - Linear scale on left and dark rectangles mean arithmetic mean of thousands transactions per second; +18. Correct update of current record in `MDBX_CURRENT` mode of +`mdbx_cursor_put()`, including sorted duplicated. - - 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. +19. Check if there is a row with data after current cursor position via +`mdbx_cursor_eof()`. -**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). +20. Additional error code `MDBX_EMULTIVAL`, which is returned by +`mdbx_put()` and `mdbx_replace()` in case is ambiguous update or delete. -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. +21. Ability to get value by key and duplicates count by `mdbx_get_ex()`. +22. Functions `mdbx_cursor_on_first()` and `mdbx_cursor_on_last()`, +which allows to know if cursor is currently on first or last position +respectively. - +23. Automatic creation of synchronization points (flush changes to +persistent storage) when changes reach set threshold (threshold can be +set by `mdbx_env_set_syncbytes()`). --------------------------------------------------------------------------------- +24. Control over debugging and receiving of debugging messages via +`mdbx_setup_debug()`. -### Async-write mode +25. Function `mdbx_env_pgwalk()` for page-walking all pages in DB. - - Linear scale on left and dark rectangles mean arithmetic mean of thousands transactions per second; +26. Three meta-pages instead of two, this allows to guarantee +consistently update weak sync-points without risking to corrupt last +steady sync-point. - - 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. +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). -**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. +28. Ability to close DB in "dirty" state (without data flush and +creation of steady synchronization point) via `mdbx_env_close_ex()`. -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. +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: + > - use-after-free; + > - double-free; + > - memory corruption and segfaults. -------------------------------------------------------------------------------- -### Cost comparison - -Summary of used resources during lazy-write mode benchmarks: +## Gotchas - - Read and write IOPS; +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. - - Sum of user CPU time and sys CPU time; +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. - - 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. +#### 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. -All benchmark data is gathered by [getrusage()](http://man7.org/linux/man-pages/man2/getrusage.2.html) syscall and by -scanning data directory. +* 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. -## Gotchas +* In _LMDB_ this results in degraded performance of all operations of +syncing data to persistent storage. -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. +* _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_. -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. +#### 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: -#### Long-time read transactions problem +* 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. -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. +* 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. -* Altering data during long read operation may exhaust available space on persistent storage. +* _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. -* If available space is exhausted then any attempt to update data - results in `MAP_FULL` error until long read operation ends. +* During DB open _libmdbx_ rollbacks to the last steady synchronization +point, this guarantees database integrity. -* Main examples of long readers is hot backup - and debugging of client application which actively uses read transactions. +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. -* In _LMDB_ this results in degraded performance of all operations - of syncing data to persistent storage. +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. -* _libmdbx_ has a mechanism which aborts such operations and `LIFO RECLAIM` - mode which addresses performance degradation. +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. -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. +Next version of _libmdbx_ will create steady synchronization points +automatically in async-write mode. -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_. +Performance comparison +====================== -#### Data safety in async-write mode +All benchmarks were done by [IOArena](https://github.com/pmwkaa/ioarena) +and multiple [scripts](https://github.com/pmwkaa/ioarena/tree/HL%2B%2B2015) +runs on Lenovo Carbon-2 laptop, i7-4600U 2.1 GHz, 8 Gb RAM, +SSD SAMSUNG MZNTD512HAGL-000L1 (DXT23L0Q) 512 Gb. -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: +### Integral performance -* 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. +Here showed sum of performance metrics in 3 benchmarks: -* 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. + - Read/Search on 4 CPU cores machine; -* _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. + - Transactions with [CRUD](https://en.wikipedia.org/wiki/CRUD) + operations in sync-write mode (fdatasync is called after each + transaction); -* During DB open _libmdbx_ rollbacks to the last steady synchronization point, this guarantees database integrity. + - 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). -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. +*Reasons why asynchronous mode isn't benchmarked here:* -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. + 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. -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. + 2. Performance gap is too high to compare in any meaningful way. -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. +### Read Scalability -2. `OOM-KICK` callback. +Summary performance with concurrent read/search queries in 1-2-4-8 +threads on 4 CPU cores machine. - `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; +### Sync-write mode - * abort or restart offending read transaction if it's running in sibling thread; + - Linear scale on left and dark rectangles mean arithmetic mean + transactions per second; - * abort current write transaction with returning error code. + - 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. -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). +**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. -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()`). +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. -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()`. +### Lazy-write mode -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()`. + - Linear scale on left and dark rectangles mean arithmetic mean of + thousands transactions per second; -9. Check if there is a row with data after current cursor position via `mdbx_cursor_eof()`. + - 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. -10. Ability to explicitly request update of present record without creating new record. Implemented as `MDBX_CURRENT` flag - for `mdbx_put()`. +**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). -11. Ability to update or delete record and get previous value via `mdbx_replace()` Also can update specific multi-value. +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. -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()`. +### Async-write mode -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()`. + - Linear scale on left and dark rectangles mean arithmetic mean of + thousands transactions per second; -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. + - 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. -18. Ability to open dbi-table with simultaneous setup of comparators for keys and values, via `mdbx_dbi_open_ex()`. +**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. -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()`. +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. -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. +### Cost comparison -23. Ability to get value by key and duplicates count by `mdbx_get_ex()`. +Summary of used resources during lazy-write mode benchmarks: -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. + - Read and write IOPS; -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. + - Sum of user CPU time and sys CPU time; -26. Sequence generation via `mdbx_dbi_sequence()`. + - 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). -27. Advanced dynamic control over DB size, including ability to choose page size via `mdbx_env_set_geometry()`, - including on Windows. +_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. -28. Three meta-pages instead of two, this allows to guarantee consistently update weak sync-points without risking to - corrupt last steady sync-point. +All benchmark data is gathered by +[getrusage()](http://man7.org/linux/man-pages/man2/getrusage.2.html) +syscall and by scanning data directory. -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. + -------------------------------------------------------------------------------- @@ -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) -``` |