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mdbx: update README.

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Leonid Yuriev 2019-12-07 21:50:19 +03:00
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### The [repository now only mirrored on the Github](https://abf.io/erthink/libmdbx) due to illegal discriminatory restrictions for Russian Crimea and for sovereign crimeans.
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-----
libmdbx
======================================
@ -161,25 +157,37 @@ without freezing writers.
## Improvements over LMDB
_libmdbx_ is superior to _legendary [LMDB](https://symas.com/lmdb/)_ in
_libmdbx_ is superior to legendary _[LMDB](https://symas.com/lmdb/)_ in
terms of features and reliability, not inferior in performance. In
comparison to LMDB, _libmdbx_ make things "just work" perfectly and
comparison to _LMDB_, _libmdbx_ make things "just work" perfectly and
out-of-the-box, not silently and catastrophically break down. The list
below is pruned down to the improvements most notable and obvious from
the user's point of view.
1. Automatic on-the-fly database size control by preset parameters, both
1. Larger limit for keys size. More than 2 larger than _LMDB_.
> For DB with default page size _libmdbx_ support keys up to 1300 bytes
> and up to 21780 bytes for 64K page size. _LMDB_ allows key size up to
> 511 bytes and may silently loses data with large values.
2. Up to 20% faster than _LMDB_ in [CRUD](https://en.wikipedia.org/wiki/Create,_read,_update_and_delete) benchmarks.
> Benchmarks of the in-[tmpfs](https://en.wikipedia.org/wiki/Tmpfs) scenarios,
> that tests the speed of engine itself, shown that _libmdbx_ 10-20% faster than _LMDB_.
> These and other results could be easily reproduced with [ioArena](https://github.com/pmwkaa/ioarena) just by `make bench-quartet`,
> including comparisons with [RockDB](https://en.wikipedia.org/wiki/RocksDB)
> and [WiredTiger](https://en.wikipedia.org/wiki/WiredTiger).
3. Automatic on-the-fly database size control by preset parameters, both
reduction and increment.
> _libmdbx_ manage the database size according to parameters specified
> by `mdbx_env_set_geometry()` function,
> ones include the growth step and the truncation threshold.
2. Automatic continuous zero-overhead database compactification.
4. Automatic continuous zero-overhead database compactification.
> _libmdbx_ logically move as possible a freed pages
> at end of allocation area into unallocated space,
> and then release such space if a lot of.
3. LIFO policy for recycling a Garbage Collection items. On systems with a disk
5. LIFO policy for recycling a Garbage Collection items. On systems with a disk
write-back cache, this can significantly increase write performance, up to
several times in a best case scenario.
> LIFO means that for reuse pages will be taken which became unused the lastest.
@ -188,63 +196,63 @@ several times in a best case scenario.
> and on disk during a series of write transactions, will be as small as possible.
> Thus creates ideal conditions for the efficient operation of the disk write-back cache.
4. Fast estimation of range query result volume, i.e. how many items can
6. Fast estimation of range query result volume, i.e. how many items can
be found between a `KEY1` and a `KEY2`. This is prerequisite for build
and/or optimize query execution plans.
> _libmdbx_ performs a rough estimate based only on b-tree pages that
> are common for the both stacks of cursors that were set to corresponing
> keys.
5. `mdbx_chk` tool for database integrity check.
7. `mdbx_chk` tool for database integrity check.
6. Guarantee of database integrity even in asynchronous unordered write-to-disk mode.
8. Guarantee of database integrity even in asynchronous unordered write-to-disk mode.
> _libmdbx_ propose additional trade-off by implementing append-like manner for updates
> in `NOSYNC` and `MAPASYNC` modes, that avoid database corruption after a system crash
> contrary to LMDB. Nevertheless, the `MDBX_UTTERLY_NOSYNC` mode available to match LMDB behaviour,
> and for a special use-cases.
7. Automated steady flush to disk upon volume of changes and/or by
9. Automated steady flush to disk upon volume of changes and/or by
timeout via cheap polling.
8. Sequence generation and three cheap persistent 64-bit markers with ACID.
10. Sequence generation and three cheap persistent 64-bit markers with ACID.
9. Support for keys and values of zero length, including multi-values
11. Support for keys and values of zero length, including multi-values
(aka sorted duplicates).
10. The handler of lack-of-space condition with a callback,
12. The handler of lack-of-space condition with a callback,
that allow you to control and resolve such situations.
11. Support for opening a database in the exclusive mode, including on a network share.
13. Support for opening a database in the exclusive mode, including on a network share.
12. Extended transaction info, including dirty and leftover space info
14. Extended transaction info, including dirty and leftover space info
for a write transaction, reading lag and hold over space for read
transactions.
13. Extended whole-database info (aka environment) and reader enumeration.
15. Extended whole-database info (aka environment) and reader enumeration.
14. Extended update or delete, _at once_ with getting previous value
16. Extended update or delete, _at once_ with getting previous value
and addressing the particular item from multi-value with the same key.
15. Support for explicitly updating the existing record, not insertion a new one.
17. Support for explicitly updating the existing record, not insertion a new one.
16. All cursors are uniformly, can be reused and should be closed explicitly,
18. All cursors are uniformly, can be reused and should be closed explicitly,
regardless ones were opened within write or read transaction.
17. Correct update of current record with `MDBX_CURRENT` flag when size
19. Correct update of current record with `MDBX_CURRENT` flag when size
of key or data was changed, including sorted duplicated.
18. Opening database handles is spared from race conditions and
20. Opening database handles is spared from race conditions and
pre-opening is not needed.
19. Ability to determine whether the particular data is on a dirty page
21. Ability to determine whether the particular data is on a dirty page
or not, that allows to avoid copy-out before updates.
20. Ability to determine whether the cursor is pointed to a key-value
22. Ability to determine whether the cursor is pointed to a key-value
pair, to the first, to the last, or not set to anything.
21. Returning `MDBX_EMULTIVAL` error in case of ambiguous update or delete.
23. Returning `MDBX_EMULTIVAL` error in case of ambiguous update or delete.
22. On **MacOS** the `fcntl(F_FULLFSYNC)` syscall is used _by
24. On **MacOS** the `fcntl(F_FULLFSYNC)` syscall is used _by
default_ to synchronize data with the disk, as this is [the only way to
guarantee data
durability](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/fsync.2.html)
@ -254,7 +262,7 @@ compared to LMDB, where the `fsync()` syscall is used. Therefore,
_libmdbx_ allows you to override this behavior by defining the
`MDBX_OSX_SPEED_INSTEADOF_DURABILITY=1` option while build the library.
23. On **Windows** the `LockFileEx()` syscall is used for locking, since
25. On **Windows** the `LockFileEx()` syscall is used for locking, since
it allows place the database on network drives, and provides protection
against incompetent user actions (aka
[poka-yoke](https://en.wikipedia.org/wiki/Poka-yoke)). Therefore
@ -433,14 +441,14 @@ recommend that you install [Homebrew](https://brew.sh/) and then execute
| -------- | ------ | ------ |
| Java | [mdbxjni](https://github.com/castortech/mdbxjni) | [Castor Technologies](https://castortech.com/) |
| .NET | [mdbx.NET](https://github.com/wangjia184/mdbx.NET) | [Jerry Wang](https://github.com/wangjia184) |
| Rust | [mdbx-rs](https://github.com/Kerollmops/mdbx-rs) | [Clément Renault](https://github.com/Kerollmops) |
--------------------------------------------------------------------------------
Performance comparison
======================
All benchmarks were done by [IOArena](https://github.com/pmwkaa/ioarena)
All benchmarks were done in 2015 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.
@ -577,16 +585,5 @@ syscall and by scanning data directory.
--------------------------------------------------------------------------------
```
$ objdump -f -h -j .text libmdbx.so
libmdbx.so: file format elf64-x86-64
architecture: i386:x86-64, flags 0x00000150:
HAS_SYMS, DYNAMIC, D_PAGED
start address 0x0000000000003710
Sections:
Idx Name Size VMA LMA File off Algn
11 .text 00015eff 0000000000003710 0000000000003710 00003710 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
```
### The [repository now only mirrored on the Github](https://abf.io/erthink/libmdbx) due to illegal discriminatory restrictions for Russian Crimea and for sovereign crimeans.
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