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mdbx-doc: уточнение ограничений в разделе "Restrictions & Caveats".

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Леонид Юрьев (Leonid Yuriev) 2023-05-25 12:54:55 +03:00
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@ -35,10 +35,11 @@ or debugging of a client application while retaining an active read
transaction. LMDB this results in `MDB_MAP_FULL` error and subsequent write transaction. LMDB this results in `MDB_MAP_FULL` error and subsequent write
performance degradation. performance degradation.
MDBX mostly solve "long-lived" readers issue by using the Handle-Slow-Readers MDBX mostly solve "long-lived" readers issue by using the
\ref MDBX_hsr_func callback which allows to abort long-lived read transactions, Handle-Slow-Readers \ref MDBX_hsr_func callback which allows to abort
and using the \ref MDBX_LIFORECLAIM mode which addresses subsequent performance degradation. long-lived read transactions, and using the \ref MDBX_LIFORECLAIM mode
The "next" version of libmdbx (\ref MithrilDB) will completely solve this. which addresses subsequent performance degradation. The "next" version
of libmdbx (\ref MithrilDB) will completely solve this.
- Avoid suspending a process with active transactions. These would then be - Avoid suspending a process with active transactions. These would then be
"long-lived" as above. "long-lived" as above.
@ -80,6 +81,19 @@ list of pages to be retired.
Both of these issues will be addressed in MithrilDB. Both of these issues will be addressed in MithrilDB.
#### Since v0.12.1 and later
Some aspects related to GC have been refined and improved, in particular:
- The search for free consecutive/adjacent pages through GC has been
significantly speeded, including acceleration using
NOEN/SSE2/AVX2/AVX512 instructions.
- The `Big Foot` feature which significantly reduces GC overhead for
processing large lists of retired pages from huge transactions. Now
libmdbx avoid creating large chunks of PNLs (page number lists) which
required a long sequences of free pages, aka large/overflow pages. Thus
avoiding searching, allocating and storing such sequences inside GC.
## Space reservation ## Space reservation
An MDBX database configuration will often reserve considerable unused An MDBX database configuration will often reserve considerable unused
@ -87,6 +101,10 @@ memory address space and maybe file size for future growth. This does
not use actual memory or disk space, but users may need to understand not use actual memory or disk space, but users may need to understand
the difference so they won't be scared off. the difference so they won't be scared off.
However, on 64-bit systems with a relative small amount of RAM, such
reservation can deplete system resources (trigger ENOMEM error, etc)
when setting an inadequately large upper DB size using \ref
mdbx_env_set_geometry() or \ref mdbx::env::geometry. So just avoid this.
## Remote filesystems ## Remote filesystems
Do not use MDBX databases on remote filesystems, even between processes Do not use MDBX databases on remote filesystems, even between processes