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Enumerations
enum	MDBX_option_t {   MDBX_opt_max_db , MDBX_opt_max_readers , MDBX_opt_sync_bytes , MDBX_opt_sync_period ,   MDBX_opt_rp_augment_limit , MDBX_opt_loose_limit , MDBX_opt_dp_reserve_limit , MDBX_opt_txn_dp_limit ,   MDBX_opt_txn_dp_initial , MDBX_opt_spill_max_denominator , MDBX_opt_spill_min_denominator , MDBX_opt_spill_parent4child_denominator ,   MDBX_opt_merge_threshold_16dot16_percent , MDBX_opt_writethrough_threshold , MDBX_opt_prefault_write_enable , MDBX_opt_gc_time_limit ,   MDBX_opt_prefer_waf_insteadof_balance , MDBX_opt_subpage_limit , MDBX_opt_subpage_room_threshold , MDBX_opt_subpage_reserve_prereq ,   MDBX_opt_subpage_reserve_limit }
	MDBX environment extra runtime options. More...
enum	MDBX_warmup_flags_t {   MDBX_warmup_default = 0 , MDBX_warmup_force = 1 , MDBX_warmup_oomsafe = 2 , MDBX_warmup_lock = 4 ,   MDBX_warmup_touchlimit = 8 , MDBX_warmup_release = 16 }
	Warming up options. More...

Functions
LIBMDBX_API int	mdbx_env_set_option (MDBX_env *env, const MDBX_option_t option, uint64_t value)
	Sets the value of a extra runtime options for an environment.
LIBMDBX_API int	mdbx_env_get_option (const MDBX_env *env, const MDBX_option_t option, uint64_t *pvalue)
	Gets the value of extra runtime options from an environment.
int	mdbx_env_set_syncbytes (MDBX_env *env, size_t threshold)
	Sets threshold to force flush the data buffers to disk, even any of MDBX_SAFE_NOSYNC flag in the environment.
int	mdbx_env_set_syncperiod (MDBX_env *env, unsigned seconds_16dot16)
	Sets relative period since the last unsteady commit to force flush the data buffers to disk, even of MDBX_SAFE_NOSYNC flag in the environment.
LIBMDBX_API int	mdbx_env_warmup (const MDBX_env *env, const MDBX_txn *txn, MDBX_warmup_flags_t flags, unsigned timeout_seconds_16dot16)
	Warms up the database by loading pages into memory, optionally lock ones.
LIBMDBX_API int	mdbx_env_set_flags (MDBX_env *env, MDBX_env_flags_t flags, bool onoff)
	Set environment flags.
LIBMDBX_API int	mdbx_env_set_geometry (MDBX_env *env, intptr_t size_lower, intptr_t size_now, intptr_t size_upper, intptr_t growth_step, intptr_t shrink_threshold, intptr_t pagesize)
	Set all size-related parameters of environment, including page size and the min/max size of the memory map.
int	mdbx_env_set_mapsize (MDBX_env *env, size_t size)
int	mdbx_env_set_maxreaders (MDBX_env *env, unsigned readers)
	Set the maximum number of threads/reader slots for for all processes interacts with the database.
int	mdbx_env_set_maxdbs (MDBX_env *env, MDBX_dbi dbs)
	Set the maximum number of named tables for the environment.
LIBMDBX_API int	mdbx_env_set_userctx (MDBX_env *env, void *ctx)
	Sets application information (a context pointer) associated with the environment.

Detailed Description

Enumeration Type Documentation

MDBX_option_t

enum MDBX_option_t

MDBX environment extra runtime options.

See also

mdbx_env_set_option()

mdbx_env_get_option()

Enumerator
MDBX_opt_max_db	Controls the maximum number of named tables for the environment. By default only unnamed key-value table could used and appropriate value should set by MDBX_opt_max_db to using any more named table(s). To reduce overhead, use the minimum sufficient value. This option may only set after mdbx_env_create() and before mdbx_env_open(). See also mdbx_env_set_maxdbs() mdbx_env_get_maxdbs()
MDBX_opt_max_readers	Defines the maximum number of threads/reader slots for all processes interacting with the database. This defines the number of slots in the lock table that is used to track readers in the environment. The default is about 100 for 4K system page size. Starting a read-only transaction normally ties a lock table slot to the current thread until the environment closes or the thread exits. If MDBX_NOSTICKYTHREADS is in use, mdbx_txn_begin() instead ties the slot to the MDBX_txn object until it or the MDBX_env object is destroyed. This option may only set after mdbx_env_create() and before mdbx_env_open(), and has an effect only when the database is opened by the first process interacts with the database. See also mdbx_env_set_maxreaders() mdbx_env_get_maxreaders()
MDBX_opt_sync_bytes	Controls interprocess/shared threshold to force flush the data buffers to disk, if MDBX_SAFE_NOSYNC is used. See also mdbx_env_set_syncbytes() mdbx_env_get_syncbytes()
MDBX_opt_sync_period	Controls interprocess/shared relative period since the last unsteady commit to force flush the data buffers to disk, if MDBX_SAFE_NOSYNC is used. See also mdbx_env_set_syncperiod() mdbx_env_get_syncperiod()
MDBX_opt_rp_augment_limit	Controls the in-process limit to grow a list of reclaimed/recycled page's numbers for finding a sequence of contiguous pages for large data items. See also MDBX_opt_gc_time_limit A long values requires allocation of contiguous database pages. To find such sequences, it may be necessary to accumulate very large lists, especially when placing very long values (more than a megabyte) in a large databases (several tens of gigabytes), which is much expensive in extreme cases. This threshold allows you to avoid such costs by allocating new pages at the end of the database (with its possible growth on disk), instead of further accumulating/reclaiming Garbage Collection records. On the other hand, too small threshold will lead to unreasonable database growth, or/and to the inability of put long values. The MDBX_opt_rp_augment_limit controls described limit for the current process. By default this limit adjusted dynamically to 1/3 of current quantity of DB pages, which is usually enough for most cases.
MDBX_opt_loose_limit	Controls the in-process limit to grow a cache of dirty pages for reuse in the current transaction. A 'dirty page' refers to a page that has been updated in memory only, the changes to a dirty page are not yet stored on disk. To reduce overhead, it is reasonable to release not all such pages immediately, but to leave some ones in cache for reuse in the current transaction. The MDBX_opt_loose_limit allows you to set a limit for such cache inside the current process. Should be in the range 0..255, default is 64.
MDBX_opt_dp_reserve_limit	Controls the in-process limit of a pre-allocated memory items for dirty pages. A 'dirty page' refers to a page that has been updated in memory only, the changes to a dirty page are not yet stored on disk. Without MDBX_WRITEMAP dirty pages are allocated from memory and released when a transaction is committed. To reduce overhead, it is reasonable to release not all ones, but to leave some allocations in reserve for reuse in the next transaction(s). The MDBX_opt_dp_reserve_limit allows you to set a limit for such reserve inside the current process. Default is 1024.
MDBX_opt_txn_dp_limit	Controls the in-process limit of dirty pages for a write transaction. A 'dirty page' refers to a page that has been updated in memory only, the changes to a dirty page are not yet stored on disk. Without MDBX_WRITEMAP dirty pages are allocated from memory and will be busy until are written to disk. Therefore for a large transactions is reasonable to limit dirty pages collecting above an some threshold but spill to disk instead. The MDBX_opt_txn_dp_limit controls described threshold for the current process. Default is 65536, it is usually enough for most cases.
MDBX_opt_txn_dp_initial	Controls the in-process initial allocation size for dirty pages list of a write transaction. Default is 1024.
MDBX_opt_spill_max_denominator	Controls the in-process how maximal part of the dirty pages may be spilled when necessary. The MDBX_opt_spill_max_denominator defines the denominator for limiting from the top for part of the current dirty pages may be spilled when the free room for a new dirty pages (i.e. distance to the MDBX_opt_txn_dp_limit threshold) is not enough to perform requested operation. Exactly max_pages_to_spill = dirty_pages - dirty_pages / N, where N is the value set by MDBX_opt_spill_max_denominator. Should be in the range 0..255, where zero means no limit, i.e. all dirty pages could be spilled. Default is 8, i.e. no more than 7/8 of the current dirty pages may be spilled when reached the condition described above.
MDBX_opt_spill_min_denominator	Controls the in-process how minimal part of the dirty pages should be spilled when necessary. The MDBX_opt_spill_min_denominator defines the denominator for limiting from the bottom for part of the current dirty pages should be spilled when the free room for a new dirty pages (i.e. distance to the MDBX_opt_txn_dp_limit threshold) is not enough to perform requested operation. Exactly min_pages_to_spill = dirty_pages / N, where N is the value set by MDBX_opt_spill_min_denominator. Should be in the range 0..255, where zero means no restriction at the bottom. Default is 8, i.e. at least the 1/8 of the current dirty pages should be spilled when reached the condition described above.
MDBX_opt_spill_parent4child_denominator	Controls the in-process how much of the parent transaction dirty pages will be spilled while start each child transaction. The MDBX_opt_spill_parent4child_denominator defines the denominator to determine how much of parent transaction dirty pages will be spilled explicitly while start each child transaction. Exactly pages_to_spill = dirty_pages / N, where N is the value set by MDBX_opt_spill_parent4child_denominator. For a stack of nested transactions each dirty page could be spilled only once, and parent's dirty pages couldn't be spilled while child transaction(s) are running. Therefore a child transaction could reach MDBX_TXN_FULL when parent(s) transaction has spilled too less (and child reach the limit of dirty pages), either when parent(s) has spilled too more (since child can't spill already spilled pages). So there is no universal golden ratio. Should be in the range 0..255, where zero means no explicit spilling will be performed during starting nested transactions. Default is 0, i.e. by default no spilling performed during starting nested transactions, that correspond historically behaviour.
MDBX_opt_merge_threshold_16dot16_percent	Controls the in-process threshold of semi-empty pages merge. This option controls the in-process threshold of minimum page fill, as used space of percentage of a page. Neighbour pages emptier than this value are candidates for merging. The threshold value is specified in 1/65536 points of a whole page, which is equivalent to the 16-dot-16 fixed point format. The specified value must be in the range from 12.5% (almost empty page) to 50% (half empty page) which corresponds to the range from 8192 and to 32768 in units respectively. See also MDBX_opt_prefer_waf_insteadof_balance
MDBX_opt_writethrough_threshold	Controls the choosing between use write-through disk writes and usual ones with followed flush by the fdatasync() syscall. Depending on the operating system, storage subsystem characteristics and the use case, higher performance can be achieved by either using write-through or a serie of usual/lazy writes followed by the flush-to-disk. Basically for N chunks the latency/cost of write-through is: latency = N * (emit + round-trip-to-storage + storage-execution); And for serie of lazy writes with flush is: latency = N * (emit + storage-execution) + flush + round-trip-to-storage. So, for large N and/or noteable round-trip-to-storage the write+flush approach is win. But for small N and/or near-zero NVMe-like latency the write-through is better. To solve this issue libmdbx provide MDBX_opt_writethrough_threshold: when N described above less or equal specified threshold, a write-through approach will be used; otherwise, when N great than specified threshold, a write-and-flush approach will be used. Note MDBX_opt_writethrough_threshold affects only MDBX_SYNC_DURABLE mode without MDBX_WRITEMAP, and not supported on Windows. On Windows a write-through is used always but MDBX_NOMETASYNC could be used for switching to write-and-flush.
MDBX_opt_prefault_write_enable	Controls prevention of page-faults of reclaimed and allocated pages in the MDBX_WRITEMAP mode by clearing ones through file handle before touching.
MDBX_opt_gc_time_limit	Controls the in-process spending time limit of searching consecutive pages inside GC. See also MDBX_opt_rp_augment_limit Задаёт ограничение времени в 1/65536 долях секунды, которое может быть потрачено в ходе пишущей транзакции на поиск последовательностей страниц внутри GC/freelist после достижения ограничения задаваемого опцией MDBX_opt_rp_augment_limit. Контроль по времени не выполняется при поиске/выделении одиночных страниц и выделении страниц под нужды GC (при обновлении GC в ходе фиксации транзакции). Задаваемый лимит времени исчисляется по "настенным часам" и контролируется в рамках транзакции, наследуется для вложенных транзакций и с аккумулированием в родительской при их фиксации. Контроль по времени производится только при достижении ограничения задаваемого опцией MDBX_opt_rp_augment_limit. Это позволяет гибко управлять поведением используя обе опции. По умолчанию ограничение устанавливается в 0, что приводит к незамедлительной остановке поиска в GC при достижении MDBX_opt_rp_augment_limit во внутреннем состоянии транзакции и соответствует поведению до появления опции MDBX_opt_gc_time_limit. С другой стороны, при минимальном значении (включая 0) MDBX_opt_rp_augment_limit переработка GC будет ограничиваться преимущественно затраченным временем.
MDBX_opt_prefer_waf_insteadof_balance	Управляет выбором между стремлением к равномерности наполнения страниц, либо уменьшением количества измененных и записанных страниц. После операций удаления страницы содержащие меньше минимума ключей, либо опустошенные до MDBX_opt_merge_threshold_16dot16_percent подлежат слиянию с одной из соседних. Если страницы справа и слева от текущей обе «грязные» (были изменены в ходе транзакции и должны быть записаны на диск), либо обе «чисты» (не изменялись в текущей транзакции), то целью для слияния всегда выбирается менее заполненная страница. Когда же только одна из соседствующих является «грязной», а другая «чистой», то возможны две тактики выбора цели для слияния: Если MDBX_opt_prefer_waf_insteadof_balance = True, то будет выбрана уже измененная страница, что НЕ УВЕЛИЧИТ количество измененных страниц и объем записи на диск при фиксации текущей транзакции, но в среднем будет УВЕЛИЧИВАТЬ неравномерность заполнения страниц. Если MDBX_opt_prefer_waf_insteadof_balance = False, то будет выбрана менее заполненная страница, что УВЕЛИЧИТ количество измененных страниц и объем записи на диск при фиксации текущей транзакции, но в среднем будет УМЕНЬШАТЬ неравномерность заполнения страниц. See also MDBX_opt_merge_threshold_16dot16_percent
MDBX_opt_subpage_limit	Задаёт в % максимальный размер вложенных страниц, используемых для размещения небольшого количества мульти-значений связанных с одном ключем. Использование вложенных страниц, вместо выноса значений на отдельные страницы вложенного дерева, позволяет уменьшить объем неиспользуемого места и этим увеличить плотность размещения данных. Но с увеличением размера вложенных страниц требуется больше листовых страниц основного дерева, что также увеличивает высоту основного дерева. Кроме этого, изменение данных на вложенных страницах требует дополнительных копирований, поэтому стоимость может быть больше во многих сценариях. min 12.5% (8192), max 100% (65535), default = 100%
MDBX_opt_subpage_room_threshold	Задаёт в % минимальный объём свободного места на основной странице, при отсутствии которого вложенные страницы выносятся в отдельное дерево. min 0, max 100% (65535), default = 0
MDBX_opt_subpage_reserve_prereq	Задаёт в % минимальный объём свободного места на основной странице, при наличии которого, производится резервирование места во вложенной. Если на основной странице свободного места недостаточно, то вложенная страница будет минимального размера. В свою очередь, при отсутствии резерва во вложенной странице, каждое добавлении в неё элементов будет требовать переформирования основной страниц с переносом всех узлов данных. Поэтому резервирование места, как правило, выгодно в сценариях с интенсивным добавлением коротких мульти-значений, например при индексировании. Но уменьшает плотность размещения данных, соответственно увеличивает объем БД и операций ввода-вывода. min 0, max 100% (65535), default = 42% (27525)
MDBX_opt_subpage_reserve_limit	Задаёт в % ограничение резервирования места на вложенных страницах. min 0, max 100% (65535), default = 4.2% (2753)

MDBX_warmup_flags_t

enum MDBX_warmup_flags_t

Warming up options.

See also

mdbx_env_warmup()

Enumerator
MDBX_warmup_default	By default mdbx_env_warmup() just ask OS kernel to asynchronously prefetch database pages.
MDBX_warmup_force	Peeking all pages of allocated portion of the database to force ones to be loaded into memory. However, the pages are just peeks sequentially, so unused pages that are in GC will be loaded in the same way as those that contain payload.
MDBX_warmup_oomsafe	Using system calls to peeks pages instead of directly accessing ones, which at the cost of additional overhead avoids killing the current process by OOM-killer in a lack of memory condition. Note Has effect only on POSIX (non-Windows) systems with conjunction to MDBX_warmup_force option.
MDBX_warmup_lock	Try to lock database pages in memory by mlock() on POSIX-systems or VirtualLock() on Windows. Please refer to description of these functions for reasonability of such locking and the information of effects, including the system as a whole. Such locking in memory requires that the corresponding resource limits (e.g. RLIMIT_RSS, RLIMIT_MEMLOCK or process working set size) and the availability of system RAM are sufficiently high. On successful, all currently allocated pages, both unused in GC and containing payload, will be locked in memory until the environment closes, or explicitly unblocked by using MDBX_warmup_release, or the database geometry will changed, including its auto-shrinking.
MDBX_warmup_touchlimit	Alters corresponding current resource limits to be enough for lock pages by MDBX_warmup_lock. However, this option should be used in simpler applications since takes into account only current size of this environment disregarding all other factors. For real-world database application you will need full-fledged management of resources and their limits with respective engineering.
MDBX_warmup_release	Release the lock that was performed before by MDBX_warmup_lock.

Function Documentation

mdbx_env_get_option()

LIBMDBX_API int mdbx_env_get_option	(	const MDBX_env *	env,
		const MDBX_option_t	option,
		uint64_t *	pvalue )

Gets the value of extra runtime options from an environment.

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	option	The option from MDBX_option_t to get value of it.
[out]	pvalue	The address where the option's value will be stored.

See also

MDBX_option_t

mdbx_env_get_option()

Returns

A non-zero error value on failure and 0 on success.

mdbx_env_set_flags()

LIBMDBX_API int mdbx_env_set_flags	(	MDBX_env *	env,
		MDBX_env_flags_t	flags,
		bool	onoff )

Set environment flags.

This may be used to set some flags in addition to those from mdbx_env_open(), or to unset these flags.

See also

mdbx_env_get_flags()

Note

In contrast to LMDB, the MDBX serialize threads via mutex while changing the flags. Therefore this function will be blocked while a write transaction running by other thread, or MDBX_BUSY will be returned if function called within a write transaction.

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	flags	The env_flags to change, bitwise OR'ed together.
[in]	onoff	A non-zero value sets the flags, zero clears them.

Returns

A non-zero error value on failure and 0 on success, some possible errors are:

Return values

MDBX_EINVAL

An invalid parameter was specified.

mdbx_env_set_geometry()

LIBMDBX_API int mdbx_env_set_geometry	(	MDBX_env *	env,
		intptr_t	size_lower,
		intptr_t	size_now,
		intptr_t	size_upper,
		intptr_t	growth_step,
		intptr_t	shrink_threshold,
		intptr_t	pagesize )

Set all size-related parameters of environment, including page size and the min/max size of the memory map.

In contrast to LMDB, the MDBX provide automatic size management of an database according the given parameters, including shrinking and resizing on the fly. From user point of view all of these just working. Nevertheless, it is reasonable to know some details in order to make optimal decisions when choosing parameters.

See also

mdbx_env_info_ex()

Both mdbx_env_set_geometry() and legacy mdbx_env_set_mapsize() are inapplicable to read-only opened environment.

Both mdbx_env_set_geometry() and legacy mdbx_env_set_mapsize() could be called either before or after mdbx_env_open(), either within the write transaction running by current thread or not:

• In case mdbx_env_set_geometry() or legacy mdbx_env_set_mapsize() was called BEFORE mdbx_env_open(), i.e. for closed environment, then the specified parameters will be used for new database creation, or will be applied during opening if database exists and no other process using it.

  If the database is already exist, opened with MDBX_EXCLUSIVE or not used by any other process, and parameters specified by mdbx_env_set_geometry() are incompatible (i.e. for instance, different page size) then mdbx_env_open() will return MDBX_INCOMPATIBLE error.

  In another way, if database will opened read-only or will used by other process during calling mdbx_env_open() that specified parameters will silently discarded (open the database with MDBX_EXCLUSIVE flag to avoid this).

• In case mdbx_env_set_geometry() or legacy mdbx_env_set_mapsize() was called after mdbx_env_open() WITHIN the write transaction running by current thread, then specified parameters will be applied as a part of write transaction, i.e. will not be completely visible to any others processes until the current write transaction has been committed by the current process. However, if transaction will be aborted, then the database file will be reverted to the previous size not immediately, but when a next transaction will be committed or when the database will be opened next time.
• In case mdbx_env_set_geometry() or legacy mdbx_env_set_mapsize() was called after mdbx_env_open() but OUTSIDE a write transaction, then MDBX will execute internal pseudo-transaction to apply new parameters (but only if anything has been changed), and changes be visible to any others processes immediately after successful completion of function.

Essentially a concept of "automatic size management" is simple and useful:

• There are the lower and upper bounds of the database file size;
• There is the growth step by which the database file will be increased, in case of lack of space;
• There is the threshold for unused space, beyond which the database file will be shrunk;
• The size of the memory map is also the maximum size of the database;
• MDBX will automatically manage both the size of the database and the size of memory map, according to the given parameters.

So, there some considerations about choosing these parameters:

• The lower bound allows you to prevent database shrinking below certain reasonable size to avoid unnecessary resizing costs.
• The upper bound allows you to prevent database growth above certain reasonable size. Besides, the upper bound defines the linear address space reservation in each process that opens the database. Therefore changing the upper bound is costly and may be required reopening environment in case of MDBX_UNABLE_EXTEND_MAPSIZE errors, and so on. Therefore, this value should be chosen reasonable large, to accommodate future growth of the database.
• The growth step must be greater than zero to allow the database to grow, but also reasonable not too small, since increasing the size by little steps will result a large overhead.
• The shrink threshold must be greater than zero to allow the database to shrink but also reasonable not too small (to avoid extra overhead) and not less than growth step to avoid up-and-down flouncing.
• The current size (i.e. size_now argument) is an auxiliary parameter for simulation legacy mdbx_env_set_mapsize() and as workaround Windows issues (see below).

Unfortunately, Windows has is a several issue with resizing of memory-mapped file:

• Windows unable shrinking a memory-mapped file (i.e memory-mapped section) in any way except unmapping file entirely and then map again. Moreover, it is impossible in any way when a memory-mapped file is used more than one process.
• Windows does not provide the usual API to augment a memory-mapped file (i.e. a memory-mapped partition), but only by using "Native API" in an undocumented way.

MDBX bypasses all Windows issues, but at a cost:

• Ability to resize database on the fly requires an additional lock and release SlimReadWriteLock during each read-only transaction.
• During resize all in-process threads should be paused and then resumed.
• Shrinking of database file is performed only when it used by single process, i.e. when a database closes by the last process or opened by the first. = Therefore, the size_now argument may be useful to set database size by the first process which open a database, and thus avoid expensive remapping further.

For create a new database with particular parameters, including the page size, mdbx_env_set_geometry() should be called after mdbx_env_create() and before mdbx_env_open(). Once the database is created, the page size cannot be changed. If you do not specify all or some of the parameters, the corresponding default values will be used. For instance, the default for database size is 10485760 bytes.

If the mapsize is increased by another process, MDBX silently and transparently adopt these changes at next transaction start. However, mdbx_txn_begin() will return MDBX_UNABLE_EXTEND_MAPSIZE if new mapping size could not be applied for current process (for instance if address space is busy). Therefore, in the case of MDBX_UNABLE_EXTEND_MAPSIZE error you need close and reopen the environment to resolve error.

Note

Actual values may be different than your have specified because of rounding to specified database page size, the system page size and/or the size of the system virtual memory management unit. You can get actual values by mdbx_env_info_ex() or see by using the tool mdbx_chk with the -v option.

Legacy mdbx_env_set_mapsize() correspond to calling mdbx_env_set_geometry() with the arguments size_lower, size_now, size_upper equal to the size and -1 (i.e. default) for all other parameters.

Parameters

[in]	env	An environment handle returned by mdbx_env_create()
[in]	size_lower	The lower bound of database size in bytes. Zero value means "minimal acceptable", and negative means "keep current or use default".
[in]	size_now	The size in bytes to setup the database size for now. Zero value means "minimal acceptable", and negative means "keep current or use default". So, it is recommended always pass -1 in this argument except some special cases.
[in]	size_upper	The upper bound of database size in bytes. Zero value means "minimal acceptable", and negative means "keep current or use default". It is recommended to avoid change upper bound while database is used by other processes or threaded (i.e. just pass -1 in this argument except absolutely necessary). Otherwise you must be ready for MDBX_UNABLE_EXTEND_MAPSIZE error(s), unexpected pauses during remapping and/or system errors like "address busy", and so on. In other words, there is no way to handle a growth of the upper bound robustly because there may be a lack of appropriate system resources (which are extremely volatile in a multi-process multi-threaded environment).
[in]	growth_step	The growth step in bytes, must be greater than zero to allow the database to grow. Negative value means "keep current or use default".
[in]	shrink_threshold	The shrink threshold in bytes, must be greater than zero to allow the database to shrink and greater than growth_step to avoid shrinking right after grow. Negative value means "keep current or use default". Default is 2*growth_step.
[in]	pagesize	The database page size for new database creation or -1 otherwise. Once the database is created, the page size cannot be changed. Must be power of 2 in the range between MDBX_MIN_PAGESIZE and MDBX_MAX_PAGESIZE. Zero value means "minimal acceptable", and negative means "keep current or use default".

Returns

A non-zero error value on failure and 0 on success, some possible errors are:

Return values

MDBX_EINVAL	An invalid parameter was specified, or the environment has an active write transaction.
MDBX_EPERM	Two specific cases for Windows: 1) Shrinking was disabled before via geometry settings and now it enabled, but there are reading threads that don't use the additional SRWL (which is required to avoid Windows issues). 2) Temporary close memory mapped is required to change geometry, but there read transaction(s) is running and no corresponding thread(s) could be suspended since the MDBX_NOSTICKYTHREADS mode is used.
MDBX_EACCESS	The environment opened in read-only.
MDBX_MAP_FULL	Specified size smaller than the space already consumed by the environment.
MDBX_TOO_LARGE	Specified size is too large, i.e. too many pages for given size, or a 32-bit process requests too much bytes for the 32-bit address space.

mdbx_env_set_mapsize()

int mdbx_env_set_mapsize ( MDBX_env * env, size_t size )

inline

Deprecated

Please use mdbx_env_set_geometry() instead.

mdbx_env_set_maxdbs()

int mdbx_env_set_maxdbs ( MDBX_env * env, MDBX_dbi dbs )

inline

Set the maximum number of named tables for the environment.

This function is only needed if multiple tables will be used in the environment. Simpler applications that use the environment as a single unnamed table can ignore this option. This function may only be called after mdbx_env_create() and before mdbx_env_open().

Currently a moderate number of slots are cheap but a huge number gets expensive: 7-120 words per transaction, and every mdbx_dbi_open() does a linear search of the opened slots.

See also

mdbx_env_get_maxdbs()

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	dbs	The maximum number of tables.

Returns

A non-zero error value on failure and 0 on success, some possible errors are:

Return values

MDBX_EINVAL	An invalid parameter was specified.
MDBX_EPERM	The environment is already open.

mdbx_env_set_maxreaders()

int mdbx_env_set_maxreaders ( MDBX_env * env, unsigned readers )

inline

Set the maximum number of threads/reader slots for for all processes interacts with the database.

This defines the number of slots in the lock table that is used to track readers in the environment. The default is about 100 for 4K system page size. Starting a read-only transaction normally ties a lock table slot to the current thread until the environment closes or the thread exits. If MDBX_NOSTICKYTHREADS is in use, mdbx_txn_begin() instead ties the slot to the MDBX_txn object until it or the MDBX_env object is destroyed. This function may only be called after mdbx_env_create() and before mdbx_env_open(), and has an effect only when the database is opened by the first process interacts with the database.

See also

mdbx_env_get_maxreaders()

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	readers	The maximum number of reader lock table slots.

Returns

A non-zero error value on failure and 0 on success, some possible errors are:

Return values

MDBX_EINVAL	An invalid parameter was specified.
MDBX_EPERM	The environment is already open.

mdbx_env_set_option()

LIBMDBX_API int mdbx_env_set_option	(	MDBX_env *	env,
		const MDBX_option_t	option,
		uint64_t	value )

Sets the value of a extra runtime options for an environment.

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	option	The option from MDBX_option_t to set value of it.
[in]	value	The value of option to be set.

See also

MDBX_option_t

mdbx_env_get_option()

Returns

A non-zero error value on failure and 0 on success.

mdbx_env_set_syncbytes()

int mdbx_env_set_syncbytes ( MDBX_env * env, size_t threshold )

inline

Sets threshold to force flush the data buffers to disk, even any of MDBX_SAFE_NOSYNC flag in the environment.

See also

mdbx_env_get_syncbytes

MDBX_opt_sync_bytes

The threshold value affects all processes which operates with given environment until the last process close environment or a new value will be settled.

Data is always written to disk when mdbx_txn_commit() is called, but the operating system may keep it buffered. MDBX always flushes the OS buffers upon commit as well, unless the environment was opened with MDBX_SAFE_NOSYNC, MDBX_UTTERLY_NOSYNC or in part MDBX_NOMETASYNC.

The default is 0, than mean no any threshold checked, and no additional flush will be made.

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	threshold	The size in bytes of summary changes when a synchronous flush would be made.

Returns

A non-zero error value on failure and 0 on success.

mdbx_env_set_syncperiod()

int mdbx_env_set_syncperiod ( MDBX_env * env, unsigned seconds_16dot16 )

inline

Sets relative period since the last unsteady commit to force flush the data buffers to disk, even of MDBX_SAFE_NOSYNC flag in the environment.

See also

mdbx_env_get_syncperiod

MDBX_opt_sync_period

The relative period value affects all processes which operates with given environment until the last process close environment or a new value will be settled.

Data is always written to disk when mdbx_txn_commit() is called, but the operating system may keep it buffered. MDBX always flushes the OS buffers upon commit as well, unless the environment was opened with MDBX_SAFE_NOSYNC or in part MDBX_NOMETASYNC.

Settled period don't checked asynchronously, but only by the mdbx_txn_commit() and mdbx_env_sync() functions. Therefore, in cases where transactions are committed infrequently and/or irregularly, polling by mdbx_env_sync() may be a reasonable solution to timeout enforcement.

The default is 0, than mean no any timeout checked, and no additional flush will be made.

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	seconds_16dot16	The period in 1/65536 of second when a synchronous flush would be made since the last unsteady commit.

Returns

A non-zero error value on failure and 0 on success.

mdbx_env_set_userctx()

LIBMDBX_API int mdbx_env_set_userctx	(	MDBX_env *	env,
		void *	ctx )

Sets application information (a context pointer) associated with the environment.

See also

mdbx_env_get_userctx()

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	ctx	An arbitrary pointer for whatever the application needs.

Returns

A non-zero error value on failure and 0 on success.

mdbx_env_warmup()

LIBMDBX_API int mdbx_env_warmup	(	const MDBX_env *	env,
		const MDBX_txn *	txn,
		MDBX_warmup_flags_t	flags,
		unsigned	timeout_seconds_16dot16 )

Warms up the database by loading pages into memory, optionally lock ones.

Depending on the specified flags, notifies OS kernel about following access, force loads the database pages, including locks ones in memory or releases such a lock. However, the function does not analyze the b-tree nor the GC. Therefore an unused pages that are in GC handled (i.e. will be loaded) in the same way as those that contain payload.

At least one of env or txn argument must be non-null.

Parameters

[in]	env	An environment handle returned by mdbx_env_create().
[in]	txn	A transaction handle returned by mdbx_txn_begin().
[in]	flags	The warmup_flags, bitwise OR'ed together.
[in]	timeout_seconds_16dot16	Optional timeout which checking only during explicitly peeking database pages for loading ones if the MDBX_warmup_force option was specified.

Returns

A non-zero error value on failure and 0 on success. Some possible errors are:

Return values

MDBX_ENOSYS	The system does not support requested operation(s).
MDBX_RESULT_TRUE	The specified timeout is reached during load data into memory.