Module lmdb_zero::open

Flags used when opening an LMDB environment.

Structs

Flags

Flags used when opening an LMDB environment.

Constants

FIXEDMAP	Use a fixed address for the mmap region. This flag must be specified when creating the environment, and is stored persistently in the environment. If successful, the memory map will always reside at the same virtual address and pointers used to reference data items in the database will be constant across multiple invocations. This option may not always work, depending on how the operating system has allocated memory to shared libraries and other uses. The feature is highly experimental.
MAPASYNC	When using WRITEMAP, use asynchronous flushes to disk. As with NOSYNC, a system crash can then corrupt the database or lose the last transactions. Calling Environment::sync() ensures on-disk database integrity until next commit. This flag may be changed at any time using Environment::set_flags().
NOLOCK	Don't do any locking. If concurrent access is anticipated, the caller must manage all concurrency itself. For proper operation the caller must enforce single-writer semantics, and must ensure that no readers are using old transactions while a writer is active. The simplest approach is to use an exclusive lock so that no readers may be active at all when a writer begins.
NOMEMINIT	Don't initialize malloc'd memory before writing to unused spaces in the data file. By default, memory for pages written to the data file is obtained using malloc. While these pages may be reused in subsequent transactions, freshly malloc'd pages will be initialized to zeroes before use. This avoids persisting leftover data from other code (that used the heap and subsequently freed the memory) into the data file. Note that many other system libraries may allocate and free memory from the heap for arbitrary uses. E.g., stdio may use the heap for file I/O buffers. This initialization step has a modest performance cost so some applications may want to disable it using this flag. This option can be a problem for applications which handle sensitive data like passwords, and it makes memory checkers like Valgrind noisy. This flag is not needed with WRITEMAP, which writes directly to the mmap instead of using malloc for pages. The initialization is also skipped if RESERVE is used; the caller is expected to overwrite all of the memory that was reserved in that case. This flag may be changed at any time using Environment::set_flags().
NOMETASYNC	Flush system buffers to disk only once per transaction, omit the metadata flush. Defer that until the system flushes files to disk, or next non-RDONLY commit or Environment::sync(). This optimization maintains database integrity, but a system crash may undo the last committed transaction. I.e. it preserves the ACI (atomicity, consistency, isolation) but not D (durability) database property. This flag may be changed at any time using Environment::set_flags().
NORDAHEAD	Turn off readahead. Most operating systems perform readahead on read requests by default. This option turns it off if the OS supports it. Turning it off may help random read performance when the DB is larger than RAM and system RAM is full. The option is not implemented on Windows.
NOSUBDIR	By default, LMDB creates its environment in a directory whose pathname is given in path, and creates its data and lock files under that directory. With this option, the path passed to EnvBuilder::open is used as-is for the database main data file. The database lock file is the path with "-lock" appended.
NOSYNC	Don't flush system buffers to disk when committing a transaction. This optimization means a system crash can corrupt the database or lose the last transactions if buffers are not yet flushed to disk. The risk is governed by how often the system flushes dirty buffers to disk and how often Environment::sync() is called. However, if the filesystem preserves write order and the WRITEMAP flag is not used, transactions exhibit ACI (atomicity, consistency, isolation) properties and only lose D (durability). I.e. database integrity is maintained, but a system crash may undo the final transactions. Note that (NOSYNC | WRITEMAP) leaves the system with no hint for when to write transactions to disk, unless Environment::sync() is called. (MAPASYNC | WRITEMAP) may be preferable. This flag may be changed at any time using Environment::set_flags().
NOTLS	Don't use Thread-Local Storage. Tie reader locktable slots to transaction objects instead of to threads. I.e. Transaction::reset() keeps the slot reseved for the transaction object. A thread may use parallel read-only transactions. A read-only transaction may span threads if the user synchronizes its use. Applications that multiplex many user threads over individual OS threads need this option. Such an application must also serialize the write transactions in an OS thread, since LMDB's write locking is unaware of the user threads.
RDONLY	Open the environment in read-only mode. No write operations will be allowed. LMDB will still modify the lock file - except on read-only filesystems, where LMDB does not use locks.
WRITEMAP	Use a writeable memory map unless RDONLY is set. This is faster and uses fewer mallocs, but loses protection from application bugs like wild pointer writes and other bad updates into the database. Incompatible with nested transactions. Do not mix processes with and without WRITEMAP on the same environment. This can defeat durability (Environment::sync etc).