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dba: lmdb: allow to override the MDB_NOSUBDIR flag #8856
Description
Description
The lmdb driver of DBA always sets the MDB_NOSUBDIR flag and does not allow PHP code to override it:
Line 43 in bc03dee
This is a problem for interoperability when sharing a single LMDB database between multiple programs written in different languages. LMDB itself defaults to using a subdirectory as a database, and will create two files data.mdb and lock.mdb in this directory. When using PHP, one has to open subdir/data.mdb explicitly, but now the PHP code will use subdir/data.mdb-lock instead of subdir/lock.mdb as a lock file, leading to two separate lock files when using different codebases:
host ~/test (master) # ls -l pkg.db
total 156K
-rw------- 1 root root 148K Jun 22 16:49 data.mdb
-rw------- 1 root root 8.0K Jun 23 19:30 lock.mdb
-rw-r--r-- 1 root root 8.0K Jun 22 18:49 data.mdb-lock
Currently, the only way to work around this issue is by changing all other codebases to use MDB_NOSUBDIR too because there is no way to disable it in PHP.
Activity
martinvonwittich commentedon Jun 23, 2022
Hm, apparently there's also no way to open the environment with
MDB_RDONLY, which causes a significant overhead when opening the environment just to read a key. In my Perl, Python and Rust implementations, enabling readonly increases the speed from ~150us to ~50us, but my PHP implementation is stuck at ~150us. Maybe it would be sensible to expose flags and the required constants completely?cmb69 commentedon Jun 24, 2022
Looks like a reasonable improvement; users should be able to at least specify some of the flags which are passed to
mdb_env_open(). PR welcome!Girgias commentedon Jun 29, 2022
I'm trying to determine what flags would be useful to support, and I'm currently thinking of just including MDB_NOSUBDIR (and probably a custom PHP constant MDB_USE_SUB_DIR) as the other ones don't seem that relevant
cmb69 commentedon Jun 29, 2022
Well,
MDB_RDONLYhas been requested here, and I think it may make sense to support that.Girgias commentedon Jun 29, 2022
This should be passed when the
"r"mode is passed todba_open()and I'm working on thatcmb69 commentedon Jun 29, 2022
Ah, right. It might make sense to set it automatically in that case.