In openldap-technical, Quanah Gibson-Mount wrote:
As per Howard Chu (Author of MDB, Primary OpenLDAP Developer):
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Full details are in the paper.
http://www.openldap.org/pub/hyc/mdm-paper.pdf
MDB assumes a unified buffer cache. See section 3.1, references 17, 18, and
19.
Note that this requirement can be relaxed in the current version of the
library. If you create the environment with the MDB_WRITEMAP option then
all reads and writes are performed using mmap, so the file buffer cache is
irrelevant. Of course then you lose the protection that the read-only map
offers.
That's not quite true. mdb_env_open() does a read() of the meta pages.
I presume this can only be a problem when other processes have the
database open? In that situation, I think the read() can be avoided
by maintaining a copy of the relevant MDB_meta information in the
lock file. The read() only needs enough info to know how to map
the data file.
This would require a version increase for the lock file and programs
using it, but not for the database file.