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(ITS#7651) LMDB: Uncontrolled database when opened from multiple processes

Full_Name: Dimitrij Denissenko
OS: Ubuntu 12.04
Submission from: (NULL) (


I found an interesting issue with LMDB. I have populated the DB with a bunch of
records and it uses ~30M on disk (after sync). Then I added a background process
to my app and populated the database again with the same record set.
Surprisingly. the resulting size on disk was >70M.

The background process is forked periodically to perform some maintenance tasks,
here is my (simplified) code:

/* Close env before forking */

if ((childpid = fork()) == 0) {
    /* Child */
    rc = mdb_env_open(env, ".", MDB_NOSYNC, 0644);
} else {
    /* Parent */
    rc = mdb_env_open(env, ".", MDB_NOSYNC, 0644);

I could narrow it down to the mdb_env_open call in the child. If I add exit(0)
before the mdb_env_open line, the DB size remains consistently at ~30M. The data
size seems to grow proportionally to the number of forks performed during data
load. What could be causing the growth? What can I do to prevent it?

Thanks in advance

PS: I tried it with MDB_FIXMAP and without, same result.