Full_Name: Scott Kevill Version: LMDB 0.9.14 OS: CentOS 5-7 URL: ftp://ftp.openldap.org/incoming/scott-kevill-150628.patch Submission from: (NULL) (60.224.160.178)

mdb_txn_begin / mdb_txn_renew will return MDB_MAP_RESIZED for the current process if another process has changed the mapsize AND the data has grown past the current process mapsize. However, by that point, in a multithreaded environment it may be difficult to wind back to a "safe" point of no active txns in order to call mdb_env_set_mapsize(env, 0) to update the mapsize.

Requesting: int mdb_env_check_mapsize(MDB_env *env); to return either MDB_SUCCESS or MDB_MAP_RESIZED. This should be lightweight enough to call frequently enough to decide whether to initiate a "safe" point to enable calling mdb_env_set_mapsize(env, 0). Return MDB_SUCCESS if the MDB has not been opened yet.

Scenario: - Process W is a writer - Process R is a reader with multiple reader threads (read-only) - Both processes are running and using a 2GB mapsize MDB - Data is currently 1GB - Ops decide W should increase mapsize to 3GB allowing for more expansion - W calls mdb_env_set_mapsize(env, 3GB) - R's mapsize remains at 2GB%0 D Data grows - R:1 thread begins a moderately long read-only txn - Data grows past 2GB - R:2 thread tries to begin a read-only txn, but fails with MDB_MAP_RESIZED - R:2 thread cannot call mdb_env_set_mapsize(env, 0), because of R:1's txn - A major control flow issue exists now, as R:2 may be deep into work, but can't continue nor back out

With mdb_env_check_mapsize(), R could periodically test it and if needed, initiate a "safe" point where none of R's threads have active txns. This could be done long before the data size actually reaches R's original mapsize (ie. when data is ~1GB rather >2GB).

- Enforcing synchronisation to a "safe" point might be relatively expensive, so it would be important to know if the mapsize has changed first. ie. Making mdb_env_set_mapsize(env, 0) lightweight for the unchanged mapsize case isn't enough

I've attached a sample patch against master (scott-kevill-150628.patch). I've tested it against 0.9.14. The code is trivial and is based on the behaviour of mdb_env_set_mapsize(env, 0). I didn't use git for the patch, so not too difficult to get working. Less than 10 new lines, but for the record, I release it to public domain. Feel free to massage it as needed.