Howard Chu writes:
It occurs to me that there is the potential to support an interesting use case with LMDB when the database resides on remote shared storage. In the context of slapd, you could run multiple read-only slapds concurrent with a single read-write slapd on a single database.
Not quite... only slap tools open an MDB (or BDB) environment in read-only mode, as far as I can tell. slapd always opens read/write. The "readonly" slapd.conf option only restricts LDAP operations.
Also there's the issue of agreeing who gets to create (and maybe reset?) a lockfile. IIRC that's where people use mkdir for atomic NFS behavior, unless modern NFS fixes that. Though maybe it's enough to omit O_CREAT for the the lockfile in the read-only slapds, if that gets supported.
The current liblmdb would need a couple small modifications to make this safe
- an option to use fcntl(LOCK) when obtaining a reader slot, and an msync()
when writing to a reader slot, to force reader lock table changes back to the server before progressing on a read txn.
And maybe another sync call for the writer to pick the change up before progressing? OTOH readers may not need to await the msync() if an older reader in the same process is still live.