Howard Chu writes: >Hallvard Breien Furuseth wrote: >> Then I'm not sure what's the point of so many speedup options like it >> has now. > > MDB_NOSYNC is perfectly safe on some filesystems like ZFS that guarantee > write > order.
Nice. Should tweak the doc a bit, then. Do you mean safe without MDB_WRITEMAP - i.e. it orders write system calls correctly? Or even with MDB_WRITEMAP - notices the order of updates to mmap pages? I imagine the latter would be rather tough. Sounds like this needs yet another mode for best performance, though: Sync after but not before writing the meta page. > Some apps want the ability to return immediately from txn_commit while > performing syncs in a background thread. MAPASYNC lets us do that. Only with MDB_WRITEMAP and if you do not care about consistency after a system crash, as mentioned in my other mail. Unless on ZFS and (MDB_NOSYNC & MDB_WRITEMAP) is safe, as above. > What you're talking about may also do that. I just want to be clear > about the motivation and the expected benefit. > > Using variably positioned meta pages seems like something we would try to cut > down on seek overhead, but looking closer it doesn't appear that it can do > that. Right, default mode will still write the file header every time. Hm, and it'll be a bit slower since some commits will need one more page: When none of the committed pages have room for the MDB_meta. Fewer seeks would only be a side effect of choosing fewer syncs. -- Hallvard
