On Mon, Jul 16, 2012 at 12:08 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Sun, Jul 15, 2012 at 2:29 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> Yeah, you have a point there. It's not real clear that switching fsync >>> from off to on is an operation that we can make any guarantees about, >>> short of executing something like the code recently added to initdb >>> to force-sync the entire PGDATA tree. Perhaps we should change fsync >>> to be PGC_POSTMASTER (ie frozen at postmaster start), and then we could >>> skip forwarding fsync requests when it's off? > >> I would argue that such a change adds no measure of safety, anyway. > > Well, yes it does, and the reason was explained further down in the > thread: since we have no particular guarantees as to how quickly > postmaster children will absorb postgresql.conf updates, there could be > individual processes still running with fsync = off long after the user > thinks he's turned it on. A forced restart solves that. I believe the > reason for the current coding in the fsync queuing stuff is so that you > only have to worry about how long it takes the checkpointer to notice > the GUC change, and not any random backend that's running a forty-hour > query.
Hrmf, I guess that's a fair point. But if we believe that reasoning then I think it's an argument for sending fsync requests even when fsync=off, not for making fsync PGC_POSTMASTER. Or maybe we could store the current value of the fsync flag in shared memory somewhere and have backends check it before deciding whether to enqueue a request. With proper use of memory barriers it should be possible to make this work without requiring a lock. >> But, at a broader level, I am not very excited about this >> optimization. It seems to me that if this is hurting enough to be >> noticeable, then it's hurting us when fsync=on as well, and we had >> maybe think a little harder about how to cut down on the IPC overhead. > > Uh, that's exactly what's under discussion. Not sending useless fsync > requests when fsync is off is just one part of it; a part that happens > to be quite useful for some test scenarios, even if not so much for > production. (IIRC, the original complainant in this thread was running > fsync off.) My point is that if sending fsync requests is cheap enough, then not sending them won't save anything meaningful. And I don't see why it can't be made just that cheap, thereby benefiting people with fsync=on as well. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers