On Thu, Jul 28, 2011 at 7:54 PM, Ants Aasma <ants.aa...@eesti.ee> wrote: > On Thu, Jul 28, 2011 at 11:54 PM, Kevin Grittner > <kevin.gritt...@wicourts.gov> wrote: >> (4) We communicate acceptable snapshots to the replica to make the >> order of visibility visibility match the master even when that >> doesn't match the order that transactions returned from commit. > > I wonder if some interpretation of 2 phase commit could make Robert's > original suggestion implement this. > > On the master the commit sequence would look something like: > 1. Insert commit record to the WAL > 2. Wait for replication > 3. Get a commit seq nr and mark XIDs visible > 4. WAL log the seq nr > 5. Return success to client > > When replaying: > * When replaying commit record, do everything but make > the tx visible. > * When replaying the commit sequence number > if there is a gap between last visible commit and current: > insert the commit sequence nr. to list of waiting commits. > else: > mark current and all directly following waiting tx's visible > > This would give consistent visibility order on master and slave. Robert > is right that this would undesirably increase WAL traffic. Delaying this > traffic would undesirably increase replay lag between master and slave. > But it seems to me that this could be an optional WAL level on top of > hot_standby that would only be enabled if consistent visibility on > slaves is desired.
I think you nailed it. An additional point to think about: if we were willing to insist on streaming replication, we could send the commit sequence numbers via a side channel rather than writing them to WAL, which would be a lot cheaper. That might even be a reasonable thing to do, because if you're doing log shipping, this is all going to be super-not-real-time anyway. OTOH, I know we don't want to make WAL shipping anything less than a first class citizen, so maybe not. At any rate, we may be getting a little sidetracked here from the original point of the thread, which was how to make snapshot-taking cheaper. Maybe there's some tie-in to when transactions become visible, but I think it's pretty weak. The existing system could be hacked up to avoid making transactions visible out of LSN order, and the system I proposed could make them visible either in LSN order or do the same thing we do now. They are basically independent problems, AFAICS. -- 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