On 2013-01-09 14:04:32 +0530, Amit Kapila wrote: > On Tuesday, January 08, 2013 8:57 PM Andres Freund wrote: > > On 2013-01-08 20:33:28 +0530, Amit Kapila wrote: > > > On Tuesday, January 08, 2013 8:01 PM Andres Freund wrote: > > > > On 2013-01-08 19:51:39 +0530, Amit Kapila wrote: > > > > > On Monday, January 07, 2013 7:15 PM Andres Freund wrote: > > > > > > On 2013-01-07 19:03:35 +0530, Amit Kapila wrote: > > > > > > > On Monday, January 07, 2013 6:30 PM Simon Riggs wrote: > > > > > > > > On 7 January 2013 12:39, Amit Kapila > > <amit.kap...@huawei.com> > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > The information that no transactions are currently running > > allows > > > > you > > > > > > to > > > > > > build a recovery snapshot, without that information the standby > > > > won't > > > > > > start answering queries. Now that doesn't matter if all > > standbys > > > > > > already > > > > > > have built a snapshot, but the primary cannot know that. > > > > > > > > > > Can't we make sure that checkpoint operation doesn't happen for > > below > > > > conds. > > > > > a. nothing has happened during or after last checkpoint > > > > > OR > > > > > b. nothing except snapshotstanby WAL has happened > > > > > > > > > > Currently it is done for point a. > > > > > > > > > > > Having to issue a checkpoint while ensuring transactions are > > > > running > > > > > > just to get a standby up doesn't seem like a good idea to me :) > > > > > > > > > > Simon: > > > > > > If you make the correct test, I'd be more inclined to accept > > the > > > > premise. > > > > > > > > > > Not sure, what exact you are expecting from test? > > > > > The test is do any one operation on system and then keep the > > system > > > > idle. > > > > > Now at each checkpoint interval, it logs WAL for SnapshotStandby. > > > > > > > > I can't really follow what you want to do here. The snapshot is > > only > > > > logged if a checkpoint is performed anyway? As recovery starts at > > (the > > > > logical) checkpoint's location we need to log a snapshot exactly > > > > there. If you want to avoid activity when the system is idle you > > need > > > > to > > > > prevent checkpoints from occurring itself. > > > > > > Even if the checkpoint is scheduled, it doesn't perform actual > > operation if > > > there's nothing logged between > > > current and previous checkpoint due to below check in > > CreateCheckPoint() > > > function. > > > if (curInsert == ControlFile->checkPoint + > > > MAXALIGN(SizeOfXLogRecord + > > sizeof(CheckPoint)) && > > > ControlFile->checkPoint == > > > ControlFile->checkPointCopy.redo) > > > > > > But if we set the wal_level as hot_standby, it will log snapshot, now > > next > > > time again when function CreateCheckPoint() > > > will get called due to scheduled checkpoint, the above check will > > fail and > > > it will again log snapshot, so this will continue, even if the system > > is > > > totally idle. > > > I understand that it doesn't cause any problem, but I think it is > > better if > > > the repeated log of snapshot in this scenario can be avoided. > > > > ISTM in that case you "just" need a way to cope with the additionally > > logged record in the above piece of code. Not logging seems to be the > > entirely wrong way to go at this. > > I think one of the ways code can be modified is as below: > > + /*size of running transactions log when there is no > active transation*/ > + if (!shutdown && XLogStandbyInfoActive()) > + { > + runningXactXLog = > MAXALIGN(MinSizeOfXactRunningXacts) + SizeOfXLogRecord; > + } > > ! if (curInsert == ControlFile->checkPoint + > ! MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) && > ! ControlFile->checkPoint == > ControlFile->checkPointCopy.redo) > > ! if (curInsert == ControlFile->checkPoint + > ! MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) && > ! ControlFile->checkPoint == > ControlFile->checkPointCopy.redo + runningXactXLog) > > Second condition is checking the last checkpoint WAL position with the > current one. > Since ControlFile->checkPointCopy.redo holds the value before "running > Xact" WAL was inserted > and ControlFile->checkPoint holds the value after "running Xact" WAL got > inserted, so if no new WAL was inserted apart from "running Xacts" and > "Checkpoint" WAL, then this condition will be true. >
I don't think thats safe, there could have been another record inserted that happens to be MinSizeOfXactRunningXacts big and we would still skip the checkpoint. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers