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IDS Forum

Re: Checkpoint blocked by a previous checkpoint

Posted By: Art Kagel
Date: Tuesday, 4 November 2014, at 2:53 p.m.

In Response To: Re: Checkpoint blocked by a previous checkpoint (JACQUES RENAUT)

My understanding is that the engine cancels the scheduled checkpoint and
simply reschedules the next timed checkpoint to start CHKPTINTVL seconds
after the completion of the long running checkpoint. Note that PLOG
checkpoints are NOT non-blocking so new transactions cannot begin and
existing transactions cannot enter a critical section of server code until
the physical log finishes flushing to disk at which point the checkpoint
reverts to non-blocking and buffer flushes are backgrounded as usual.


Art S. Kagel, President and Principal Consultant
ASK Database Management


Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on the IIUG, nor any other organization with which I am
associated either explicitly, implicitly, or by inference. Neither do
those opinions reflect those of other individuals affiliated with any
entity with which I am affiliated nor those of the entities themselves.

On Tue, Nov 4, 2014 at 12:17 PM, JACQUES RENAUT <> wrote:

> Original post:
> Hi folks.
> I have a situation that I suspect but can't get IBM to confirm or deny.
> Thus,
> I'm requesting a peer review that my logic is correct and a likely
> explanation
> of a blocked checkpoint.
> Solaris 10
> IDS 11.50.FC3X5
> This is what I think happened:
> My client's production box has a checkpoint interval of 10 minutes and
> ~1.5GB
> physical log. During one such interval there was a violent flurry of table
> updates, the kind that put many pages into the plog. About 1 minute before
> the
> end of the checkpoint interval, the plog reached 75% full and the engine
> initiated a PLOG-checkpoint. That checkpoint, with about 600,000 dirty
> pages
> to flush, the checkpoint lasted about 84 seconds.
> Whoops! That creeps into the scheduled CKPTINTVL-checkpoint, which tried to
> start about 14 seconds before the PLOG-checkpoint finished.
> How did I piece together this scenario?
> My client called in with a blocked checkpoint. While breaking our heads
> over
> it I ran onstat -g ckp and noticed that the last checkpoint that had
> completed
> (over an hour ago by this time) was the PLOG and it had lasted 84 seconds,
> consistent with the last "Checkpoint completed" message in the online log.
> It
> had started about 58 seconds before he next schedule CKPTINTVL-checkpoint
> would have started.
> What should the engine do if it finds a checkpoint already in progress
> when it
> wants to start one? It could either:
> - Queue itself to the checkpoint latch until the current checkpoint is
> done,
> and then run the seconds checkpoint. Perhaps issue a warning in the online
> log
> that a checkpoint had to await another.
> - Cancel the new one, since there's one already running. And issue a
> warning
> in online log that it did so.
> IMO, the latter is the more logical and efficient course. But what did my
> engine do? (Remember, this is still *my* forensic reconstruction of the
> story.) The second checkpoint blocked itself and never came out of that
> state,
> effectively halting the server.
> Is this consistent with any bug that someone may know about? My client *is*
> running an old release. Or is my logic discombobulated?
> Thanks for clarity here.
> -- Jacob
> Reply:
> CKPTINTVL is the amount of time to kick off a checkpoint since the last
> checkpoint finished (any type of checkpoint not just CKPTINTVL type
> checkpoints). So when the PLOG checkpoint fired off, there would not be
> another checkpoint until CKPTINTVL passed after that checkpoint finished.
> Jacques Renaut
> IBM Informix Advanced Support
> APD Team
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.


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