On Tue, 26 Jun 2007, Gregory Stark wrote:

What exactly happens if a checkpoint takes so long that the next checkpoint
starts. Aside from it not actually helping is there much reason to avoid this
situation? Have we ever actually tested it?

More segments get created, and because of how they are cleared at the beginning this causes its own mini-I/O storm through the same buffered write channel the checkpoint writes are going into (which way or may not be the same way normal WAL writes go, depending on whether you're using O_[D]SYNC WAL writes). I've seen some weird and intermittant breakdowns from the contention that occurs when this happens, and it's certainly something to be avoided.

To test it you could just use a big buffer cache, write like mad to it, and make checkpoint_segments smaller than it should be for that workload. It's easy enough to kill yourself exactly this way right now though, and the fact that LDC gives you a parameter to aim this particular foot-gun more precisely isn't a big deal IMHO as long as the documentation is clear.

* Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD

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