Re: Load Distributed Checkpoints, final patch

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 gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD