Re: Why so many xlogs?

On Mon, Nov 01, 2010 at 08:31:10PM +0100, Cédric Villemain wrote:
> It should stick at a maximum of 3 * checkpoint_segments + 1, if it
> exceed it will remove the extra files after.

if you'd look at the graph you'd notice that it never goes down to 2n+1.
And really - so far I have not yet heard/seen/read any solid reasoning
for 3n instead of 2n.

> > also - can you explain why "fraction of total time" (time!) would
> > directly relate to number of xlog files existing in pg_xlog? I mean -
> > you're not the first person to suggest it, but I don't see any way that
> > these two could be related.
> It's guess that while your checkpoint is longer by this factor(X%),
> the number of wal files needed might be multiplied by the same ratio.
> (1+X%) To handle extra files created while the checklpoint is still
> running.

I'm not sure I understand. Will need to run some tests. Yet - even
assuming (2 + checkpoint_completion_target ) * n - it doesn't explain
why there was no difference in number of segments after decreasing from
0.9 to 0.5.

Best regards,

depesz


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