Re: Why so many xlogs?

2010/11/1 hubert depesz lubaczewski <depesz(at)depesz(dot)com>:
> On Mon, Nov 01, 2010 at 08:13:49PM +0100, Cédric Villemain wrote:
>> 2010/11/1 hubert depesz lubaczewski <depesz(at)depesz(dot)com>:
>> > On Mon, Nov 01, 2010 at 07:22:50PM +0100, Cédric Villemain wrote:
>> >> > as I understand, max number of xlog files in pg_xlog should be ( 1 + 2 *
>> >> > checkpoint_segments ).
>> >> (2 + checkpoint_completion_target) * checkpoint_segments + 1
>> >> => 291
>> >
>> > this is formula gave to me by rhodiumtoad on irc, but we tested with lower
>> > checkpoint_completion_target and it didn't change *anything*.
>>
>> You'll have between 211 and 291 files for 0.1 to 0.9
>> checkpoint_completion_target.
>> You'd have more than the number of files given by formula used during
>> your tests ?
>
> yes. we decreased checkpoint_completion_target to 0.5, and the numbers
> of xlog segments *did not change*.

It should stick at a maximum of 3 * checkpoint_segments + 1, if it
exceed it will remove the extra files after.

> 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.

>
> Best regards,
>
> depesz
>
> --
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Cédric Villemain               2ndQuadrant
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