Re: Spread checkpoint sync

On Mon, Jan 31, 2011 at 11:51 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Mon, Jan 31, 2011 at 11:29 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> That sounds like you have an entirely wrong mental model of where the
>>> cost comes from.  Those times are not independent.
>
>> Yeah, Greg Smith made the same point a week or three ago.  But it
>> seems to me that there is potential value in overlaying the write and
>> sync phases to some degree.  For example, if the write phase is spread
>> over 15 minutes and you have 30 files, then by, say, minute 7, it's a
>> probably OK to flush the file you wrote first.
>
> Yeah, probably, but we can't do anything as stupid as file-by-file.

Eh?

> I wonder whether it'd be useful to keep track of the total amount of
> data written-and-not-yet-synced, and to issue fsyncs often enough to
> keep that below some parameter; the idea being that the parameter would
> limit how much dirty kernel disk cache there is.  Of course, ideally the
> kernel would have a similar tunable and this would be a waste of effort
> on our part...

It's not clear to me how you'd maintain that information without it
turning into a contention bottleneck.

--
Robert Haas
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