Horton

Re: Spread checkpoint sync

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>, Itagaki Takahiro <itagaki.takahiro@gmail.com>, Greg Smith <greg@2ndquadrant.com>, Ron Mayer <rm_pg@cheapcomplexdevices.com>, Josh Berkus <josh@agliodbs.com>, pgsql-hackers@postgresql.org
Date: 2011-01-31T16:51:13Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add new buffers_backend_fsync field to pg_stat_bgwriter.

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.

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...

			regards, tom lane