Horton

Re: Spread checkpoint sync

Itagaki Takahiro <itagaki.takahiro@gmail.com>

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From: Itagaki Takahiro <itagaki.takahiro@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Greg Smith <greg@2ndquadrant.com>, Ron Mayer <rm_pg@cheapcomplexdevices.com>, Josh Berkus <josh@agliodbs.com>, pgsql-hackers@postgresql.org
Date: 2011-01-31T08:04:22Z
Lists: pgsql-hackers

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  1. Add new buffers_backend_fsync field to pg_stat_bgwriter.

On Mon, Jan 31, 2011 at 13:41, Robert Haas <robertmhaas@gmail.com> wrote:
> 1. Absorb fsync requests a lot more often during the sync phase.
> 2. Still try to run the cleaning scan during the sync phase.
> 3. Pause for 3 seconds after every fsync.
>
> So if we want the checkpoint
> to finish in, say, 20 minutes, we can't know whether the write phase
> needs to be finished by minute 10 or 15 or 16 or 19 or only by 19:59.

We probably need deadline-based scheduling, that is being used in write()
phase. If we want to sync 100 files in 20 minutes, each file should be
sync'ed in 12 seconds if we think each fsync takes the same time.
If we would have better estimation algorithm (file size? dirty ratio?),
each fsync chould have some weight factor.  But deadline-based scheduling
is still needed then.

BTW, we should not sleep in full-speed checkpoint. CHECKPOINT command,
shutdown, pg_start_backup(), and some of checkpoints during recovery
might don't want to sleep.

-- 
Itagaki Takahiro