Re: checkpoint_completion_target and Ext3

Josh Berkus wrote:
> Given your analysis of fsync'ing behavior on Ext3, would you say that it
> is better to set checkpoint_completion_target to 0.0 on Ext3?
>

Setting that to 0.0 gives the same basic behavior as in 8.2 and earlier
versions. Those had even worst I/O spikes issues. Even on ext3, there
is value to spreading the writes around over time, particularly if you
have a large setting for checkpoint_segments. Ideally the write phase
will be spread out over 2.5 minutes, if you've set the segments high
enough that checkpoints are being driven by checkpoint_timeout. The
original testing myself and Heikki did settled on the default of 0.5 for
checkpoint_completion_target on ext3, so that part hasn't really
changed. It's still better than just writing everything in one big
dump, as you'd see with it set to 0.0.

While Linux and ext3 aren't great about getting stuff to disk, doing
some writing in advance of sync will improve things at least a little.
The thing that I don't ever expect to work on ext3 is spreading the sync
phase out over time.

P.S. those of you who are into filesystem trivia but don't read
pgsql-hackers normally may enjoy
http://blog.2ndquadrant.com/en/2011/01/tuning-linux-for-low-postgresq.html
and
http://archives.postgresql.org/message-id/4D4C4610.1030109@2ndquadrant.com
which has the research Josh is alluding to here. I also just wrote a
rebuttal today to the "PostgreSQL doesn't have hints" meme at
http://blog.2ndquadrant.com/en/2011/02/hinting-at-postgresql.html

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books