| From: | Greg Stark <stark(at)mit(dot)edu> |
|---|---|
| To: | Bruce Momjian <bruce(at)momjian(dot)us> |
| Cc: | Greg Smith <greg(at)2ndquadrant(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andres Freund <andres(at)2ndquadrant(dot)com>, Ants Aasma <ants(at)cybertec(dot)at>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Enabling Checksums |
| Date: | 2013-03-20 12:13:51 |
| Message-ID: | CAM-w4HO8jQ_+fzRiA5_VHx8zxd41pf7gTpbD=o7kbu+bjyYWWQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Mar 18, 2013 at 5:52 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> With a potential 10-20% overhead, I am unclear who would enable this at
> initdb time.
For what it's worth I think cpu overhead of the checksum is totally a
red herring.. Of course there's no reason not to optimize it to be as
fast as possible but if we say there's a 10% cpu overhead due to
calculating the checksum users will think that's perfectly reasonable
trade-off and have no trouble looking at their cpu utilization and
deciding whether they have that overhead to spare. They can always buy
machines with more cores anyways.
Added I/O overhead, especially fsync latency is the performance impact
that I think we should be focusing on. Uses will be totally taken by
surprise to hear that checksums require I/O. And fsync latency to the
xlog is very very difficult to reduce. You can buy more hard drives
until the cows come home and the fsync latency will hardly change.
--
greg
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