From: | Peter Eisentraut <peter_e(at)gmx(dot)net> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Greg Stark <gsstark(at)mit(dot)edu>, "A(dot)M(dot)" <agentm(at)themactionfaction(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: O_DSYNC broken on MacOS X? |
Date: | 2010-10-05 12:11:43 |
Message-ID: | 1286280703.16817.15.camel@vanquo.pezone.net |
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Thread: | |
Lists: | pgsql-hackers |
On mån, 2010-10-04 at 23:41 -0400, Robert Haas wrote:
> > Well, it's not really useful, but that's how it works "everywhere". On
> > Linux, fsync carries the stuff from the kernel's RAM to the disk
> > controller's RAM, and then it depends on some hdparm magic or something
> > what happens next.
>
> That's a bit vaguer than I'd like. TFD says "The aim of WAL is to
> ensure that the log is written before database records are altered,
> but this can be subverted by disk drives that falsely report a
> successful write to the kernel, when in fact they have only cached the
> data and not yet stored it on the disk. A power failure in such a
> situation might lead to irrecoverable data corruption. Administrators
> should try to ensure that disks holding PostgreSQL's WAL log files do
> not make such false reports." This leaves open the question of how
> they should attempt to do this; we should say what we know about that.
That is explained in section 29.1 "Reliability".
> I also notice the following sentence in our documentation, which now
> appears to me to be flat-out wrong: "The wal_sync_method parameter
> determines how PostgreSQL will ask the kernel to force WAL updates
> out to disk. All the options should be the same in terms of
> reliability, but it's quite platform-specific which one will be the
> fastest." Obviously, we know now (if we didn't before) that this
> isn't the case, per my OP.
Right. It was true before fsync_writethrough was invented.
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