Re: O_DSYNC broken on MacOS X?

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Peter Eisentraut <peter_e(at)gmx(dot)net>
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: 2010-10-07 15:11:16
Message-ID: AANLkTimCOYA6DxyjNxNw7O_taHuNDOrARO2Eo1jkYQ7+@mail.gmail.com
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On Tue, Oct 5, 2010 at 8:11 AM, Peter Eisentraut <peter_e(at)gmx(dot)net> wrote:
> 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.

Proposed doc patch attached.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company

Attachment Content-Type Size
document-wal-caveats.patch application/octet-stream 2.5 KB

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