On Tue, Oct 5, 2010 at 8:11 AM, Peter Eisentraut <peter_e@gmx.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
