Re: SCSI vs. IDE performance test - Mailing list pgsql-general

On Mon, 2003-10-27 at 17:18, Rick Gigger wrote:
> ahhh. "lies about write order" is the phrase that I was looking for.  That
> seemed to make sense but I didn't know if I could go directly from "lying
> about fsync" to that.  Obviously I don't understand exactly what fsync is
> doing.  I assume this means that if you were to turn fsync off you would get
> considerably better performance but introduce the possibility of corrupting
> the files in your database.

Yes.

There was a recent thread (in -general or -performance) regarding
putting the WAL files on a different disk, and changing wal_sync_-
method to open_sync (or open_datasync, don't remember).

This will allow the device(s) that the database is on to
run asynchronously, while the WAL is synchronous, for safety.

> Thank you.  This makes a lot more sense now.
>
>
>
> ----- Original Message -----
> From: "Tom Lane" <tgl@sss.pgh.pa.us>
> To: "Rick Gigger" <rick@alpinenetworking.com>
> Cc: <pgsql-general@postgresql.org>
> Sent: Monday, October 27, 2003 3:39 PM
> Subject: Re: [GENERAL] SCSI vs. IDE performance test
>
>
> > "Rick Gigger" <rick@alpinenetworking.com> writes:
> > > It seems to me file system journaling should fix the whole problem by
> giving
> > > you a record of what was actually commited to disk and what was not.
> >
> > Nope, a journaling FS has exactly the same problem Postgres does
> > (because the underlying "WAL" concept is the same: write the log entries
> > before you change the files they describe).  If the drive lies about
> > write order, the FS can be screwed just as badly.  Now the FS code might
> > have a low-level way to force write order that Postgres doesn't have
> > access to ... but simply uttering the magic incantation "journaling file
> > system" will not make this problem disappear.
> >
> > regards, tom lane
> >
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>
>
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--
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Ron Johnson, Jr. ron.l.johnson@cox.net
Jefferson, LA USA

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says Mr. Taylor, now head of the Washington
http://www.csmonitor.com/2002/0829/p01s03-wosc.html