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Date: Wed, 7 Mar 2001 21:59:31 +0100
From: Jens Axboe <axboe@suse.de>
To: "Stephen C. Tweedie" <sct@redhat.com>
Cc: David Balazic <david.balazic@uni-mb.si>, torvalds@transmeta.com,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: scsi vs ide performance on fsync's
Message-ID: <20010307215931.J4653@suse.de>
In-Reply-To: <3AA53DC0.C6E2F308@uni-mb.si> <20010306213720.U2803@suse.de> <20010307135135.B3715@redhat.com> <20010307151241.E526@suse.de> <20010307150556.L7453@redhat.com> <20010307195152.C4653@suse.de> <20010307191044.M7453@redhat.com> <20010307211536.G4653@suse.de> <20010307205659.E9080@redhat.com>
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In-Reply-To: <20010307205659.E9080@redhat.com>; from sct@redhat.com on Wed, Mar 07, 2001 at 08:56:59PM +0000
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On Wed, Mar 07 2001, Stephen C. Tweedie wrote:
> On Wed, Mar 07, 2001 at 09:15:36PM +0100, Jens Axboe wrote:
> > On Wed, Mar 07 2001, Stephen C. Tweedie wrote:
> > > 
> > > For most fs'es, that's not an issue.  The fs won't start writeback on
> > > the primary disk at all until the journal commit has been acknowledged
> > > as firm on disk.
> > 
> > But do you then force wait on that journal commit?
> 
> It doesn't matter too much --- it's only the writeback which is doing
> this (ext3 uses a separate journal thread for it), so any sleep is
> only there to wait for the moment when writeback can safely begin:
> users of the filesystem won't see any stalls.

Ok, but even if this is true for ext3 it may not be true for other
journalled fs. AFAIR, reiser is doing an explicit wait_on_buffer
which would then amount to quite a performance hit (speculation,
haven't measured).

> > A barrier operation is sufficient then. So you're saying don't
> > over design, a simple barrier is all you need?
> 
> Pretty much so.  The simple barrier is the only thing which can be
> effectively optimised at the hardware level with SCSI anyway.

True

-- 
Jens Axboe

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