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Date: Fri, 25 Nov 2011 12:02:35 +0100
From: Pawel Jakub Dawidek <pjd@FreeBSD.org>
To: Kostik Belousov <kostikbel@gmail.com>
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References: <1957615267.20111123230026@serebryakov.spb.ru>
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X-OS: FreeBSD 9.0-CURRENT amd64
User-Agent: Mutt/1.5.21 (2010-09-15)
Cc: freebsd-fs@freebsd.org
Subject: Re: Does UFS2 send BIO_FLUSH to GEOM when update metadata (with
 softupdates)?
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On Wed, Nov 23, 2011 at 09:44:44PM +0200, Kostik Belousov wrote:
> On Wed, Nov 23, 2011 at 11:00:26PM +0400, Lev Serebryakov wrote:
> > Hello, Freebsd-fs.
> >=20
> >   Does UFS2 with softupdates (without journal) issues BIO_FLUSH to
> > GEOM layer when it need to ensure consistency on on-disk metadata?
> No. Softupdates do not need flushes.

Well, they do for two reasons:
1. To properly handle sync operations (fsync(2), O_SYNC).
2. To maintain consistent on-disk structures.

The second point is there, because BIO_FLUSH is the only way to avoid
reordering (apart from turning off disk write cache).

SU assumes no I/O reordering will happen, which is very weak assumption.

--=20
Pawel Jakub Dawidek                       http://www.wheelsystems.com
FreeBSD committer                         http://www.FreeBSD.org
Am I Evil? Yes, I Am!                     http://yomoli.com

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