From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=37522 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1Ou3yS-0006Y4-Tl
	for qemu-devel@nongnu.org; Fri, 10 Sep 2010 09:49:06 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69)
	(envelope-from <hch@lst.de>) id 1Ou3yR-0007Eb-1l
	for qemu-devel@nongnu.org; Fri, 10 Sep 2010 09:49:04 -0400
Received: from verein.lst.de ([213.95.11.210]:51946)
	by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from <hch@lst.de>)
	id 1Ou3yQ-0007EL-Oj
	for qemu-devel@nongnu.org; Fri, 10 Sep 2010 09:49:03 -0400
Date: Fri, 10 Sep 2010 15:48:59 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format
Message-ID: <20100910134859.GB28831@lst.de>
References: <4C86BC6B.5010809@codemonkey.ws> <4C874812.9090807@redhat.com>
	<4C87860A.3060904@codemonkey.ws> <4C888287.8020209@redhat.com>
	<4C88D7CC.5000806@codemonkey.ws> <4C8A1311.8070903@redhat.com>
	<4C8A15C4.40201@redhat.com>
	<AANLkTi=tt+Zh_i15LFz36kOdD8sFBKpJ2wsPRGM=3zkV@mail.gmail.com>
	<4C8A19CA.3040000@redhat.com> <4C8A3106.8050501@codemonkey.ws>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <4C8A3106.8050501@codemonkey.ws>
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

On Fri, Sep 10, 2010 at 08:22:14AM -0500, Anthony Liguori wrote:
> fsck will always be fast on qed because the metadata is small.  For a 
> 1PB image, there's 128MB worth of L2s if it's fully allocated (keeping 
> in mind, that once you're fully allocated, you'll never fsck again).  If 
> you've got 1PB worth of storage, I'm fairly sure you're going to be able 
> to do 128MB of reads in a short period of time.  Even if it's a few 
> seconds, it only occurs on power failure so it's pretty reasonable.

I don't think it is.  Even if the metadata is small it can still be
spread all over the disks and seek latencies might kill you.  I think
if we want to make qed future proof it needs to provide transactional
integrity for metadata updates, just like a journaling filesystem.
Given that small amount of metadata and less different kinds it will
still be a lot simpler than a full filesystem of course.