From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=54958 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PsGfQ-0005nd-M0 for qemu-devel@nongnu.org; Wed, 23 Feb 2011 10:30:18 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PsGfL-0000aT-HI for qemu-devel@nongnu.org; Wed, 23 Feb 2011 10:30:12 -0500 Received: from mail-vx0-f173.google.com ([209.85.220.173]:37954) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PsGfL-0000aH-Eb for qemu-devel@nongnu.org; Wed, 23 Feb 2011 10:30:11 -0500 Received: by vxb41 with SMTP id 41so2812003vxb.4 for ; Wed, 23 Feb 2011 07:30:10 -0800 (PST) Message-ID: <4D6527F4.2010101@codemonkey.ws> Date: Wed, 23 Feb 2011 09:29:56 -0600 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: Strategic decision: COW format References: <4D5BC467.4070804@redhat.com> <4D5E4271.80501@redhat.com> <4D5E8031.5020402@codemonkey.ws> <4D637A20.9020307@redhat.com> <4D650F10.3060900@redhat.com> <4D651858.9040106@codemonkey.ws> <4D651BD2.3040500@redhat.com> In-Reply-To: <4D651BD2.3040500@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Chunqiang Tang , Markus Armbruster , qemu-devel@nongnu.org, Stefan Hajnoczi , Avi Kivity On 02/23/2011 08:38 AM, Kevin Wolf wrote: > Am 23.02.2011 15:23, schrieb Anthony Liguori: > >> On 02/23/2011 07:43 AM, Avi Kivity wrote: >> >>> On 02/22/2011 10:56 AM, Kevin Wolf wrote: >>> >>>> *sigh* >>>> >>>> It starts to get annoying, but if you really insist, I can repeat it >>>> once more: These features that you don't need (this is the correct >>>> description for what you call "misfeatures") _are_ implemented in a way >>>> that they don't impact the "normal" case. And they are it today. >>>> >>>> >>> Plus, encryption and snapshots can be implemented in a way that >>> doesn't impact performance more than is reasonable. >>> >> We're still missing the existence proof of this, but even assuming it >> > Define "reasonable". I sent you some numbers not too long for > encryption, and I consider them reasonable (iirc, between 25% and 40% > slower than without encryption). > I was really referring to snapshots. I have absolutely no doubt that encryption can be implemented with a reasonable performance overhead. >> existed, what about snapshots? Are we okay having a feature in a >> prominent format that isn't going to meet user's expectations? >> >> Is there any hope that an image with 1000, 1000, or 10000 snapshots is >> going to have even reasonable performance in qcow2? >> > Is there any hope for backing file chains of 1000 files or more? I > haven't tried it out, but in theory I'd expect that internal snapshots > could cope better with it than external ones because internal snapshots > don't have to go through the whole chain all the time. > I don't think there's a user expectation of backing file chains of 1000 files performing well. However, I've talked to a number of customers that have been interested in using internal snapshots for checkpointing which would involve a large number of snapshots. In fact, Fabrice originally added qcow2 because he was interested in doing reverse debugging. The idea of internal snapshots was to store a high number of checkpoints to allow reverse debugging to be optimized. I think the way snapshot metadata is stored makes this not realistic since they're stored in more or less a linear array. I think to really support a high number of snapshots, you'd want to store a hash with each block that contained a refcount > 1. I think you quickly end up reinventing btrfs though in the process. Regards, Anthony Liguori > What are the points where you think that performance of internal > snapshots suffers? > > The argument that I would understand is that internal snapshots are > probably not as handy in all scenarios. > > Kevin > >