From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=53199 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PsGnw-0000q7-1v for qemu-devel@nongnu.org; Wed, 23 Feb 2011 10:39:08 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PsGnp-0002Ni-7Q for qemu-devel@nongnu.org; Wed, 23 Feb 2011 10:38:59 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58162) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PsGno-0002NV-S1 for qemu-devel@nongnu.org; Wed, 23 Feb 2011 10:38:57 -0500 Message-ID: <4D652984.90401@redhat.com> Date: Wed, 23 Feb 2011 17:36:36 +0200 From: Avi Kivity 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> <4D6527F4.2010101@codemonkey.ws> In-Reply-To: <4D6527F4.2010101@codemonkey.ws> 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: Anthony Liguori Cc: Kevin Wolf , Chunqiang Tang , qemu-devel@nongnu.org, Markus Armbruster , Stefan Hajnoczi On 02/23/2011 05:29 PM, Anthony Liguori wrote: > >>> 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 don't see how that works, since the memory image is duplicated for each snapshot. So thousands of snapshots = terabytes of storage, and hours of creating the snapshots. Migrate-to-file with block live migration, or even better, something based on Kemari would be a lot faster. > > 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. Can you elaborate? What's the problem with a linear array of snapshots (say up to 10,000 snapshots)? -- error compiling committee.c: too many arguments to function