From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60677) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cyahR-0007Rn-6p for qemu-devel@nongnu.org; Thu, 13 Apr 2017 05:05:58 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cyahO-0005wu-2z for qemu-devel@nongnu.org; Thu, 13 Apr 2017 05:05:57 -0400 From: Alberto Garcia In-Reply-To: <20170413090210.GA5095@noname.redhat.com> References: <20170406150148.zwjpozqtale44jfh@perseus.local> <9d848582-8c76-4d88-2b31-e0e4c63b61d4@redhat.com> <5cb5f7fb-aadd-d8b5-7cf5-d677db045105@redhat.com> <20170413090210.GA5095@noname.redhat.com> Date: Thu, 13 Apr 2017 11:05:49 +0200 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [Qemu-block] [RFC] Proposed qcow2 extension: subcluster allocation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Max Reitz , Eric Blake , qemu-devel@nongnu.org, Stefan Hajnoczi , qemu-block@nongnu.org On Thu 13 Apr 2017 11:02:10 AM CEST, Kevin Wolf wrote: > I think you still need it if you don't want to look at the refcount > blocks for every write. When you take an internal snapshot, you just > increase the refcount of the L2 tables at first and keep the contents > the same, including the subcluster information. On the first write to > the cluster, like with normal images you need to copy the whole > cluster, and whether this is the case is determined with the COPIED > flag. (The copy can actually keep unallocated/zeroed subclusters that > way, but data has to be copied for the whole cluster.) > > Of course, in all cases the COPIED flag is just an optimisation > because it's always possible to look at the refcount blocks, but I > don't think there is any difference between subclustered and > traditional images in this respect. Exactly :) Berto