From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53717) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XoE9K-0007Fp-N7 for qemu-devel@nongnu.org; Tue, 11 Nov 2014 11:18:41 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XoE9E-0006JJ-HK for qemu-devel@nongnu.org; Tue, 11 Nov 2014 11:18:34 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40817) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XoE9E-0006JB-9P for qemu-devel@nongnu.org; Tue, 11 Nov 2014 11:18:28 -0500 Message-ID: <546236CD.30301@redhat.com> Date: Tue, 11 Nov 2014 17:18:21 +0100 From: Max Reitz MIME-Version: 1.0 References: <1415627159-15941-1-git-send-email-mreitz@redhat.com> <1415627159-15941-6-git-send-email-mreitz@redhat.com> <54612A27.7000801@redhat.com> <5461C751.3080607@redhat.com> <5462359D.4040503@redhat.com> In-Reply-To: <5462359D.4040503@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 05/21] qcow2: Refcount overflow and qcow2_alloc_bytes() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eric Blake , qemu-devel@nongnu.org Cc: Kevin Wolf , Peter Lieven , Stefan Hajnoczi On 2014-11-11 at 17:13, Eric Blake wrote: > On 11/11/2014 01:22 AM, Max Reitz wrote: >> On 2014-11-10 at 22:12, Eric Blake wrote: >>> On 11/10/2014 06:45 AM, Max Reitz wrote: >>>> qcow2_alloc_bytes() may reuse a cluster multiple times, in which case >>>> the refcount is increased accordingly. However, if this would lead to an >>>> overflow the function should instead just not reuse this cluster and >>>> allocate a new one. >>> So if recount_order is 1 (2 bits per refcount, max refcount of 4 >> *max refcount of 3 (0b11) > Oh right, because 0 is special. Although I think I figured that out... > >>> ), and >>> we encounter the same cluster 6 times (say by 5 back-to-back internal >>> snapshots), does this code optimize to only 2 clusters (both with >>> refcount 3) or does it result in each of the last 3 clusters spilling to > ...when talking about 3 shares of a cluster. > >>> its own 1-ref cluster for a total of 4 clusters? Short of Benoit's work >>> on deduplication, is there even a way to avoid inefficient use of >>> spilled clusters? >> I'm not sure what you're referring to; maybe I should add that >> qcow2_alloc_bytes() is used for allocating compressed clusters (which >> ideally don't take up a full host cluster), so "reuse" in this context >> just means that several compressed clusters share one host cluster. > No, I was thinking about internal snapshots rather than compressed > clusters (although there's probably some overlap on what happens). > >> Maybe you're referring to the following situation: We have the default >> cluster size of 64k. Now we're trying to allocate 16k for each of the >> compressed clusters A, B, C and D. D won't fit into that cluster because >> the maximum refcount is three, so it will be put into a newly allocated >> host cluster. Finally, we're trying to allocate 32k for a compressed >> cluster E, which will then be put into the same cluster as D. We >> therefore have the following allocation (each sub-box representing 16k): >> >> +---+---+---+---+ +---+---+---+---+ >> |A |B | C | | | D | E | | >> +---+---+---+---+ +---+---+---+---+ >> >> whereas the ideal allocation would be: >> >> +---+---+---+---+ +---+---+---+---+ >> |A |B | E | | C | D | | | >> +---+---+---+---+ +---+---+---+---+ >> >> This is a problem, but I think first it's a minor one (just use a >> sufficiently large refcount width if you're going to use compressed >> clusters) and second it's about compressed clusters, whose performance I >> could hardly care less about, frankly. > No, I was envisioning that we have a brand new image with one cluster > allocated (cluster 1 has refcount 1), then 5 times in a row we do > 'savevm' to take an internal snapshot. If I understand your code > correctly, the first two snapshots increase the refcount, so cluster 1 > has a refcount of 3. Then the next snapshot can't increase the refcount, > so it instead copies the contents to cluster 2. No, it just errors out. qcow2_alloc_bytes() is only used for allocating space for a compressed cluster. When taking a snapshot, update_refcount() will be called to increase the clusters' refcounts, and that function will simply throw an error. Max > The fourth and fifth > snapshots also see that cluster 1 is full, and allocate cluster 3 and 4; > whereas a more efficient usage would increase the refcount of cluster 2 > instead of allocating. >