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. 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. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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