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Re: [Qemu-devel] [RFC] Proposed qcow2 extension: subcluster allocation

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [RFC] Proposed qcow2 extension: subcluster allocation
Date: Tue, 18 Apr 2017 20:27:33 +0300
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On 04/18/2017 02:52 PM, Alberto Garcia wrote:
> On Thu 13 Apr 2017 05:17:21 PM CEST, Denis V. Lunev wrote:
>> On 04/13/2017 06:04 PM, Alberto Garcia wrote:
>>> On Thu 13 Apr 2017 03:30:43 PM CEST, Denis V. Lunev wrote:
>>>> Yes, block size should be increased. I perfectly in agreement with
>>>> your.  But I think that we could do that by plain increase of the
>>>> cluster size without any further dances. Sub-clusters as sub-clusters
>>>> will help if we are able to avoid COW. With COW I do not see much
>>>> difference.
>>> I'm trying to summarize your position, tell me if I got everything
>>> correctly:
>>> 1. We should try to reduce data fragmentation on the qcow2 file,
>>>    because it will have a long term effect on the I/O performance (as
>>>    opposed to an effect on the initial operations on the empty image).
>> yes
>>> 2. The way to do that is to increase the cluster size (to 1MB or
>>>    more).
>> yes
>>> 3. Benefit: increasing the cluster size also decreases the amount of
>>>    metadata (L2 and refcount).
>> yes
>>> 4. Problem: L2 tables become too big and fill up the cache more
>>>    easily. To solve this the cache code should do partial reads
>>>    instead of complete L2 clusters.
>> yes. We can read full cluster as originally if L2 cache is empty.
>>> 5. Problem: larger cluster sizes also mean more data to copy when
>>>    there's a COW. To solve this the COW code should be modified so it
>>>    goes from 5 OPs (read head, write head, read tail, write tail,
>>>    write data) to 2 OPs (read cluster, write modified cluster).
>> yes, with small tweak if head and tail are in different clusters. In
>> this case we
>> will end up with 3 OPs.
>>> 6. Having subclusters adds incompatible changes to the file format,
>>>    and they offer no benefit after allocation.
>> yes
>>> 7. Subclusters are only really useful if they match the guest fs block
>>>    size (because you would avoid doing COW on allocation). Otherwise
>>>    the only thing that you get is a faster COW (because you move less
>>>    data), but the improvement is not dramatic and it's better if we do
>>>    what's proposed in point 5.
>> yes
>>> 8. Even if the subcluster size matches the guest block size, you'll
>>>    get very fast initial allocation but also more chances to end up
>>>    with a very fragmented qcow2 image, which is worse in the long run.
>> yes
>>> 9. Problem: larger clusters make a less efficient use of disk space,
>>>    but that's a drawback you're fine with considering all of the
>>>    above.
>> yes
>>> Is that a fair summary of what you're trying to say? Anything else
>>> missing?
>> yes.
>> 5a. Problem: initial cluster allocation without COW. Could be made
>>       cluster-size agnostic with the help of fallocate() call. Big
>> clusters are even
>>       better as the amount of such allocations is reduced.
>> Thank you very much for this cool summary! I am too tongue-tied.
> Hi Denis,
> I don't have the have data to verify all your claims here, but in
> general what you say makes sense.
> Although I'm not sure if I agree with everything (especially on whether
> any of this applies to SSD drives at all) it seems that we all agree
> that the COW algorithm can be improved, so perhaps I should start by
> taking a look at that.
> Regards,
> Berto
I understand. I just wanted to raise another possible (compatible)
approach, which could help.


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