Re: [Qemu-block] QCOW2 support for LZO compression

[Denis V. Lunev]

On 06/26/2017 12:20 PM, Peter Lieven wrote:
>
> Am 26.06.2017 um 10:28 schrieb Kevin Wolf:
>> [ Cc: qemu-devel; don't post to qemu-block only! ]
>>
>> Am 26.06.2017 um 09:57 hat Peter Lieven geschrieben:
>>> Hi,
>>>
>>> I am currently working on optimizing speed for compressed QCOW2
>>> images. We use them for templates and would also like to use them for
>>> backups, but the latter is almost infeasible because using gzip for
>>> compression is horribly slow. I tried to experiment with different
>>> options to deflate, but in the end I think its better to use a
>>> different compression algorithm for cases where speed matters. As we
>>> already have probing for it in configure and as it is widely used I
>>> would like to use LZO for that purpose. I think it would be best to
>>> have a flag to indicate that compressed blocks use LZO compression,
>>> but I would need a little explaination which of the feature fields I
>>> have to use to prevent an older (incompatible) Qemu opening LZO
>>> compressed QCOW2 images.
>>>
>>> I also have already some numbers. I converted a fresh Debian 9 Install
>>> which has an uncomressed QCOW2 size of 1158 MB with qemu-img to a
>>> compressed QCOW2.  With GZIP compression the result is 356MB whereas
>>> the LZO version is 452MB. However, the current GZIP variant uses 35
>>> seconds for this operation where LZO only needs 4 seconds. I think is
>>> is a good trade in especially when its optional so the user can
>>> choose.
>>>
>>> What are your thoughts?
>> We had a related RFC patch by Den earlier this year, which never
>> received many comment and never got out of RFC:
>>
>> https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg04682.html
>
> I was not aware of that one. Thanks for pointing out.
>
>>
>> So he chose a different algorithm (zstd). When I asked, he posted a
>> comparison of algorithms (however a generic one and not measured in the
>> context of qemu) that suggests that LZO would be slightly faster, but
>> have a considerable worse compression ratio with the settings that were
>> benchmarked.
>
> My idea to choose LZO was that it is widely available and available in
> any distro you can think of. We already have probing for it in configure.
> My concern with ZSTD would be that it seems there are no packages
> available for most distros and that it seems to be multi-threaded. I
> don't
> know if this will cause any trouble?
>
We have had that compression working in multithreaded process.

>>
>> I think it's clear that if there is any serious interest in compression,
>> we'll want to support at least one more algorithm. What we still need to
>> evaluate is which one(s) to take, and whether a simple incompatible flag
>> in the header like in Den's patch is enough or whether we should add a
>> whole new header field for the compression algorithm (like we already
>> have for encryption).
>
> From my side there clearly is interest in optimizing the compression. Its
> even possible to speed up zlib by 3-4x times by choosing other parameters
> for deflate which unfortunately are not compatible with our inflate
> settings.
>
> I don't know if its worth creating a new header field. Even if we
> spent to bits
> in the end (one for LZO and one for ZSDT). I think this wouldn't hurt.
> However,
> there are likely to pop up new compression algorithms in the future and
> a header would be more flexible.
>
> I just don't want to make it too complicated and as you pointed out
> compression is
> not that interesting for most people - maybe due to its speed.
>
I think we need something generic but simple. I think that we should not
support compression with the different algorithm in the single file.

Speaking about compression, we do have different constraints for
different situation, f.e. backups are written once and rarely read while
generic compression in backing store is read frequently but never
read. Thus the exact algorithm should be selectable.

Den