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

[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

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.

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).

Kevin