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


From: Peter Lieven
Subject: Re: [Qemu-devel] QCOW2 support for LZO compression
Date: Mon, 26 Jun 2017 12:20:23 +0200
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Am 26.06.2017 um 12:12 schrieb Daniel P. Berrange:
On Mon, Jun 26, 2017 at 12:08:01PM +0200, Peter Lieven wrote:
Am 26.06.2017 um 11:57 schrieb Kevin Wolf:
Am 26.06.2017 um 11:20 hat Peter Lieven geschrieben:
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?
The availability and that we already link against LZO is a good point. I
think we want to avoid a situation where compressed qcow2 files can't be
read by binaries of popular distributions - after all, downloadable
images are an important use case for compressed images.
As long as the default remains gzip I don't see any issues. If you choose
a different algorithm, you should know what you are doing.
The problem comes if Debian were to choose to only link in ZSTD, and RHEL
were choose to only link LZO. Images distributed by one distro, with this
new compression would be unusuable on other distros. So whatever compression
format we choose to add should be something we are confident that all distros
will be happy enabling by default. This favours libraries are already widely
included in distros, especially if QEMU already links to them indirectly.

This is very strong argument for LZO. I checked with Debian 9 and Ubuntu 16.04
they have liblzo installed in the base system. I think maybe for btrfs 
compression support.

Peter




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