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

[Peter Lieven <pl@kamp.de>]

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