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Re: ImageInfo oddities regarding compression

From: Markus Armbruster
Subject: Re: ImageInfo oddities regarding compression
Date: Fri, 27 Nov 2020 17:52:09 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)

Kevin Wolf <kwolf@redhat.com> writes:

> Am 27.11.2020 um 13:21 hat Markus Armbruster geschrieben:
>> >> I fell down this (thankfully shallow) rabbit hole because we also have
>> >> 
>> >>     { 'enum': 'MultiFDCompression',
>> >>       'data': [ 'none', 'zlib',
>> >>                 { 'name': 'zstd', 'if': 'defined(CONFIG_ZSTD)' } ] }
>> >> 
>> >> I wonder whether we could merge them into a common type.
>> Looks like we could: current code would never report the additional
>> value 'none'.  Introspection would show it, though.  Seems unlikely to
>> cause trouble.  Observation, not demand.
> Forgot to comment on this one...
> Technically we could probably, but would it make sense? Support for
> compression formats has to be implemented separately for both cases, so
> that they currently happen to support the same list is more of a
> coincidence.
> If we ever add a third compression format to qcow2, would we add the
> same format to migration, too, or would we split the schema into two
> types again?

I figure if a compression method is worth adding to one, it's probably
worth adding to the other.

Having two separate enums isn't too bad.  A third has been proposed[*],
but I hope we can reuse migration's existing enum there.

[*] [PATCH 1/6] migration: Add multi-thread compress method

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