Re: ImageInfo oddities regarding compression

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Nov 27, 2020 at 05:52:09PM +0100, Markus Armbruster wrote:
> 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.

I'm not entirely sure about that, as compression algorithms have different
tradeoffs.

For migration compression we're streaming large volumes of data in a live
session, both compression and decompression speed is very important.

For qcow2 we're handling relative small discrete blocks (a qcow2
cluster at a time) and decompression speed is much more important that
compression speed, since we generally compress once out of band, and
decompress many times while live.

So we could see new compression methods desired for migration that are
not relevant for qcow2, or vica-verca.

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

Regards,
Daniel
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