Re: [PATCH v12 2/3] qcow2: Allow writing compressed data of multiple clusters

[Alberto Garcia]

On Thu 09 Apr 2020 08:39:12 PM CEST, Vladimir Sementsov-Ogievskiy wrote:
>> Because of this a test cannot expect that running the same commands on
>> an empty image produces always the same results.
>> 
>> Is this something that we should be concerned about?
>
> Parallel writing compressed clusters is significant improvement, as it
> allow compressing in really parallel threads.

I see, I just wasn't sure if you were aware of this side effect.

> So, I don't think we should make specific workaround for
> testing... What exactly is the case?

I noticed this while writing some tests for the subcluster allocation
feature, but this is not a problem for me. Many of our iotests make
assumptions about the location of L2 and refcount tables so changing
those would break a lot of them. This thing only changes the offset of
the compressed data clusters (and their L2 entries), but as far as I'm
aware no one relies on them being predictable. I just need to make sure
that I don't do it either.

Berto