Re: [PATCH v4 24/30] qcow2: Clear the L2 bitmap when allocating a compressed cluster

[Alberto Garcia]

On Fri 24 Apr 2020 07:44:33 PM CEST, Eric Blake <address@hidden> wrote:
>>> at the same time, I can see where you're coming from in stating that
>>> if it makes management of extended L2 easier to allow zero subclusters
>>> on top of a compressed cluster, then there's no reason to forbid it.
>> 
>> I'm not sure if it makes it easier. Some operations are definitely going
>> to be easier but maybe we have to add and handle _ZERO_COMPRESSED in
>> addition to _ZERO_PLAIN and _ZERO_ALLOC (the same for unallocated
>> subclusters). Or maybe replace QCow2SubclusterType with something
>> else. I need to evaluate that.
>
> Reading the entire cluster will be interesting - you'll have to
> decompress the entire memory, then overwrite the zeroed portions.

I don't think so, qcow2_get_host_offset() would detect the number of
contiguous subclusters of the same type at the given offset. In this
case they would be _ZERO subclusters so there's no need to decompress
anything, or even read it (it works the same with uncompressed
clusters).

Berto