Re: [PATCH v4 07/30] qcow2: Document the Extended L2 Entries feature

[Alberto Garcia]

On Tue 14 Apr 2020 06:19:18 PM CEST, Vladimir Sementsov-Ogievskiy 
<address@hidden> wrote:
>>> It still may cache information about zeroed subclusters: gives more
>>> detailed block-status. But we should mention somehow external
>>> files. Hm. not only for raw external files, but it is documented that
>>> cluster can't be unallocated when an external data file is used.
>> 
>> What do you mean by "cluster can't be unallocated" ?
>
> I mean this sentence from qcow2.txt:
>
>                     "The offset may only be 0 with
>                      bit 63 set (indicating a host cluster offset of 0) when 
> an
>                      external data file is used."
>
> In other words, cluster can't be unallocated with data file in use.

I still don't follow... clusters can be unallocated, and when you create
a new image they are indeed unallocated.

Bit 63 (QCOW_OFLAG_COPIED) is what indicates if a cluster is allocated
or not, and you can unmap an allocated cluster with 'write -z -u'.

Berto