Re: [Qemu-devel] [PATCH v2] qcow2: Add bdrv_discard support

[Jun Li]

Hi Kevin and Stefan,

  As the realize of bdrv_discard, and this patch has been received, but 
I have some question.

This adds a bdrv_discard function to qcow2 that frees the discarded clusters.
It does not yet pass the discard on to the underlying file system driver, but
the space can be reused by future writes to the image.

Above content is on url: 
https://lists.gnu.org/archive/html/qemu-devel/2011-01/msg03100.html

I have following question:

Why bdrv_discard does not yet pass the discard on to the underlying file 
system driver? And this exist a potential issue: host cluster leak for 
qcow2. As qcow2 image can be created on RAW disk,
so this is not just "does not yet pass the discard on to the underlying 
file system driver".  Could you give some explanation why do not realize 
this function? Thx.

As I am trying to realize qcow2 shrinking, but hit bdrv_discard can not 
release the free cluster on host.


Best Regards,
Jun Li