Re: [Qemu-devel] [PATCH] qemu-img: Allow unmap backing image for zeroed clusters

[Kevin Wolf]

Am 27.09.2016 um 04:20 hat Fam Zheng geschrieben:
> We already specified BDRV_O_UNMAP when opening images in 'qemu-img
> commit', but didn't turn on the "unmap" in the active commit job. This
> patch fixes that so that zeroed clusters in top image can be discarded
> which is desired in the virt-sparsify use case, where a temporary
> overlay is created and fstrim'ed before commiting back, to free space in
> the original image.
> 
> The block-commit keeps the existing behavior.
> 
> Signed-off-by: Fam Zheng <address@hidden>

Is there a good reason for not using discard in an active commit if the
image was opened with BDRV_O_UNMAP? That is, wouldn't it make sense to
just set the unmap option for the mirror unconditionally and also for
block-commit?

If there is a reason, we should probably add an option to block-commit,
but I still feels that enabling it should be the default then.

Kevin