Re: [Qemu-block] [Qemu-devel] [PATCH v2 0/2] Truncate the tail of the image file in qcow2 shrinking

[Pavel Butsykin]

On 22.09.2017 12:50, Daniel P. Berrange wrote:
> On Fri, Sep 22, 2017 at 12:39:24PM +0300, Pavel Butsykin wrote:
> > Now after shrinking the qcow2 image, at the end of the image file, there might
> > be a tail that probably will never be used. Although it will not bring any
> > tangible benefit, we can cut the tail if it is. Yes, it will not free up disk
> > space, but if the blocks were be allocated sequentially and the image is not
> > heavily fragmented then the virtual size of the image file will be commensurate
> > with the real size. It also doesn't look like a great plus.. Well, at least we
> > can discuss it.
>
> If the block backend has discard support enabled, can't we get the tail
> to be discarded rather than merely truncated ?


It has already been implemented. (see 
https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg04581.html)
Sorry, I just forgot to mention that this patch rebased on Max's block
branch (https://github.com/XanClic/qemu/commits/block). Actually the
truncation will always be done on the already discarded area. It can
be useful only if the block backend doesn't support discard or a file
system doesn't support sparse files.

> Regards,
> Daniel