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Re: qemu-img measure

From: Stefan Hajnoczi
Subject: Re: qemu-img measure
Date: Fri, 24 Jul 2020 15:00:10 +0100

On Thu, Jul 23, 2020 at 07:31:13PM +0300, Nir Soffer wrote:
> On Thu, Jul 23, 2020 at 6:12 PM Arik Hadas <ahadas@redhat.com> wrote:
> > [root@lion01 8c98c94d-bc14-4e24-b89c-4d96c820056d]# qemu-img measure -O 
> > qcow2 /tmp/arik.qcow2
> > required size: 9359720448
> > fully allocated size: 16108814336
> Now we have qcow2 image, so we don't depend on the file system capabilities.
> This is the advantage of using advanced file format.
> > shouldn't the 'measure' command be a bit smarter than that? :)
> I think it cannot be smarter, but maybe qemu folks have a better answer.

qemu-img measure checks that allocation status of blocks. As Nir said,
if the file system (older NFS versions) doesn't support that then it
doesn't know about zero blocks.

It would be possible to add a slow loop that reads the entire input
image but I'm not sure how useful it would be to read many gigabytes of
data from the disk. The process would be slow and degrade performance of
VMs by consuming I/O bandwidth.


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