Re: [Qemu-devel] Should 'qemu-img commit' sparsify the backing file?

[Kevin Wolf]

Am 05.11.2015 um 10:46 hat Fam Zheng geschrieben:
> On Thu, 11/05 08:56, Richard W.M. Jones wrote:
> > On Thu, Nov 05, 2015 at 03:14:49PM +0800, Fam Zheng wrote:
> > > I don't think this the purpose of "qemu-img commit". Committing
> > > "new" data in overlay.qcow2 has little to do with discarding backing
> > > image's fragments where there are only explict zeroes.
> > 
> > In that case, why didn't this step sparsify the backing file?
> 
> Commit should be (by default, at least) as fast as possible, so committing
> overlay.qcow2 shouldn't involve scanning the whole backing file.

But if we do need to commit some data to the backing file, we should try
to be efficient (i.e. use zero clusters in the backing file). I think
your mirror fixes achieve this.

This should actually be enough for virt-sparsify because IIUC it fills
the overlay with zero clusters in the right places before committing.

Kevin