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

[Richard W.M. Jones]

This doesn't work (I tested both qemu 2.3 and qemu from git).  Should it?

(1) Create a non-sparse guest image:

$ virt-builder fedora-22
$ mv fedora-22.img fedora-22.img.sparse
$ cp --sparse=never fedora-22.img.sparse fedora-22.img
$ du -sh fedora-22.img
6.1G fedora-22.img

(2) Add a snapshot on top:

$ qemu-img create -f qcow2 -o compat=1.1 -b fedora-22.img overlay.qcow2
$ du -sh fedora-22.img overlay.qcow2 
6.1G fedora-22.img
196K overlay.qcow2

(3) Sparsify the overlay:

$ virt-sparsify --in-place overlay.qcow2 
$ du -sh fedora-22.img overlay.qcow2 
6.1G fedora-22.img
3.2M overlay.qcow2

(4) Commit to the backing file:

$ qemu-img commit overlay.qcow2 
Image committed.
$ du -sh fedora-22.img overlay.qcow2 
6.1G fedora-22.img                         <------
260K overlay.qcow2

Notice that the backing file (fedora-22.img) doesn't get any smaller.

I'm expecting the backing file to shrink to around 800 MB, which is
does if you run virt-sparsify directly on the backing file.

For background, `virt-sparsify --in-place' mounts each filesystem
(using a small Linux/qemu appliance), and runs fstrim on each one.

Rich.

-- 
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