shrink option for discard (for bad host-filesystems and -backup solutions)

[kolAflash]

When using discard=unmap for virtio or scsi devices with QCOW2 images, space discarded by the guest will be unmaped on the host, which is basically great!

This will turn the QCOW2 image into a sparse file which is efficient for most scenarios. But it may be that you need to avoid big sparse files on your host. For example because you need to use a backup solution which doesn't support sparse files well. Or maybe the QCOW2 image is on a filesystem mount which doesn't support sparse files at all.

For those scenarios an alternative option for the discard setting (discard=shrink) would be great, so that the QCOW2 file itself gets shrunken again.
I'm not sure about how the initial growing* of QCOW2 images is implemented and if there are maybe limitations. But I hope it may be possible do the inverse and actually shrink (not sparse) an QCOW2 image with internally discarded blocks.

I'm using Qemu-5.2.0 and Linux >= 5.3 (host and guest).

*If you use "qemu-img create -f qcow2 ..." withOUT the "preallocation" option.

[Thomas Huth (th-huth)]

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