[Bug 1920211] [NEW] shrink option for discard (for bad host-filesystems and -backup solutions)

[kolAflash]

Public bug reported:

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.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1920211

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

Status in QEMU:
  New

Bug description:
  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.

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