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

[Thomas Huth]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
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2) If you don't have an account on gitlab.com and don't intend to get
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the state back to "New" or "Confirmed" within the next 60 days (other-
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the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes
anymore).

Thank you and sorry for the inconvenience.


** Tags added: feature-request

** Changed in: qemu
       Status: New => Incomplete

** Changed in: qemu
   Importance: Undecided => Wishlist

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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:
  Incomplete

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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