Comment 14 for bug 1846427

In reply to Kevin's comment#13:

> I find Laszlo's case with a preallocated image particularly surprising
> because the behaviour isn't supposed to have changed at all for
> preallocated images, at least if the heuristics still detects them as
> such.

But isn't that "if" at the core of this problem? What happens if the
detection misfires? (This is not a loaded question, I'm not implying any
particular circumstances; I'm just surprised that heuristics could be
considered at all.)

> Once a preallocated image becomes almost fully allocated, it's
> expected that we won't detect it any more. So, Laszlo, do you know how
> much of your images was allocated? 'qemu-img check' prints the
> allocation statistics.

I don't have the images any longer, and since then, I've been running
qemu 4.0 (for my upstream QEMU binaries).

However, I can say some things (with both affected VMs being Fedora
installations):

- As noted earlier, the images were formatted for 100GB, with
  preallocation=metadata.

- I always install Fedora from Live ISOs (never starting with
  pre-installed images), and right after installation, "du" on the host
  side always reports 5-8 GB usage. Definitely never more than 10GB. So
  I'd say these images were very sparsely populated.

- I always use qcow2 images like this, in the domain XMLs:

  <driver name='qemu' type='qcow2' cache='writeback'
   error_policy='enospace' discard='unmap'/>

  and I always use virtio-scsi so that discard='unmap' actually have an
  effect.

- I occasionally run "fstrim" in the guest, and / or "virsh domfstrim"
  on the host. (And re-run "du" on the host side in every such case.)

- Right after installation (with the VM powered down), I might compress
  the image with "qemu-img convert -c"; but I don't believe I've done
  that too recently.

- The general idea on my end is that I'd like to limit guest disk usage
  by the *host* disk's free space, and not by an arbitrary pre-set disk
  image size. Hence 100GB stands for "infinity" (I might have used 1TB
  just as well), and error_policy='enospace' lets me act, should a guest
  actually run out of space, on the host disk. Finally, discard='unmap'
  prevents waste. I use "preallocation=metadata" because the initial
  size cost is negligible, but I perceive writes to be faster.

Hopefully this helps at least a tiny bit... Thanks!