volume: allow extend cow preallocated

[aesteve-rh]

Currently we assume in the code that cow
volumes do not need to be extended in any case.
However, all preallocated volumes, both
cow and raw, should be extended when requested.
Otherwise, cow preallocated volumes have bigger
capacity than their truesize. Thus, when the size
of the disk reaches the truesize, the VM will
pause, which is an undesired effect.

To avoid this, we relax this assumption so that
only cow sparse volumes are skipped on extend calls.

Bug-Url: https://bugzilla.redhat.com/2170689
Signed-off-by: Albert Esteve aesteve@redhat.com

[aesteve-rh]

/ost

[ahadas]

lgtm but I'd also rename _extendSizeRaw then

[aesteve-rh]

Verified in my local setup doing:

1. Create a new 10GiB Preallocated disk with Enable Incremental Backup
2. Virtual Size and Actual Size are 10GiB
3. Edit disk and extend by 5GiB
4. Results:
   a) Without the fix: Actual Size is 15GiB, but Virtual Size remains at 10GiB
   b) With the fix: both Actual Size and Virtual Size are expanded to 15GiB

[mz-pdm]

Much better now, thanks!

[aesteve-rh]

/ost

[nirs]

Verified in my local setup doing:

1. Create a new 10GiB Preallocated disk with Enable Incremental Backup
2. Virtual Size and Actual Size are 10GiB
3. Edit disk and extend by 5GiB
4. Results:
   a) Without the fix: Actual Size is 15GiB, but Virtual Size remains at 10GiB

This is wrong - extending cow disk must increase the virtual size, but
the actual size does not have to change, since cow volume grow as needed.
We can argue that when using preallocated volume extending must also
preallocated the disk size, looks like the existing ocde already does this.

b) With the fix: both Actual Size and Virtual Size are expanded to 15GiB

Sound correct.

But I don't see how this issue can cause a VM to pause. If the virtual size did not
change (a), the vm does not see the new virtual size so it wil not try to write after
the end of the actual lv, and will not pause.

[ahadas]

@nirs the part you comnented on was supposed to be: Without the fix: Virtual Size is 15GiB, but Actual Size remains at 10GiB

[nirs]

Change looks safe, but it cannot fix an issue of vm pausing, since this cannot be caused by smaller virtual size.

The fix also does not affect the code changing the virtual size of
cow volume, it only affects the code changing the actual lv.

[nirs]

@nirs the part you comnented on was supposed to be: Without the fix: Virtual Size is 15GiB, but Actual Size remains at 10GiB

But this is fine, the disk will be extended as needed while writing.

[nirs]

Looking at the code path changed, this affects HSM.extendVolumeSize, which extend the
amount of space allocated (e.g. file size or lv size). This virtual size is changed
in HSM.updateVolumeSize.

Bad API on vdsm side, instead of one API that does the right thing we have 2 APIs
that engine must call in some cases, but too late to fix this now.

So the fix is just a nice to have allocation fix, keep preallocated volumes
preallocated after user extend the size on the engine side.

If this fix an issue of pausing vms, it means thin provisioning code is broken
and should be fixed. This change can hide the thin proviioning issue by preallocating,
but if the thin provioning is broken, the vm will still pause when the disk will
become full, becuase qcow2 metadata need extra space.

Testing extension of preallocated volumes without this fix will tell if more work is
needed for thin volumes.

[ahadas]

So the fix ... keep preallocated volumes preallocated after user extend the size on the engine side.

right, without this fix our tools show customers that something is wrong in their storage domains after extending preallocated qcow disks on block storage, as they expect virtual size == actual size for preallocated disks

[nirs]

OK, so the issue is not pausing vms, but incorrect allocation reported by the tools.