With the older (pre 4.3) libvirt in place this creates it with metadata
preallocation.
/usr/bin/qemu-img create -f qcow2 -o
preallocation=metadata,compat=1.1,lazy_refcounts
/var/lib/libvirt/images/ubuntu18.04.qcow2 15728640K
In the related code this looks like:
834 if (info.size_arg > info.allocation)
835 virBufferAddLit(&buf, "preallocation=metadata,");
836 else
837 virBufferAddLit(&buf, "preallocation=falloc,");
And size_arg == capacity (from xml).
Per definition [1] we have three cases for "allocation":
a) allocation can be bigger than capacity for high-metadata overhead cases
b) If allocation is omitted when creating a volume, the volume will be fully
allocated at time of
creation.
c) If set to a value smaller than the capacity, the pool has the option of
deciding to sparsely
allocate a volume.
But as we see in the XMLs above virt-manager asks to fully allocate. We hit
neither of (a)/(b).
Therefore the pool has -NOT- the option to do a sparse allocation as we also do
not hit (c)
It isn't written/defined explicitly in those statements, but our case of
capacity==allocation should (form the text) behave like (b) at least in
my reading. It didn't do so in the past and the libvirt change [2] fixed
that unclarity.
I don't think we'd want to change libvirt to go back to not allocate in this
case.
But we should report and ask at virt-manager if it should/want to submit a
smaller allocation 0<=allocation<=capacity to get back to the old behavior.
virt-manager upstream then has a chance to agree and fix it (which we
would backport) or discuss pointing to a different reading of [1] that
would end up in a libvirt discussion and fix then.
[1]: https://libvirt.org/formatstorage.html#StorageVol
[2]:
https://libvirt.org/git/?p=libvirt.git;a=commit;h=c9ec7088c7a3f4cd26bb471f1f243931fff6f4f9
** Changed in: virt-manager (Ubuntu)
Importance: Undecided => Medium
** Changed in: virt-manager (Ubuntu Disco)
Importance: Undecided => Medium
** Changed in: libvirt (Ubuntu Disco)
Importance: Undecided => Low
** Changed in: libvirt (Ubuntu)
Importance: Undecided => Low
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https://bugs.launchpad.net/bugs/1847105
Title:
very slow disk creation, snapshotting
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