[libvirt-users] Reintroduce "allocate entire disk" checkbox on virt-manager

[Gionatan Danti]

Il 28-06-2018 12:44 Daniel P. Berrangé ha scritto:
> There is always a performance distinction between raw and qcow2, but it 
> is
> much less these days with qcow2v3 than it was with the original qcow2
> design.

Sure, but especially with random reads/writes over large LBA range the 
difference is noticeable [1]. Moreover, if something goes wrong, a RAW 
file can be inspected with standard block device tools. As a reference 
point, both oVirt and RHEL uses RAW files for base disk images.

It's not only performance related, but it regards thin-provision also. 
Why the wizard should automatically select fat provisioning based on 
image format? What if I want thin-provisioning using filesystem's sparse 
file support via RAW files?

> This is really tangential. virt-manager chose to use internal snapshots
> because they were easy to support, but it could equally use external
> snapshots. This shouldn't have a bearing on other choices - if the
> internal snapshotting is unacceptable due to the guest pause, this
> needs addressing regardless of allocation.

I agree, but currently the wizard force you to do a choice between:
a) sparse Qcow2 file, with (sometime dangerous?) internal snapshot 
support;
b) fully allocated RAW files, with *no* external snapshot support.
As you can see, it is virt-manager itself that entangles the choices 
regarding file format/allocation/snapshot support.
And external snapshot support in virt-manager would be *super* cool ;)

> Using qcow2 doesn't require you to use cow at the disk image layer - it
> simply gives you the ability, should you want to. So you don't get 
> double
> cow by default

I badly expressed the idea, sorry. Writing to a *snapshotted* Qcow2 file 
causes double CoW; on the other hand, writing to an un-snapshotted Qcow2 
file only causes double block allocation.

> Which widely used modern filesystems still don't support fallocate. It 
> is
> essentially a standard feature on any modern production quality 
> filesystem
> these days.

True, with an exception: ZFS. And it is a *big* exception. Moreover, why 
allocate all data by default when using RAW files? What about thin 
images?

What really strikes me is that the checkbox *was* here in previous 
virt-manager releases. Did it caused confusion or some other problems?

Thanks.

[1] https://www.linux-kvm.org/images/9/92/Qcow2-why-not.pdf

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Danti Gionatan
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