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

[Cole Robinson]

On 06/19/2018 03:37 PM, Gionatan Danti wrote:
> Il 19-06-2018 20:14 Cole Robinson ha scritto:
>> If you change the disk image format from qcow2 to raw in
>> Edit->Preferences, then new disk images are set to fully allocated raw.
>> Check the image details with 'qemu-img info $filename' to confirm. So I
>> think by default we are doing what you want?
>>
>> - Cole
> 
> Er, the point is that I would really like to have a *sparse* RAW image
> file. On older virt-manager, unchecking "allocate entire disk" was what
> I normally used. Auto-allocating all disk space without a mean to avoid
> that has two main drawbacks:
> - you can't have sparse/thin volumes;
> - allocating on a fallocate-less filesystem is excruciatingly slow and
> cause of unneeded wear on SSDs.
> 
> Why using a sparse RAW image rather than a Qcow2 image? Basically:
> - RAW disks are easier to handle/inspect in case something goes wrong;
> - avoid double CoW on CoW-enabled filesystems (eg: ZFS, btrfs);
> - better performance (no Qcow2 L2 chunk cache range, etc).
> 
> It is worth nothing that oVirt (and RHEV) uses (sparse or allocated,
> based on user selection) base RAW files with eventual Qcow2 overlays for
> snapshots.
> 

Sorry, I misunderstood. You can still achieve what you want but it's
more clicks: new vm, manage storage, add volume, and select raw volume
with whatever capacity you want but with 0 allocation.

qcow2 is the default for virt-manager because it enables features like
snapshots out of the box. The main motivation I have largely heard for
wanting raw over qcow2 is performance, but then using sparse raw
actually makes raw less performant, so it's kind of a weird middle
ground. For that reason I don't think it warrants adding back the
checkbox to the new VM UI since I think it's a fairly obscure use case,
and it can be achieved through the 'manage storage' wizard albeit with
more clicks

- Cole