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

[Cole Robinson]

On 06/19/2018 07:06 PM, Gionatan Danti wrote:
> Il 19-06-2018 22:16 Cole Robinson ha scritto:
>> 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.
> 
> Sure, but the automatic disk creation is very handy and much less error
> prone.
> As it is now, if using a fallocate-less filesystem (eg: ZFS) and *not*
> selecting to create a custom disk, you risk waiting minutes or hours for
> libvirt to fully allocate the image by writing 0s to the disk file. This
> can wreck havok on SSDs and other eundurance-limited medium.
> 
>> 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
> 
> On CoW filesystems, sparse RAW files are faster then Qcow2 ones.
> Moreover, avoiding double CoW is important for SSDs (which have limited
> lifespan). Even on XFS, sparse RAW files should be faster in the long
> run than Qcow2 files, due to no weird limitation on L2 chunk cache size.
> 
> I found the checkbox quite self-explanatory and very handy. Any chances
> to reconsider your decision?

I see it as another test case and larger UI surface in the common path
for something that will save clicks for a corner case. I still don't see
it asworth exposing in the UI.

- Cole