libvirt users - Jun 2018 - Re: Reintroduce "allocate entire disk" checkbox on virt-manager

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?
Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

Il 20-06-2018 01:06 Gionatan Danti ha scritto:
> 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?
> Thanks.
Hi, sorry for the bump... any feedback about that?
Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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

Il 26-06-2018 23:49 Cole Robinson ha scritto:
> 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
I can not force this decision, obviously. However, let me recap why I 
found it important to have the "allocate disk now" checkbox:

- RAW files, even sparse one, are faster than Qcow2 files in the long 
run (ie: when block allocation is >8 GB);
- Qcow2 snapshots have significant gotchas (ie: the guest is suspended 
during the snapshot), while using RAW files will at least prevent using 
virt-manager snapshot feature without thinking;
- on CoW filesystems, using Qcow2 files will means *double* CoW with a) 
reduced performance and b) more wear on SSDs;
- on filesystems not supporting fallocate, libvirtd reverts to "write 
zeroes to the entire file) which is both a) very slow and b) detrimental 
to SSDs life;
- most other virtualization platform (old virt-manager and current oVirt 
included) split the choice of file format from the allocation policy.

I 100% agree that, using the custom disk creation mask, what I ask it 
entirely possible with virt-manager today. However, it would be *very* 
handy to have the checkbox back in the VM wizard itself.

Would opening a BZ ticket at least reopen the possibility to reconsider 
that decision?
Thanks anyway.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8