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

Hi list,
on older virt-manager versions (ie: what shipped with RHEL 6), a 
checkbox called "allocate entire disk" was selectable when configuring
a
new virtual machine. When checked, it means that the RAW disk image file 
was entirely allocated, generally issuing a fallocate() call. When 
unchecked, the disk image was a sparse file, with on-demand space 
allocation.

On new virt-manager versions (ie: what ships with RHEL 7), the checkbox 
is gone. This means that for creating a sparse allocated file from 
within the "new vm" wizard, one is forced to use a Qcow2 file 
(selectable in the global preferences). No sparse RAM images can be 
created within such wizard.

As a heavy consumer of RAW disk files, I would really like to have the 
checkbox back, especially in RHEL/CentOS 7.x
Do you plan to reintroduce it? For RHEL/CentOS, should I open a Bugzilla 
ticket?
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 06/18/2018 04:05 PM, Gionatan Danti wrote:
> Hi list,
> on older virt-manager versions (ie: what shipped with RHEL 6), a
> checkbox called "allocate entire disk" was selectable when
configuring a
> new virtual machine. When checked, it means that the RAW disk image file
> was entirely allocated, generally issuing a fallocate() call. When
> unchecked, the disk image was a sparse file, with on-demand space
> allocation.
> 
> On new virt-manager versions (ie: what ships with RHEL 7), the checkbox
> is gone. This means that for creating a sparse allocated file from
> within the "new vm" wizard, one is forced to use a Qcow2 file
> (selectable in the global preferences). No sparse RAM images can be
> created within such wizard.
> 
> As a heavy consumer of RAW disk files, I would really like to have the
> checkbox back, especially in RHEL/CentOS 7.x
> Do you plan to reintroduce it? For RHEL/CentOS, should I open a Bugzilla
> ticket?
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

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.

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