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