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

[Daniel P. Berrangé]

On Thu, Jun 28, 2018 at 12:35:56PM +0200, Gionatan Danti wrote:
> 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);

There is always a performance distinction between raw and qcow2, but it is
much less these days with qcow2v3 than it was with the original qcow2
design. 

> - 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;

This is really tangential. virt-manager chose to use internal snapshots
because they were easy to support, but it could equally use external
snapshots. This shouldn't have a bearing on other choices - if the
internal snapshotting is unacceptable due to the guest pause, this
needs addressing regardless of allocation.

> - on CoW filesystems, using Qcow2 files will means *double* CoW with a)
> reduced performance and b) more wear on SSDs;

Using qcow2 doesn't require you to use cow at the disk image layer - it
simply gives you the ability, should you want to. So you don't get double
cow by default

> - 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;

Which widely used modern filesystems still don't support fallocate. It is
essentially a standard feature on any modern production quality filesystem 
these days.


Regards,
Daniel
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