Re: [Qemu-devel] Use of PreallocMode in block drivers

[Peter Krempa <pkrempa@redhat.com>]

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On Thu, May 09, 2019 at 10:26:46 +0200, Stefano Garzarella wrote:
> On Wed, May 08, 2019 at 01:44:27PM +0200, Markus Armbruster wrote:
> > Stefano Garzarella <sgarzare@redhat.com> writes:
> > 
> > > On Tue, May 07, 2019 at 08:34:51AM +0200, Markus Armbruster wrote:
> > >> Cc: Peter for a libvirt perspective.
> > >> 
> > >> Stefano Garzarella <sgarzare@redhat.com> writes:
> > >> 
> > >> > This patch adds the support of preallocation (off/full) for the RBD
> > >> > block driver.
> > >> > If available, we use rbd_writesame() to quickly fill the image when
> > >> > full preallocation is required.
> > >> >
> > >> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > >> > ---
> > >> >  block/rbd.c          | 149 ++++++++++++++++++++++++++++++++++++++-----
> > >> >  qapi/block-core.json |   4 +-
> > >> >  2 files changed, 136 insertions(+), 17 deletions(-)

[...]


> > >> >  ##
> > >> >  # @BlockdevVmdkSubformat:
> > >> 
> > >> The non-support of values 'metadata' and 'falloc' is not visible in
> > >> introspection, only in documentation.  No reason to block this patch, as
> > >> the other block drivers have the same introspection weakness (only
> > >> sheepdog and vdi bother to document).
> > >> 
> > >> Should we address the introspection weakness?  Only if there's a use for
> > >> the information, I think.
> > >
> > > If the management applications will use that information (or maybe also
> > > our help pages), could be useful to have an array of 'PreallocMode'
> > > supported per-driver.
> > 
> > Ideally, query-qmp-schema would show only the supported values.
> > 
> > Not hard to do, just tedious: we'd get a number of sub-enums in addition
> > to the full one, and we'd have to map from sub-enum to the full one.
> > 
> > QAPI language support for sub-enums would remove most of the tedium.
> > Not worthwhile unless the need for sub-enums is actually common.
> 
> I should study better the QMP and QAPI to understand how to implement
> the sub-enums.
> 
> If you agree, I'll put it as a background task, until somebody from
> management applications tell us his interest.

Sorry for the late response. Libvirt currently does not deal that much
with the preallocation settings. Preallocation isn't in current state
implemented at all for 'blockdev-create' and only the 'metadata' and
'falloc' modes are used in the storage driver via qemu-img.

We currently hardcode the knowledge for which formats actually support
it internally.

I'd say it's not criticall to expose this in the QMP schema but
obviously if we'll ever need to use it for a recent enough qemu it's
welcome to have a way to check.

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