[ovirt-devel] In-place sparsify support in VDSM

[Richard W.M. Jones]

On Tue, May 17, 2016 at 09:50:07PM +0300, Nir Soffer wrote:
> On Tue, May 17, 2016 at 7:14 PM, Shmuel Melamud <smelamud at redhat.com> wrote:
> 
> > Hi!
> >
> > There is an RFE being implemented currently (
> > https://bugzilla.redhat.com/show_bug.cgi?id=734120) to use --inplace
> > option in virt-sparsify to sparsify a disk image in-place, without copying
> > it.
> >
> > The problem is that in-place sparsify works on NFS only starting from NFS
> > 4.2, while the copying implementation supposedly works with any storage.
> >
> > From my point of view, it is better to remove the old code and start with
> > the new code that uses --inplace and just add to document that one will
> > need NFS >= 4.2 for sparsify feature to work. Although the old
> > implementation exists, it is not used currently so there are no actual
> > users that may be affected by the change. If it will be a crying need to
> > use sparsify on older NFS or some other incompatible storage, we can add it
> > later on the base of the new code. The old code is far from ideal and we
> > will need to rewrite it in any case, and there is not sense to do this
> > without real need.
> >
> 
> Richard,
> 
> Currently vdsm is using:
> 
>     virt-sparsify --tmp prebuilt:tmp_vol --format src_format --convert
> dst_format src_vol dst_vol

> (--format and --dst_format are optional)
> 
> tmp_vol, src_vol, and dst_vol can be either file on nfs/glusterfs/other
> posix shared filesystem,
> or an lv created on top of lun (iscsi/fc).
> 
> can you confirm this method works on all the storage types I mentioned? or
> this depends
> on the underlying storage server?

Copying mode requires that the dst_vol supports sparseness.  The most
common case where this would *not* be true is partitions on local hard
disks.  You can't make a partition and/or local hard disk sparse.

However all of the ones you've mentioned above support sparseness, so
you should be good.

*If* you were using --inplace only, you could nuke the --tmp option,
and indeed all the code associated with "prebuilt" qcow2 files for
scratch space.

That's because --inplace mode uses hardly any temporary storage, so
you can just have it use regular /var/tmp or $TMPDIR.

However ...
 
> The new inplace method is much nicer, but something that works only on
> latest NFS version
> is not useful for our most important users, using block storage.

... it's a shame, but yes.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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