[Libguestfs] [Qemu-block] v2v: -o rhv-upload: Long time spent zeroing the disk

[Richard W.M. Jones]

On Tue, Apr 10, 2018 at 03:48:11PM +0200, Kevin Wolf wrote:
> Am 10.04.2018 um 15:03 hat Nir Soffer geschrieben:
> > On Tue, Apr 10, 2018 at 1:44 PM Richard W.M. Jones <rjones at redhat.com>
> > wrote:
> > 
> > > We now have true zeroing support in oVirt imageio, thanks for that.
> > >
> > > However a problem is that ‘qemu-img convert’ issues zero requests for
> > > the whole disk before starting the transfer.  It does this using 32 MB
> > > requests which take approx. 1 second each to execute on the oVirt side.
> > 
> > 
> > > Two problems therefore:
> > >
> > > (1) Zeroing the disk can take a long time (eg. 40 GB is approx.
> > >     20 minutes).  Furthermore there is no progress indication while this
> > >     is happening.
> > >
> > 
> > >     Nothing bad happens: because it is making frequent requests there
> > >     is no timeout.
> > >
> > > (2) I suspect that because we don't have trim support that this is
> > >     actually causing the disk to get fully allocated on the target.
> > >
> > >     The NBD requests are sent with may_trim=1 so we could turn these
> > >     into trim requests, but obviously cannot do that while there is no
> > >     trim support.
> > >
> > 
> > It sounds like nbdkit is emulating trim with zero instead of noop.
> > 
> > I'm not sure why qemu-img is trying to do, I hope the nbd maintainer on
> > qemu side can explain this.
> 
> qemu-img tries to efficiently zero out the whole device at once so that
> it doesn't have to use individual small write requests for unallocated
> parts of the image later on.
> 
> The problem is that the NBD block driver has max_pwrite_zeroes = 32 MB,
> so it's not that efficient after all. I'm not sure if there is a real
> reason for this, but Eric should know.
> 
> > However, since you suggest that we could use "trim" request for these
> > requests, it means that these requests are advisory (since trim is), and
> > we can just ignore them if the server does not support trim.
> 
> What qemu-img sends shouldn't be a NBD_CMD_TRIM request (which is indeed
> advisory), but a NBD_CMD_WRITE_ZEROES request. qemu-img relies on the
> image actually being zeroed after this.

Yup it's actually sending NBD_CMD_WRITE_ZEROES with the flag
NBD_CMD_FLAG_NO_HOLE clear (not set).

I think Eric needs to comment here ..

Rich.

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