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 <rjo...@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 Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org _______________________________________________ Libguestfs mailing list Libguestfs@redhat.com https://www.redhat.com/mailman/listinfo/libguestfs