On Tue, Apr 10, 2018 at 02:07:33PM +0000, Nir Soffer wrote: > This makes sense if the device is backed by a block device on oVirt side, > and the NBD support efficient zeroing. But in this case the device is backed > by an empty sparse file on NFS, and oVirt does not support yet efficient > zeroing, we just write zeros manually. > > I think should be handled on virt-v2v plugin side. When zeroing a file raw > image, > you can ignore zero requests after the highest write offset, since the > plugin > created a new image, and we know that the image is empty. > > When the destination is a block device we cannot avoid zeroing since a block > device may contain junk data (we usually get dirty empty images from our > local > xtremio server).
(Off topic for qemu-block but ...) We don't have enough information at our end to know about any of this. > > 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. > > > > We support zero with unlimited size without sending any payload to oVirt, > so > there is no reason to limit zero request by max_pwrite_zeros. This limit may > make sense when zero is emulated using pwrite. Yes, this seems wrong, but I'd want Eric to comment. > > > 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. > > > > So it seems that may_trim=1 is wrong, since trim cannot replace zero. Note that the current plugin ignores may_trim. It is not used at all, so it's not relevant to this problem. However this flag actually corresponds to the inverse of NBD_CMD_FLAG_NO_HOLE which is defined by the NBD spec as: bit 1, NBD_CMD_FLAG_NO_HOLE; valid during NBD_CMD_WRITE_ZEROES. SHOULD be set to 1 if the client wants to ensure that the server does not create a hole. The client MAY send NBD_CMD_FLAG_NO_HOLE even if NBD_FLAG_SEND_TRIM was not set in the transmission flags field. The server MUST support the use of this flag if it advertises NBD_FLAG_SEND_WRITE_ZEROES. * qemu-img convert uses NBD_CMD_WRITE_ZEROES and does NOT set this flag (hence in the plugin we see may_trim=1), and I believe that qemu-img is correct because it doesn't want to force preallocation. Rich. * https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top