On Tue, Apr 10, 2018 at 5:50 PM Richard W.M. Jones <rjo...@redhat.com> wrote:
> 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. > Can't use use this logic in the oVirt plugin? file based storage -> skip initial zeroing block based storage -> use initial zeroing Do you think that publishing disk capabilities in the sdk will solve 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. > So once oVirt will support efficient zeroing, this flag may be translated to (for file based storage): may_trim=1 -> fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE) may_trim=0 -> fallocate(FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE) We planed to choose this by default on oVirt side, based on disk type. For preallocated disk we never want to use FALLOC_FL_PUNCH_HOLE, and for sparse disk we always want to use FALLOC_FL_PUNCH_HOLE unless it is not supported. Seems that we need to add a "trim" or "punch_hole" flag to the PATCH/zero request, so you can hint oVirt how do you want to zero. oVirt will choose what to do based on storage type (file/block), user request(trim/notrim), and disk type (thin/preallocated). I think we can start the use this flag when we publish the "trim" feature. Nir