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

> 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.