search for: nbd_flag_init_zeroes

...rt) it does everything we need. However the original proposal you put here seems reasonable. I have only one comment about it: Should the new error (ENOTSUP) be submitted as a separate patch to the spec? > [1] https://lists.debian.org/nbd/2016/12/msg00015.html and following > (doc: Propose NBD_FLAG_INIT_ZEROES extension) > > > > > I will not push this without both: > > - a positive review (for example, we may decide that burning another > > NBD_FLAG_* is undesirable, and that we should instead have some sort > > of NBD_OPT_ handshake for determining when the server supp...

...umentation and a reference implementation of that optimization as well (as qemu-img convert can completely skip the zeroing, whether the bulk wipe or per-hole writing, when it knows the destination is already zero). [1] https://lists.debian.org/nbd/2016/12/msg00015.html and following (doc: Propose NBD_FLAG_INIT_ZEROES extension) > > I will not push this without both: > - a positive review (for example, we may decide that burning another > NBD_FLAG_* is undesirable, and that we should instead have some sort > of NBD_OPT_ handshake for determining when the server supports > NBD_CMF_FLAG_FAST_ZE...

While it may be counterintuitive at first, the introduction of NBD_CMD_WRITE_ZEROES and NBD_CMD_BLOCK_STATUS has caused a performance regression in qemu [1], when copying a sparse file. When the destination file must contain the same contents as the source, but it is not known in advance whether the destination started life with all zero content, then there are cases where it is faster to request