> On 6 Dec 2016, at 09:25, Kevin Wolf <kw...@redhat.com> wrote: > > Am 06.12.2016 um 00:42 hat Eric Blake geschrieben: >> While not directly related to NBD_CMD_WRITE_ZEROES, the qemu >> team discovered that it is useful if a server can advertise >> whether an export is in a known-all-zeroes state at the time >> the client connects. > > Does a server usually have the information to set this flag, other than > querying the block status of all blocks at startup?
The server may have other ways of knowing this, for instance that it has just created the file (*), or that it stat'd the file before opening it (not unlikely) and noticed it had 0 allocated size. The latter I suspect would be trivial to implement in nbd-server (*) = e.g. I had one application where nbd use the export path to signify it wanted to open a temporary file, the path consisting of a UUID and an encoded length. If the file was not present already it created it with ftruncate(). That could trivially have used this. > If so, the client could just query this by itself. Well there's no currently mainlined extension to do that, but yes it could. On the other hand I see no issue passing complete zero status back to the client if it's so obvious from a stat(). -- Alex Bligh
