> On 24 Sep 2016, at 17:20, Vladimir Sementsov-Ogievskiy > <vsement...@virtuozzo.com> wrote: > > Also, accordingly to documentation, NBD_CMD_TRIM is not appropriate for disk > clearing: > > * `NBD_CMD_TRIM` (4) > > A hint to the server that the data defined by len and offset is no > longer needed. A server MAY discard len bytes starting at offset, but > is not required to. > > After issuing this command, a client MUST NOT make any assumptions > about the contents of the export affected by this command, until > overwriting it again with `NBD_CMD_WRITE`. > > - it may do nothing.. So, what to do with this? add flag FORCE_TRIM for this > command? Or add FORCE_HOLES flag to WRITE_ZEROES?
You cannot force a hole, because NBD the is not guaranteed to support holes. Use NBD_CMD_WRITE_ZEROES without NBD_CMD_FLAG_NO_HOLE and you can pretty much assume that a server that supports holes will write holes. A server that does not support holes will write zeroes. If you don't care whether the resultant data is zero, just use NBD_CMD_TRIM. But as you do care (see above) you must be prepared for a 'thick' write of zeroes on servers that don't support it. -- Alex Bligh
