On 24.09.2016 19:35, Alex Bligh wrote: >> On 24 Sep 2016, at 17:20, Vladimir Sementsov-Ogievskiy >> <[email protected]> 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. >
No, holes are critical. Concreate case: incremental backup to delta file. If we write zeroes instead of holes, we will lose underlying data (from previous incremental). -- Best regards, Vladimir ------------------------------------------------------------------------------ _______________________________________________ Nbd-general mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/nbd-general
