Re: [Qemu-devel] write_zeroes/trim on the whole disk

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

On 24.09.2016 19:35, Alex Bligh wrote:
>> On 24 Sep 2016, at 17:20, Vladimir Sementsov-Ogievskiy <vsementsov@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.
>

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