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

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

On 24.09.2016 19:44, Vladimir Sementsov-Ogievskiy wrote:
> 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).
>
hmm, no, sorry, that is not needed.

-- 
Best regards,
Vladimir