Re: [PATCH] dm-raid.txt: document discard support

[Heinz Mauelshagen <heinzm@redhat.com>]

On 08/08/2015 06:27 PM, Martin K. Petersen wrote:
>>>>>> "Heinz" == heinzm  <heinzm@redhat.com> writes:
> Heinz,
>
>> +Discard Support
>> +---------------
>> +The implementation of discard support amoung hardware vendors varies. When
>> +a block is discarded, some storage devices will return zeros when it is
>> +read.  These devices set the 'discard_zeros_data' attribute.  Other devices
>> +will return random data.  Confusingly, some devices will set
>> +'discard_zeros_data', but will still not always return zeros when
>> +discarded blocks are read!
> This used to be correct but that's no longer the case. We only set
> discard_zeroes_data when we have hard guarantees. We no longer
> exclusively rely on what the device reports.

Martin,

so can we rely on discard_zeroes_data if reported 100% now?

Heinz

>
> discard_zeroes_data is only set if the device reports LBPME/LBPRZ and
> uses WRITE SAME 10 or 16 (SCSI) or if it reports DRAT/RZAT and is
> explicitly whitelisted (ATA).
>