Re: [PATCH 0/2] Change type-2 dif to use rq embedded 32 byte cdb

[Rob Evers <revers@redhat.com>]

On 11/26/2012 06:58 PM, Martin K. Petersen wrote:
>>>>>> "Rob" == Rob Evers<revers@redhat.com>  writes:
> Rob>  These patches replace the original t10 type 2 dif mempool
> Rob>  implementation by increasing the block and scsi cdb maximum sizes
> Rob>  from 16 to 32 bytes.  The cdb embedded in the request structure can
> Rob>  then be used for type 2 dif commands, or other 32 byte cdbs as
> Rob>  required.
>
> Rob>  Motivation for this is that type-2 dif commands should be treated
> Rob>  as any other read/writes generally, without any performance
> Rob>  penalty.
>
> We did the mempool because we did not want to penalize everybody else by
> always allocating 32-byte CDBs. Type 2 is a really rare corner case.

I didn't see a penalty in the non dif case, in the sanity checking I did.
If someone has results that indicate otherwise, please indicate.

> So while I agree we should handle the UA scenario correctly, I'm not
> sure I agree with the fix. Why are you messing with Type 2 devices in
> the first place? These should really only be used inside disk arrays.
>

The previous fix for the mempool race condition was found when
one of our partners reported a problem in a system that had
type 2 dif storage attached to the host.

Use of BLK_MAX_CDB and MAX_COMMAND_SIZE could
use some feedback.