On 11/26/2012 06:58 PM, Martin K. Petersen wrote:
"Rob" == Rob Evers<revers@xxxxxxxxxx> 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.
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