A SAS Outstanding Xfer_RDY Question.

Nixon, Bob Bob.Nixon at emulex.com
Mon Jun 30 16:12:07 PDT 2003

* From the T10 Reflector (t10 at t10.org), posted by:
* "Nixon, Bob" <Bob.Nixon at Emulex.Com>
Thanks, Rob, "misplaced" was exactly our concern...I didn't want to polish
this tu...rkey, I wanted to simply and reliably disable it. 

I appreciate your confirmation that a first burst is not very useful and add
that supporting all the possible pathological behavior in a multi initiator
environment, where it MAY change unpredictably, makes it exceedingly
expensive in code complexity.  That's why I was looking for a cheap reliable
way for us transport providers to be SURE it didn't apply.  The mode page
alone doesn't give any way to assure that since it is changeable by an
uncontrolled population of other initiators (and by the target itself, for
that matter).

It's possible I'm missing some important point...Is there already a way for
an initiator to opt out of first burst behavior independent of what other
initiators choose?  (Or am I misinterpreting that the first burst size is
writable by initiators?)

   - Bob

-----Original Message-----
From: Elliott, Robert (Server Storage) [mailto:elliott at hp.com]
Sent: Monday, June 30, 2003 3:47 PM
To: Nixon, Bob; t10 at t10.org
Subject: RE: A SAS Outstanding Xfer_RDY Question.

I think the mode page is more than enough for this misplaced feature.

First Burst Size is not useful for the low latency environment which SAS
is supposed to be.  SAS COMMAND frames are interlocked, requiring
waiting for an ACK before sending another frame.  There's not much
benefit for the target to send just an ACK after receiving a COMMAND
frame rather than send an ACK followed by an XFER_RDY frame.  It's not
like iSCSI, where the data can be pipelined right behind the command
frame because they are likely to be in transit over a very long
Rob Elliott, elliott at hp.com
Hewlett-Packard Industry Standard Server Storage Advanced Technology
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