RE: sector alignment for DataOut PDUs?

["Buck Landry" <blandry@crossroads.com>]

>>> Also, I think we should consider making the "alignment" any
multiple of 512 the target wants
<<<

Well.. if we put no restrictions on it, and I decided my target should only handle one data-out pdu per scsi write cmd, I might abuse this and force an alignment of (512 * ReallyHugeNumber).  This might demand more resources or complexity from the initiator than is wished for, although I can't think of any reasons off the top of my head (perhaps I should think harder).  OTOH I am fully aware that the target is the entity that is supposed to be cheap n' easy to implement...

Perhaps you could limit the alignment size by the target's MaxRecvPDULength?  Or...  

I believe this "multiple-of" idea also needs to be reconciled with MaxBurstSize.  If MaxBurstSize is negotiated by the initiator to 512 bytes, then by my understanding the target still must send 512-byte R2Ts for writes (and hence, expect 512-byte data-out pdus), even if the target's sector size is 4k.

--buck 




-----Original Message-----
From: Rod Harrison [mailto:rod.harrison@windriver.com]
Sent: Tuesday, February 26, 2002 5:51 PM
To: Paul Koning
Cc: ips@ece.cmu.edu
Subject: RE: sector alignment for DataOut PDUs?



	I was talking about DATA-OUT PDUs.

	I would suggest we also restrict any target that wants to
negotiate for the 512 byte DATA-OUT option to not doing anything
weird with FirstBurstSize or MaxRecvPDULength. I think it would
unreasonable to allow a target to require 512 byte aligned
DATA-OUTs and have those other two parameters not multiples of
512 bytes.

	How do we handle the case where the iSCSI target node has
multiple LUNs and not all of them are disks? Perhaps the rule for
transfers lengths that are not multiples of 512 bytes works OK
here?

	Also, I think we should consider making the "alignment" any
multiple of 512 the target wants, not just 512. Otherwise it
seems to me you are back where you started if you have a disk
with 4096 byte sectors and the initiator chooses to send you
DATA-OUT PDUs that are not a multiple of 4k.

	- Rod

-----Original Message-----
From: Paul Koning [mailto:ni1d@arrl.net]
Sent: Tuesday, February 26, 2002 11:05 PM
To: rod.harrison@windriver.com
Cc: ips@ece.cmu.edu
Subject: RE: sector alignment for DataOut PDUs?


>>>>> "Rod" == Rod Harrison <rod.harrison@windriver.com> writes:

 Rod> I have no objection in principle to this but I have a
concern
 Rod> over non-media access commands. Things like mode select
 Rod> payloads, i.e. defect lists etc are not necessarily 512
byte
 Rod> multiples. The wording would need to allow "odd" sizes when
the
 Rod> SCSI transfer length is not a 512 byte multiple.

Are those DataOut or DataIn?  The target can arrange inbound PDU
sizes
to be whatever multiple is helpful if it wants to, that isn't the
case
I was talking about.

In any case, if there are cases where the total transfer length
for
data out isn't a multiple of 512, then the rule would have to
allow
the last transfer to have an odd length.  The earlier transfers
could
still be constrained if the constraint is enabled.

      paul