Subject: Re: iSCSI:Request/Response Ordering

From: "Edward A. Gardner" <[email-masked]>
Date: Sat, 29 Sep 2001 21:16:07 -0600

The simple answer is that an initiator may not make any assumptions about
the order of requests to the same blocks (by itself or other initiators)
that may be outstanding at the same time.  If you care about ordering, an
initiator must wait until previous requests are complete before issuing a
request that references the same block(s).

This assumes that all commands are issued as simple tasks, which is the most
common situation today (one suspects the only situation).

People have suggested more complex schemes in the past, amounting to
exporting some portion of the transfer dependency graph to the target.  The
ordered task attribute is one approach to this.  None have proved practical
in practice.

In practice, if a target receives references to the same block from multiple
initiators, it can perform the operations in whatever order it wishes.
There is no "correct" order, all are equally valid.  (Again, I'm assuming
all are issued as simple tasks).

Edward A. Gardner               [email-masked]
Ophidian Designs                719 593-8866 voice
1262 Hofstead Terrace           719 593-8989 fax
Colorado Springs, CO  80907     719 210-7200 cell
-----Original Message-----
From: Sanjeev Bhagat (TRIPACE/Zoetermeer) <[email-masked]>
To: 'IPS Reflector' <[email-masked]>; [email-masked] <[email-masked]>
Date: Saturday, September 29, 2001 7:03 PM
Subject: iSCSI:Request/Response Ordering

Hello All (T10, IPS),

The SAM-2 specifications makes no assumption about, or places any
requirement on the ordering of requests or responses between tasks or task
management functions received from different SCSI initiator ports.

In this scenario how can a SCSI target make correctly handle the read/write
requests made on same blocks by different intiators at the same time.

Sanjeev

---

From: "Sanjeev Bhagat \(TRIPACE/Zoetermeer\)" <[email-masked]>
Date: Sun, 30 Sep 2001 13:08:07 +0200

Edward,

Thanks for the answer, Yes i guess locking in combination with ordered
requests can help in solving this problem out.

Howevere i think that there should be some way in which write access to the
target can be reserved for some initiators. ( i dont know if that already
exists) or there should be some way of ordering of requests fos a target LU
in task router/ task manager

any comments?

sanjeev

----- Original Message -----
From: "Edward A. Gardner" <[email-masked]>
To: "Sanjeev Bhagat (TRIPACE/Zoetermeer)" <[email-masked]>;
"'IPS Reflector'" <[email-masked]>; <[email-masked]>
Sent: Sunday, September 30, 2001 5:16 AM
Subject: Re: iSCSI:Request/Response Ordering