iSCSI:Request/Response Ordering

Sanjeev Bhagat (TRIPACE/Zoetermeer) iscsi_t10 at
Sun Sep 30 04:08:07 PDT 2001

* From the T10 Reflector (t10 at, posted by:
* "Sanjeev Bhagat \(TRIPACE/Zoetermeer\)" <iscsi_t10 at>

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?


----- Original Message -----
From: "Edward A. Gardner" <eag at>
To: "Sanjeev Bhagat (TRIPACE/Zoetermeer)" <iscsi_t10 at>;
"'IPS Reflector'" <ips at>; <T10 at>
Sent: Sunday, September 30, 2001 5:16 AM
Subject: Re: iSCSI:Request/Response Ordering

> 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
> 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.
> ordered task attribute is one approach to this.  None have proved
> in practice.
> In practice, if a target receives references to the same block from
> 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               eag at
> 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) <iscsi_t10 at>
> To: 'IPS Reflector' <ips at>; T10 at <T10 at>
> 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
> requests made on same blocks by different intiators at the same time.
> Sanjeev

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