Re: iSCSI:Request/Response Ordering

["Sanjeev Bhagat \(TRIPACE/Zoetermeer\)" <iscsi_t10@sanjeevbhagat.com>]

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" <eag@ophidian.com>
To: "Sanjeev Bhagat (TRIPACE/Zoetermeer)" <iscsi_t10@sanjeevbhagat.com>;
"'IPS Reflector'" <ips@ece.cmu.edu>; <T10@t10.org>
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
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               eag@ophidian.com
> 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@sanjeevbhagat.com>
> To: 'IPS Reflector' <ips@ece.cmu.edu>; T10@t10.org <T10@t10.org>
> 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
>
>
>
>