Subject: iSCSI: SCSI Cmd PDU larger than 48 bytes

From: Sanjay Goyal <[email-masked]>
Date: Wed, 30 Jan 2002 09:33:04 -0500

what is the probability that the SCSI Cmd PDU will be more than 48
bytes? 

	the cases are:

	1. Bi-Di command

	2. CDB larger than 16 bytes

	Regards

	Sanjay G

---

From: "Martin, Nick" <[email-masked]>
Date: Wed, 30 Jan 2002 12:01:03 -0600

Sanjay,
 
This is a reasonable question.  Here is my opinion.
 
Firstly the occurrence of such commands in most iSCSI sessions is not a
random function with predictable distribution.
 
For a target, the likelihood of a bi-directional command or a CDB larger
than 16 bytes is near zero unless the target supports such commands.
Many of today's targets will support (understand and execute) neither
bi-directional nor large CDB commands.
 
For an initiator (HBA), the probability is zero until support is added
to upper layer (device class) software to be able to send such commands.
Further the OS's interface to the HBA must comprehend and permit such
requests to be passed from the device class drivers to the HBA drivers.
The HBA might not know whether device drivers will send such commands
until they do so.
 
Still these can not be effectively used unless the same customer has
both.  For a bridge device which might be translating between iSCSI and
Fibre Channel, again there must be support in both end points.
Otherwise the command will not be generated, or when generated and
delivered, will be rejected.  Hopefully after the first such rejected
command, additional similar commands will be very few and far between.
 
There are SCSI commands defined today which require long CDBs or
bi-directional data transfers and there are more commands being proposed
which would require one or both.  At least one major vendor has already
built devices which use them.  They may be widely used someday.  They
are not yet "main-stream" or "common place".
 
If a customer has a device which supports or depends on this
functionality and an initiator which can use them, iSCSI must be able to
deliver them.  In sessions with such a device, a high percentage of the
commands could fall into these categories. 
 
Thanks,
Nick

-----Original Message-----
From: Sanjay Goyal [mailto:[email-masked]]
Sent: Wednesday, January 30, 2002 8:33 AM
To: [email-masked]'
Cc: [email-masked]
Subject: iSCSI: SCSI Cmd PDU larger than 48 bytes

	what is the probability that the SCSI Cmd PDU will be more than
48 bytes? 

	the cases are:

	1. Bi-Di command

	Regards

	Sanjay G