RE: Enhancements for the iSCSI

[Jim McGrath <Jim.McGrath@quantum.com>]

Actually, in SCSI there are no such things as initiator devices and target
devices.  There are just devices, and they can play the role of a target or
initiator with respect to a given command (nexus).  Any device and be both a
target and an initiator - just not for the same command.

Third part copy is a good example.  Another is any subsystem that operates
as a target on one physical bus and an initiator on another physical bus
(like a disk array controller).  While the traditional separation into two
distinct (normally parallel) buses make the device a target or initiator
based on the bus, that quickly goes away when you start looking at other
physical buses (like Fibre Channel), where the two buses may be one.  A good
example here is RAID functions distributed to disk drive controllers (making
them both targets of the initial commands and initiators of multiple
commands to other drives on the same bus).  This became interesting with the
advent of Fibre Channel.

The reason why people think that a device must be one or the other is a
result of market conditions - traditionally there has not been a big market
for devices that perform both roles on the same bus.  But that has nothing
to do with SCSI, transports, and the like - but rather with the higher level
functions these devices perform.  So unless iSCSI changes what functions
people use the devices for, putting in all the support for target/initiator
capability in one device won't change the products people will make .

Although, like SCSI, I would argue that the architecture should support that
sort of dual role (note that other protocols do not - like ATA).

Jim


-----Original Message-----
From: Nelson Nahum [mailto:nnahum@store-age.com]
Sent: Friday, September 08, 2000 3:21 PM
To: ips@ece.cmu.edu
Subject: Enhancements for the iSCSI


I think that the iSCSI is a special opportunity to enhance the current SCSI
protocol especially it networking capabilities.
SCSI was designed with the concept of Initiator for the HBA and Target for
the devices. The Initator sends commands to the Targets that are passive.
But the networking world supposes every one talking with every one. The fact
that iSCSI is implemented over TCP/IP can help to develop these enhacements.

Modern SAN appliances need to "talk" with the storage devices but also need
a way to update information in the HBAs in a asynchronous manner.

My questions are the following:

1 - Is iSCSI designed in such a way that every Target can also be Initiator
and viceversa or it supposes different charasterisitics (memory, CPU power
etc.) between the Initiator and the Target?

2 - Is possible to add asynchronous events that the Target could send to the
Initiators at any time?

3 - Another important enhancement could be the possibility to "broadcast"
information to many iSCSI devices or HBAs.

Nelson Nahum
CTO
StoreAge Networking Technologies