Re: SCSI/ISCSI, hardware/software

[Bryan Henderson]

>There is some confusion in the SCSI world between SCSI as a transport 
>and SCSI as a commands set and software communication protocol, which 
>works above the transport. So, you can implement SCSI transport at any 
>software (eg iSCSI) or hardware (parallel SCSI, Fibre Channel, SATA, 
>etc.) way, but if the SCSI message passing protocol is used overall 
>system remains SCSI with all protocol obligations like task management. 

The above doesn't really resolve the confusion, since it uses some 
ambiguous terms and constructions.  I'm not sure what it's supposed to 
say, but let me try to state in the terminology of the SCSI standards what 
SCSI is:

SCSI is a family of separate specifications.  Some are specifications of 
transports, and others are specifications of command sets (a layer above 
the transports).  A SCSI device must implement a SCSI transport spec and a 
SCSI command set spec -- and also contain a piece that actually does the 
work (e.g. a disk drive), the details of which aren't specified by SCSI.

Examples of SCSI transport specification are (I'm paraphrasing the names) 
parallel SCSI, Fibre Channel, and ISCSI.  Examples of command sets are the 
disk device command set and the tape device command set.

>So, pure software SCSI solution is possible. BTW, there are pure 
>hardware iSCSI implementations as well.

I don't think it's even meaningful to talk about whether an implementation 
is hardware or software.  The "pure hardware" implementations contain 
megabytes of software, which was written in languages like C, contains 
operating systems like Linux, and can be transmitted across a network and 
updated easily.  The "pure software" implementation involve kilograms of 
hardware in every SCSI command -- CPUs, power supplies, etc.

Not only that, but the "all hardware" ISCSI initiators people talk about, 
which are PCI cards with Ethernet jacks, are not complete initiators.  The 
computer you plug the card into, on which you run Linux and some 
application programs, is the initiator.  The card is just the 
ISCSI-specific core of it.

There's really two distinctions people mean to make when talking about 
hardware vs software:

1) Is it preassembled?  Can you lift it out of box whole, or do you have 
to acquire some special software and some more generic parts separately 
and manage their combination?

2) Does it involve a general purpose computing system, particularly one 
that you share with some other computing, or a faster special purpose 
dedicated one?

In the context of a Linux SCSI discussion, I'd just talk about how much of 
the implementation is in or above the Linux kernel, and how much is below. 
 And then we can say that ISCSI-specific function (initiator or target) 
can be implemented 1) entirely above the Linux kernel; 2) entirely inside 
the Linux kernel; 3) entirely below the Linux kernel; or 4) a combination 
of these.

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