RE: Command Queue Depth (was asymmetric/Symmetric)

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

The issue of buffer space allocation for multiple initiators has a long and
troubled history in SCSI.  We have never been able to come up with a good
answer.  

Fibre Channel tried to fix this with the notion of "login BB credit" - when
you login you get a minimum number of credits you are always guaranteed when
you start data transfers.  The problem with this is that storage devices had
no realistic ability to discriminate between initiators or to change the
login BB credit.  In addition, the expectation is that all possible
initiators would get these credits on login.  So storage devices vendors
have played it safe and kept this number low (at 0 until recently, now
around 2).  For iSCSI the number of initial credits you need to "prime the
pump" until normal data flow is established is probably large (given the
latencies are higher than in Fibre Channel, especially FC-AL), and the
number of potential initiators larger than in Fibre Channel, making this a
whole lot worse for the storage device.

As soon as we allow the devices to start adjusting these credits, then you
have the protocol problem of making sure people know when their credits are
adjusted and the policy problem of how, who, and when to adjust the credits.
Changing everyone's credit when you add a new initiator can get into a
notification nightmare, although it is "fair."  Any policy brings up all
sorts of nasty issues regarding fairness vs efficient use of the
transmission media.

Jim

Note: the same problem has plagued other attempts to allocate device
resources between multiple initiators, like command queue space.  In general
policies with respect to multiple initiators are not really standard in the
SCSI world.


-----Original Message-----
From: Matt Wakeley [mailto:matt_wakeley@agilent.com]
Sent: Wednesday, September 06, 2000 2:56 PM
To: ips
Subject: Re: Command Queue Depth (was asymmetric/Symmetric)


Joshua Tseng wrote:

> James,
>
> I agree with others that there may be an issue with the
> command windowing mechanism in the existing iSCSI spec.  It is like
> "TCP in reverse", in that the target determines the size of the window,
and
> not the initiator as in TCP.  Rather, I believe that everything that this
> windowing mechanism is attempting to achieve can be more easily obtained
> by having the target communicate its buffer size to the initiator at
> iSCSI login.  It should be the role of the initiator to determine how
> many commands to put in flight simultaneously, given this input on
available
> buffer size from the target.

As more initiators connect to a target, it may need to scale back the amount
of
this buffering it has allocated to each previously logged in initiator (to
prevent rejecting new logins).

>
>
> As far as multiple initiators, could this not be resolved by the target
> refusing additional logins beyond the number of initiators it can safely
> support?  Not being a storage expert, this is my best guess/suggestion
> at how to do it.

I believe John already answered this...

-Matt