Re: iSCSI: Use of the A bit

["John Hufferd" <hufferd@us.ibm.com>]

I am not sure that was the problem.  The point was if they have such a
device that has such a shortage of memory that it can never complete the
I/O without running out of buffer.  It wants to free up the buffer so that
it can send some more data so that it can fulfill the Read request.

If the Initiator supports the A-bit SNACK (which is optional), then some
buffer control would be possible.  But as was pointed out by Matthew, with
ErrorRecoveryLevel=0, there is no need to even use the A-bit, since there
is no error recovery within command, you can just send the data, then
refill the buffer.  If there is an error, the session has to be torn down.

However, if you have that same type of device but support
ErrorRecoveryLevel=1, then SNACK with A-bit support seems like an strong
preference for a corresponding initiator.  If the device detects that the
A-bit is not supported at the initiator (because A-bit was set, but no
response was received at the approprate after a period of time), then it
probably should "promote" the recovery to level=0, and start refilling the
buffers as mentioned above.


.
.
.
John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/SSG San Jose Ca
Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
Home Office (408) 997-6136, Cell: (408) 499-9702
Internet address: hufferd@us.ibm.com


Santosh Rao <santoshr@hpcuhe.cup.hp.com>@ece.cmu.edu on 03/13/2002 07:25:21
PM

Sent by:    owner-ips@ece.cmu.edu


To:    ips@ece.cmu.edu (ips)
cc:
Subject:    Re: iSCSI: Use of the A bit



>
> Actually it does not actually say that.  It says "This is limited to
> sessions that support error recovery and is implemented through the A bit
> ..."
>
> I know what I am about to say, is nit picking, but ....
> ErrorRecoveryLevel=0 is an error recovery technique.
>
> Having said that, we probably should just change that statement to "This
is
> implemented through the A bit ..."

John,

Initiators that use "ErrorRecoveryLevel=0" must not be required to respond
to
an A bit setting with a DataACK. "ErrorRecoveryLevel=0" implies the
initiator
need not support/use SNACK pdu.

A target can recycle its data buffers once it has completed sending the
data,
since it knows that the initiator will not issue Data SNACK while operating
as
"ErrorRecoveryLevel=0".

- Santosh

--
##################################
Santosh Rao
Software Design Engineer,
HP-UX iSCSI Driver Team,
Hewlett Packard, Cupertino.
email : santoshr@cup.hp.com
Phone : 408-447-3751
##################################