Re: iSCSI & Linked Commands

[julian_satran@il.ibm.com]

According to your logic no FCP implementation can use linked commands?

Is this true for all OS's?  Is it a verified fact or foloklor?   Is it so
also for the new MS StorPort driver?

JUlo

Santosh Rao <santoshr@cup.hp.com> on 18/04/2001 20:07:39

Please respond to Santosh Rao <santoshr@cup.hp.com>

To:   Julian Satran/Haifa/IBM@IBMIL
cc:   santoshr@cup.hp.com
Subject:  Re: iSCSI & Linked Commands




julian_satran@il.ibm.com wrote:
>
> Santosh,
>
> iSCSI HBAs are being designed now and they will get a way to convey the
> tags.

Julian,

Perhaps, I should try to make a better effort to come across more
clearly :-) The linked commands require task tags to be generated by the
SCSI ULP (which is an O.S. component, out of the control of HBA vendors
usually). Most O.S. SCSI ULPs do not generate task tags (or OX_IDs) but
leave this responsibility to the LLPs.

Hence, iSCSI HBAs being designed now makes no difference to this
picture. The O.S. ULP implementations don't need to be re-written for
iSCSI (one would hope!) and therefore, O.S. ULP architectures [that
exist today] prevent the usage of linked commands. (Also, the common
feeling is that since linked commands are not used, why change ULPs to
do otherwise.)


> The question I asked myself when introducing a restriction was always not
> why not restricting but rather why restricting.

Targets need a relaible guarantee that once an initiator issues an Abort
Task for a task/command, it will receive no further PDUs for that task
upon completion of the Abort Task.

In the absence of a mandate that forces initiators to comply to the
above, targets cannot reliably release & re-use their I/O resource since
they could get stale PDUs on that task later.

That is the reason I am requesting that initiators be mandated to apply
connection allegiance to their abort task.

> As for abort as linked command can
> exist only one at a time send the abort task wherever the current command
> is and don't initiate the next. Did I miss something?

I agree that linked commands treatment would be no different than normal
commands. IOW, connection allegiance per command should suffice, as long
as the abort task is included within its purview.


> The tags will be needed for recovery as well (I know that you think that
> isn't necessary either!).

On the contrary, I do believe that task tags need to be sent in the
Abort Task and this should not be an issue. The LLP is aware of the I/O
that timed out, its task tag and it generates the Abort Task PDU and
populates the task tag. This is standard practise and nothing different
here for iSCSI.

Regards,
Santosh
 - santoshr.vcf