RE: iSCSI & Linked Commands

[Charles Monia <cmonia@NishanSystems.com>]

Hi:

> Mark has already replied to your question on why linked commands won't
> work given today's implementations. In most cases, the initiator task
> tag is either generated in the HBA driver or in the HBA firmware. 

The fact that some adapter implementations on some interconnects may not
support linked commands does not grant a license to delete the capability
from the protocol specification.

Incidentally, in my opinion the problem of linked command support has more
to do with whether or not the adapter has enough intelligence to handle the
linked command semantics and retain its internal task state following the
receipt of status.  Making the tag accessible in some way is required if for
no other reason that to support the ABORT TASK function  

Charles

> -----Original Message-----
> From: Santosh Rao [mailto:santoshr@cup.hp.com]
> Sent: Tuesday, April 17, 2001 11:19 AM
> To: ips@ece.cmu.edu
> Subject: Re: iSCSI & Linked Commands
> 
> 
> Julian,
> 
> Mark has already replied to your question on why linked commands won't
> work given today's implementations. In most cases, the initiator task
> tag is either generated in the HBA driver or in the HBA firmware. 
> 
> Since there is no task tag generated from the SCSI ULP while 
> handing an
> I/O request to the SCSI LLP, making the association across different
> commands within the same task (linked command) becomes impossible.
> 
> Is there any reason why iSCSI would want to apply connection 
> allegiance
> per command and not per task (?). If someone did use linked 
> commands and
> wanted a clean way of aborting the task on some error, an abort task
> sent down the same connection as the rest of the task would ensure
> flushing of stale PDUs. 
> 
> I agree that there is no need to say linked commands are unsupported
> since this is a ULP feature and support or lack thereof is decided at
> the initiator and target ULPs. 
> 
> - Santosh
> 
> 
> julian_satran@il.ibm.com wrote:
> > 
> > Mark,
> > 
> > The argument about the HBAs generating tags is pretty weak 
> as iSCSI will
> > have it's own HBA's and iSCSI will generate the tags in any 
> implementation.
> > As for the utility - the sequential and conditional 
> execution of the linked
> > commands is guaranteed regardless of delivery or queuing 
> order.  The only
> > reason they might get obsolete is their inability to hide 
> latency but I
> > don't see any compelling reason to have them unsupported by iSCSI.
> > 
> > Julo
> > 
> > Mark Mokryn <mark@sangate.com> on 17/04/2001 12:55:05
> > 
> > Please respond to Mark Mokryn <mark@sangate.com>
> > 
> > To:   Julian Satran/Haifa/IBM@IBMIL
> > cc:   ips@ece.cmu.edu
> > Subject:  Re: iSCSI & Linked Commands
> > 
> > Julian,
> > 
> > Santosh is right. Linked commands require an identical 
> I_T_L_x nexus,
> > but many Fibre Channel (and possibly) SCSI adapters 
> generate a queue tag
> > on-board, with no possibility of host software control. On such
> > adapters, the generation of linked commands is impossible, 
> and clearly
> > today's SCSI layers are aware of this.
> > 
> > This raises the entire issue of task management in iSCSI: Linked
> > commands are dated back to SCSI-2, where they indeed served 
> a purpose.
> > In the SCSI bus protocol, the target controlled all SCSI bus phases
> > (following selection). Thus, in a linked command sequence, 
> the target
> > may drive the command phase immediately following the 
> status phase, thus
> > saving bus cycles (i.e. arbitration, selection, etc.). 
> However, in the
> > serial protocols, I don't see how linked commands are of 
> any use, since
> > there are no bus phases to save. In contrast with popular 
> belief, linked
> > commands offer no atomicity. Even in SCSI bus protocol, a 
> linked command
> > may be disconnected at any time (at the target's 
> discretion), and a new
> > command (from any initiator) may be started. Linked 
> commands have always
> > been optional, and indeed many target implementations today do not
> > support them. For instance, looking at the Shark SCSI 
> reference manual,
> > according to the inquiry data, Shark does not support 
> linked commands.
> > 
> > So, perhaps the wise thing to do is to not support linked 
> commands in
> > iSCSI. It has always been an optional feature for logical units, and
> > today is outdated and often unsupported, both by targets 
> and initiators.
> > 
> > -mark
> > 
> > julian_satran@il.ibm.com wrote:
> > >
> > > Santosh,
> > >
> > > Sorry to interrupt this captivating thread. Why do you 
> think linked
> > > commands won't work?
> > >
> > > Julo
> > >
> > > Santosh Rao <santoshr@cup.hp.com> on 16/04/2001 20:13:04
> > >
> > > Please respond to Santosh Rao <santoshr@cup.hp.com>
> > >
> > > To:   Douglas Otis <dotis@sanlight.net>
> > > cc:   Ips <ips@ece.cmu.edu>
> > > Subject:  Re: iSCSI:flow control, acknowledgement, and a 
> deterministic
> > >       recovery
> > >
> > > Doug,
> > >
> > > You seem to be referring to linked commands as a case wherein the
> > > approach of Abort Task will not flush stale PDUs.
> > >
> > > Linked Commands cannot work the way SCSI implementations 
> are defined
> > > today, since linked commands require the initiator task 
> tag (I_T_L_x
> > > nexus identifier in SAM-2 Execute Command terminology) to 
> be generated
> > > by the SCSI ULP. However, in practice, the Initiator Task 
> Tag (or the FC
> > > OX_ID) is typically generated in the SCSI LLP (or in some 
> cases in the
> > > adapter firmware). IOW, there is no common reference 
> handle like the
> > > task tag sent down from the ULP that allows for 
> association of multiple
> > > commands to a task in several/most implementations today.
> > >
> > > When this is fixed up to get linked commands to work [& 
> there exist
> > > examples of its usage], there is no reason connection 
> allegiance could
> > > not be applied to all the commands within the task.
> > >
> > > I fail to see why you think Abort Task will not work with 
> sequential
> > > devices (?).
> > >
> > > - Santosh
> > >
> > > Douglas Otis wrote:
> > > >
> > > > Santosh,
> > > >
> > > > I see a few problems with this approach.  Tasks as 
> defined in iSCSI do
> > > not
> > > > maintain connection allegiance.  The driver binds all 
> SCSI commands to
> > > their
> > > > connection for the most resent association.  Although 
> there are several
> > > > places within the iSCSI proposal that make reference to 
> a task having a
> > > > connection allegiance, this is in error.  Commands and 
> not tasks carry
> > > such
> > > > allegiance.  Your recovery scheme will not allow a satisfactory
> > recovery
> > > > with a sequential device.  In this case, repeating the 
> command is not a
> > > > solution.  As a result, one connection falter and it 
> will become a
> > > difficult
> > > > situation.  In addition, you have no clue from iSCSI 
> your delivery
> > > status.
> > > > You do not know if you are waiting for the target or if 
> you are waiting
> > > for
> > > > the connection.  Some sequential devices have rather 
> long time-outs
> > with
> > > > these complications of deducing status created by the multiple
> > > connections.
> > > >
> > > > The application will not know about these connection allegiance
> > problems.
> > > > The iSCSI layer does not define interaction to provide 
> additional
> > > > application status to allow these applications to 
> respond in a manner
> > > that
> > > > may aid this situation nor should such additional information be
> > > required.
> > > > With your scheme the SCSI driver must examine the 
> content of these
> > > commands
> > > > to make a guess as to the connection allegiance 
> assignments.  Now the
> > > driver
> > > > is expected to understand what the intended action is 
> of this SCSI
> > > > management command.  What signal is used to indicate a 
> need for the
> > iSCSI
> > > > immediate treatment?  The only obvious seems to be the 
> task attribute
> > > > argument.  With the way iSCSI has defined iSCSI 
> immediate, I would
> > expect
> > > > those commands to be treated in a LIFO rather than the 
> normal FIFO
> > > fashion.
> > > >
> > > > Doug
> > > >
> > > > > Douglas Otis wrote:
> > > > > >
> > > > > > With multiple connections, if you are not going to 
> use a valid
> > > > > > CmdSN, or in your case a null CmdSN for all 
> commands, then there
> > > > > > would be a need to include a timestamp to meet a 
> timely delivery
> > > > > > requirement in the same manner as used in FC 
> encapsulation.  IP
> > > > > > can deliver over any time period.  A command could 
> arrive at any
> > > > > > time with respect to other connections.  With all 
> of your feedback
> > > > > > now from just the SCSI layer, the SCSI layer is 
> likely to have
> > timed
> > > > > > out and restarted and now stray commands finally 
> make an appearance
> > > > > > (the technician re-inserted the cable).  What did 
> that do?  Yes,
> > > > > > if this were on a single connection, then TCP could 
> provide some
> > > > > > assurances, (ignoring digests errors) but you must 
> not make that
> > > > > > assumption nor can you assume all disruptions are symmetric.
> > > > >
> > > > > Doug,
> > > > >
> > > > > The below snippet from my last mail answered your 
> above concern. The
> > > > > Abort Task is sent on the same connection as the 
> command. (connection
> > > > > allegiance applied to the abort task as well). The 
> Abort task pushes
> > > the
> > > > > stale data PDUs. There is no need for a timestamp on 
> iSCSI PDUs.
> > > > >
> > > > > > > As for your second concern regarding I/O 
> timeouts, there is
> > > > > no need for
> > > > > > > any timestamp. An I/O timeout is dealt with by an 
> Abort Task.
> > > > > The abort
> > > > > > > task response guarantees that the abort reached 
> the target and
> > > pushed
> > > > > > > all intermediate stale frames. Failure to 
> complete Abort Task
> > leads
> > > to
> > > > > > > higher level error recovery (ex : Logout, or some 
> higher form of
> > > task
> > > > > > > mgmt).
> > > > >
> > > > > - Santosh
> > >  - santoshr.vcf
>