Re: Target Reset handling

["Mallikarjun C." <cbm@rose.hp.com>]

James,

I am in complete agreement with you on all these concerns as they parallel
mine as noted in an earlier posting (http://ips.pdl.cs.cmu.edu/mail/msg00348.html),
the relevant portion of which is reproduced below -

o Section 3.7.1 on page 23.  For the Target Reset task management
  function, the target is not expected to provide a response - and
  this is concerning to me.
       - how would the initiator confirm the successful completion of
         the target reset?

       - not having a response effectively makes the target reset
         an operation iSCSI hardware cannot assist.  Having a response
         would make it no different from the rest of the iSCSI transactions
         and the hardware can gracefully deal with it.

       - SAM-2 (section 6.6, page 63) specifies what a target should
         do "Before returning a FUNCTION COMPLETE response".  This seems
         to me as an implicit requirement that a response be returned
         on a Target Reset task management function.

       - I am also unclear as to why the sessions are allowed to be
         terminated.  In the FC world, as far as I can recall, the
         process login sessions remain intact after a target reset.
         If it is required that the sessions and the associated TCP
         connections be cleared in iSCSI, it is helpful to mandate (as
         opposed to leaving it up to the implementation) that an Async
         event shall be reported to all the initiators currently logged in.

--
Mallikarjun 
M/S 5601			
Networked Storage Architecture
HP Storage Organization
Hewlett-Packard, Roseville.
cbm@rose.hp.com




>In reading the iscsi-01 draft, I was bothered by several things in the
>handling of Target Reset.
>
>a) The lack of at least a basic ACCept on the Target Reset. If the target
>can send an async event, why not at least notify reception of the function ?
>
>Given connections with lots of outstanding traffic, I'd see this as a more
>graceful reset procedure. It allows any outstanding i/o that may be
>completing while the TR is in transit (or queued for processing on the
>target) to do so, possibly lightening the load of i/o that has to error to
>complete. This would potentially quicken the recovery time post reset. I
>would expect this to be more important as the "network" gets larger and
>longer.
>
>Note: FCP does support this behavior.
>
>b) Why not require async events to all initiators ?
>
>The biggest headache with Target Reset is how long it takes for the other
>initiators to recognize the device has been reset. The 1st new i/o will get
>a Unit Attention CA, but this status is typically seen only by the SCSI
>class driver (e.g. disk/tape/etc). Unless instructed by the class driver,
>the port level driver (e.g. scsi/fc/iscsi hba) will have to timeout the
>i/o's (if timing was requested) to recover their context. If the class
>driver does try to tell the port driver, it typically will do so in a crude
>fashion - issuing abort requests on the i/o's it knows about.
>
>Perhaps, if the TCP connections are gracefully shutdown between the
>initiator and target, the initiator will be to abort the i/o on the
>connections quickly (in this case, it looks like a pseudo async event).
>However, if there is no handshaking on the connections, my limited
>experience with TCP says it takes a long time for the connection to error
>out and reset. And during this process, we'll be sending i/o abort requests
>down the terminated-on-one-end connection. All this would make the recovery
>time on these other intiators very large.
>
>Note: this point assumes that if async events are required - they are ack'd.
>
>c) Is there something inherent that requires the TCP connections to be
>terminated ?
>
>The TCP connections look very similar to (but not the same as) FCP Process
>logins between the initiator and target. In FCP, the reset did not
>necessarily disrupt the port or process logins. It only had to affect the
>FPC/SCSI task manager. (note: a device was free to really reset, thus indeed
>tearing down the logins - with the FC port machine handling it as an error)
>
>What is the background that required the TCP sessions to be broken ?
>
>Obviously, if they are not broken, it affects the answers to (a) and (b)
>above.
>
>d) Given the history of long error recovery times in multi-initiator
>environments in both parallel scsi and fibre channel on BDR's/Target
>Reset's, any speed up in this area would be advantageous.
>
>-- James
>
>
>
>--------------------
>James Smart
>Trebia Networks, Inc                  Ph:   978-318-9547
>35 Forest Ridge Rd                    Cell: 603-674-3687
>Concord,  MA   01742                  james.smart@trebia.com
>
>