iSCSI: error recovery

[Matt Wakeley <matt_wakeley@agilent.com>]

There has been a lot of discussion on how the Status reference numbers are
(can be) used for error detection and recovery.  There is even a (optional)
method for numbering the Data PDUs now.

Let's clarify what "errors" we are trying to recover from, and how the RNs are
meant to be used.  The example is as follows.  An iSCSI session has multiple
TCP connections over *separate* physical links.  If one of the physical links
fails, it is desirable to "recover" the SCSI I/Os that were occurring on the
TCP connection(s) that were established over that link.  We should *not*
attempt to recover "errors" that are caused due to data being discarded after
it has been delivered from TCP to the upper layers (iSCSI, SCSI, whatever).

Now there has already been discussion on how the TCP timeouts are (generally)
longer than most SCSI command timeouts, so I'm only discussing link errors
that can be detected fairly quickly.  For example, if the physical link gets
yanked, the MAC can relatively quickly determine the link is down and notify
the appropriate management entity.

The goal is to have a mechanism for the initiator to determine what commands
are outstanding on the failed connection. Likewise, it's desirable for the
target to retain the data and/or status of I/Os until they are acknowledged by
the initiator, so that in the event of a link failure, the target can "replay"
the I/O.

>From the ExpCmdRN, the initiator knows which commands it sent on the failed
TCP connection where received by the target and those that were not.  Any
commands received by the target, but not completed (no status pdu received
before the failure) should be resent on another TCP connection with the
"retry" bit set.  Any commands not received by the target are resent on
another TCP connection without the "retry" bit.

The target keeps the context (status and maybe data) of SCSI I/Os it's
executed until it has positive acknowledgment from the initiator that the I/O
is complete at the initiators end.  This acknowledgment is indicated in the
ExpStatRN received from the initiator.  Acknowledged I/Os are then deallocated
in the target.

Now for some issues I have with the (current) iSCSI draft:

In section 2.2.2 it states "As the only cause for long delays in responses can
be failed connections and received responses free-up resources, we felt that
score boarding responses at the initiator could be accomplished by simple
bitmaps and there is no need to flow-control responses."

Score boarding, especially with bit maps,  is an operation that can be
somewhat CPU heavy in the normal "performance path" of the iSCSI layer. If the
ExpStatRN was local to each TCP connection, rather than global across the
iSCSI session, then there would be no requirement for score boarding.  The
initiator would simply increment the StatRN received on each connection for
use in the ExpStatRN for that connection.

>From an earlier email: "1.1.1.3   Data PDU numbering
Incoming Data PDUs MAY be numbered by a target to enable fast recovery of long
running READ commands. Data PDUs are numbered with DataRN.  NOP command PDUs
carrying the same Initiator Tag as the Data PDUs are used to acknowledge the
incoming Data PDUs."

Since the only "error" we are trying to recover from is the very rare event
that a physical link fails, I fail to see what the benefit is to be able to
"recover" at the PDU level.  Plus, you'll have to build into the protocol a
mechanism to request retransmission of particular data PDUs.  Let's simplify
and just send the command with the "retry" bit set.

Also from an earlier email:


> >Mallikarjun,
> >
> >Thanks for your comments.
> >
> >Initiator scoreboarding is not considered. I will try to emphasize this
> >even more in the new draft.
> >The party responsible for reporting length is the target.  As overlapping
> >ranges are not explicitly
> >forbidden this would be a harder task than apparent. Reporting counts
> >becomes entirely a question of faith!
>
> I didn't realize that (what FC calls as) data overlay is allowed, FCP
> requires this initiator capability to be explicitly stated in session
> establishment (process login).  Is there a particular reason why this
> is chosen to be allowed by default in iSCSI?
>

Again, in the interests of simplicity, I request that data overlay be
forbidden.  Period.  Otherwise, the initiator would have to perform score
boarding at the byte level to be positively sure that each byte was really
received.

>
> Given that the physical number of bytes transferred could be more (data
> overlay case) or less (command retry case), may I suggest that the discussion
> about Residual under/overflow (section 3.3.1) and Residual Count (section
> 3.3.2) make it explicitly clear that it's the status of "logical" # of
> bytes that is being reported?  That way, initiator implementations can
> always rely on those fields regardless of the history of the task.
>
> I assume you took note of my comments on the need to change wording and
> payload definition of NOP PDU.
>
-Matt Wakeley