iSCSI : Command Ordering Proposal.

[Santosh Rao <santoshr@cup.hp.com>]

Julian & All,

Proposal :
=======
CmdSN should be used to enforce ordering only when the Ordered Task Tag
attribute is set in the SCSI Command PDU.

Reference :
=========
Current iSCSI model for enforcing command ordering, based on striping
commands across multiple TCP connections in an iSCSI Session and using a

CmdSN to order the commands received out of order. Currently, CmdSN is
used to enforce ordering for all commands.

Objective of multi-connection sessions :
==============================
Multiple TCP connections per session were defined in order to
allow for :
a) bandwidth aggregation by the creation of a fat pipe between the
target and initiator.
b) to optimize against single connection failures.
c) to allow for iSCSI layer failover mechanisms within a session.


Problems with the current ordering model :
================================
1) When one TCP connection is faulty in an iSCSI session and is causing
command transmission failures, all the commands being sent on other
connections in the session will stall [due to the CmdSN ordering]
at the receiving iSCSI layer of  the target, until TCP
timeouts + re-transmissions for the commands on the faulty
connection are exhausted and a "link ping" of the faulty
connection fails, thereby, detecting connection failure and initiating
connection failure recovery.

While operating under such conditions, the commands sent on
other TCP connections within the session will be stalled until the
initiator commences connection failure recovery and re-sends the
commands with their original CmdRN on a new connection.

This can result in a large number of stalled I/Os that can remain
stalled for the period it takes to detect a faulty TCP connection and
complete connection failure recovery.

2) This strict command ordering introduces a lot of complexity into
error recovery conditions like connection failures, QUEUE FULL, BUSY &
CHECK CONDITION handling due to the attempts iSCSI makes to enforce
strict ordering of command delivery.

3) This model depends on the retry of ALL the failed commands on a new
connection in order to bridge the missing CmdSNs at the target. If a
subset of the failed commands were not to be retried [due to a retry
policy that forbids this, such as non-idempotent media, or a
ULP timeout],
then, the CmdSNs at the target waiting for the out-of-order commands to
arrive, need bridging which results in additional complexity.

Requirements for Ordering :
=====================
1) SCSI ordering is only required on a per-LUN basis.

2) Strict SCSI ordering is only required when requested through the
Ordered Task Tag attribute. For all the other types of tasks, no command

ordering is required. Strict Command ordering is not enforced in other
SCSI transport protocols. Command ordering in other SCSI transport
protocols is violated when a QUEUE FULL, BUSY or CHECK
CONDITION SCSI Status is returned. Command ordering can also
be violated when the SCSI transport encounters transport failures
or resource allocation failures on intermittent commands.

3) SAM Section 4.6.2 assumes the service delivery subsystem to provide
in-order delivery for convinience. "This assumption is only made to
simplify
description of behaviour and does NOT constitute a requirement."

Solution :
========
iSCSI should only enforce strict ordering when requested to do so by the

SCSI ULP through the Ordered Task Tag attribute.

Benefits :
========
1) Optimizes iSCSI performance by avoiding stalls on other TCP
connections within a session when one connection is faulty.

2) Simplifies error recovery on connection failure, digest error,
CHECK CONDITION, QUEUE FULL and BUSY scsi status as
well as initiator resource allocation errors.

 -------------------------------------------------------

Regards,
Santosh
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