Re: iSCSI : Command Ordering Proposal.

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

Douglas Otis wrote:

> Santosh,
>
> If there is a task attribute, the complexity is when you allow CmdSN = 0;
>
> Perhaps CmdSN = 0 should not be allowed or treated as just another in the
> sequence.

The (CmdSN==0) does have its uses. One example is the use of NOP-OUT for
a "connection ping."


>  If a retry flag is set, the CmdSN below the
> ExpCmdSN should be viewed as an exception much as Head of Queue should be an
> exception for MaxCmdSN.  Both of these cases imply exceptional behavior by
> the initiator in response to a problem.

Accepting a command outside the (ExpCmdSN, MaxCmdSN) window with the retry
bit set then opens up windows for other types of stale and duplicate commands
to be
processed.

Here's the deal (as I view it) :

Commands are sent with the "retry" bit under 2 conditions :
- Connection Failures
- Digest Errors

In both these cases, an explicit handshake is required regarding the
(ExpCmdSN, MaxCmdSN) window, prior to starting to send "retry"
commands. This ensures that genuine "retry" commands do not slip
behind the (ExpCmdSN, MaxCmdSN) window.

In the case of a connection failure, the Logout Response provides this
handshake. For digest errors, there is no such hand-shake.
Hence, a command that encountered a digest error and was retried could
likely slip behind the (ExpCmdSN, MaxCmdSN) window and then be never
processed.

Regards,
Santosh
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