Re: iSCSI: retries and SCSI

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

Black_David@emc.com wrote:

> When the operation is not idempotent
> (doing it over has ill effects - tape reads and writes
> are examples), then this optimization is not applicable
> and if the target hasn't retained the stuff required
> to respond to the retry, it has to fail/reject the retry.

David,

Thanks for the clarification on this. This approach of
rejecting the retry, rather than not retrying for sequential
devices, poses a couple of problems :

a) The responsibility of dealing with the reject will lie
    with a iSCSI-pSCSI or iSCSI-FC bridge in most of
    the tape connectivity cases.

b) The target iSCSI layer now needs to share SCSI state
     information with its ULP on the device's class and abilities.
     (something that would normally be exchanged b/n the peer
      SCSI ULPs on either side.)

I believe the draft already intends to address the issue of retries
on a connection failure being made optional
[if the initiator detects ULP timeout and wishes not to retry].

I would like to suggest that the same policy be applied to digest
errors as well. (as initiators may not wish to retry on a digest error,
when it sees a ULP timeout.).
This allows initiators the flexibility to not retry the I/O to
non-idempotent target media.

Thanks & Regards,
Santosh

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