Re: iSCSI: DataRN

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

julian_satran@il.ibm.com wrote:

> Santosh,
>
> The ACK for a response is when status is ACKed (by the status numbering).
>
> Before this ack a target has several options for restart:
>
>    to keep all the data (recall we have dropped DataRN) and "restart
>    sending".

This may work for a READ if the iSCSI layer has buffered the data until it
gets a StatSN ACK. Note that even for the READ, status recovery is optional
and a tape may not implement status / data recovery and just retry the
command. In such a case, a retry results in the target re-doing the operation.

For a WRITE however, I don't see how this is going to work. Are you suggesting
that the iSCSI layer NOT deliver data to the SCSI layer until the Response is
acknowleged ??? This is ruled out, since the Response itself comes from the
SCSI layer upon completion of the SCSI command.

If, on the WRITE, the data was delivered to the SCSI layer which then wrote it
to media and advanced the tape, what good does the above do ?? The tape has
advanced following the WRITE and the initiator can corrupt the tape by
retrying the WRITE.


>    not to keep data and reject the command restart with a service response
>    (that we have to specify) of restart reject (your tape target may want
>    to do just this)

You seem to suggest moving the responsibility of ensuring data integrity from
the host to the target !! This is not suitable for the following reasons :
a) Often, tape is the last class of storage products to migrate to new storage
transport technologies. IOW, the majority of tape backup is going to attain
iSCSI connectivity through a iSCSI-FC bridge or iSCS-pSCSI bridge. In such a
case, the service response management is done in the bridge and this level of
intelligence is now being moved into the bridge.

The responsiblity of ensuring data integrity on a SCSI target lies with the
initiator issuing the I/O.

iSCSI MUST not retry a command to sequential media when there is the
possiblity that some or all of the data phase has completed.

Regards,
Santosh
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