Re: iSCSI: DataRN

[julian_satran@il.ibm.com]

Santosh,

The current draft has status numbering as SHOULD and that is (as it was
pointed out to me) almost mandatory.
Considering that the consensus at Orlando and on the list was to make CmdRN
(now CmdSN) mandatory even for one connection I don't see any reason not to
make StatSN support also mandatory .

On Writes our assumptions was that the target can at any point in time
either reject the restart (if as you outline it can't do it) or if it can
reacquire the data it misses trough R2T and resend the status (on writes
the target is assumed to know what it needs). In case it rejects the
restart you can report a "transport failure" and you are as good as a
parallel packetized SCSI.

Julo



Santosh Rao <santoshr@cup.hp.com> on 25/01/2001 00:13:53

Please respond to Santosh Rao <santoshr@cup.hp.com>

To:   Julian Satran/Haifa/IBM@IBMIL
cc:   ips@ece.cmu.edu
Subject:  Re: iSCSI: DataRN




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

 - santoshr.vcf