Re: iSCSI: revised error recovery hierarchy

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

Arpakorn Boonkongchuen wrote:

> At 06:42 PM 9/14/2001 -0700, Santosh Rao wrote:
> > > Arpakorn Boonkongchuen wrote:
> > > >
> > > > I have been following the iSCSI error recovery discussion
> > > > and have one question.  Is it a valid procedure for an
> > > > initiator who doesn't implement SNACK and does not want to
> > > > tear down the session and all connections when it gets a
> > > > data digest error to simply aborts the task (with task
> > > > management function request) associated with the bad
> > > > data digest received and tries to recover by resending the
> > > > command?
> >
> >Any initiator that does not implement SNACK will have to perform session
> >logout & re-login as the error recovery on digest errors that occurs on
> >the status PDU.
>
> ok but do you see a problem if it is a data PDU?

None. An Abort Task should be usable to recover from a data digest error on
a Data-In PDU.

One could also let the task run to completion and determine that an I/O
underrun has occurred based on a comparision of the ExpDataSN in the status
pdu and a count within the driver of the number of data pdu's received on
that scsi cmd. This method is simplet and does not require any abort task
usage.

The above recovery scheme is not documented in Section 7.4 as a means of
recovering from data digest errors on iscsi data pdu's. This mechanism
should be considered for inclusion in Section 7.4, since it is a simple
technique to recover from data digest errors and does not require any abort
task to be issued.


> >This is because of the current SNACK mechanism which will not allow the
> >initiator to acknowledge any further Status PDUs once a hole occurs in
> >the StatSN. The only way to workaround a hole in the StatSN sequence is
> >to either use a Status SNACK or to do a session logout.
>
> A session logout is a pretty drastic measure. I wonder whether it
> suffices to just log out and relogin on that particular connection
> only?

Either a "connection logout for recovery" or a session logout is usable to
recover from digest errors on status pdu's when the initiator does not
support SNACK usage.

Section 7.4 is missing the usage of session logout as a recovery mechanism
for digest errors on Status PDUs.

Regards,
Santosh
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