Re: iSCSI question

[Mark Bakke <mbakke@cisco.com>]

Julian-

Your comment is accurate, but I just want to make sure that the
readers take "last resort" properly.  From a technical, iSCSI
point of view, it's the last resort.

However, from a practical (implementation/deployment) point of
view, session recovery is not all that drastic.  We have been
shipping iSCSI drivers and targets that do session recovery for
quite a while, and it works just quite well for disks (the OS
SCSI layer handles any retries), and for tape applications that
support re-positioning, which are becoming more common.  We
have spent a significant amount of time testing this both in
our lab and in the field.

<SOAPBOX>
Once tape device and driver vendors support SSC-2, there should be
no need to do more than session recovery, except perhaps for
some performance advantages environments where a lot of connection
recovery needs to take place.  But for most iSCSI environments,
doing more than session recovery is not necessary.
</SOAPBOX>

--
Mark

Julian Satran wrote:
> 
> Session recovery means just creating a NEW session and forgetting about all old commands.
> It is the last resort recovery where everything else fails and as such it is the most basic
> function - that anybody has to have.
> 
> Julo
> 
>   Shahram Davari <Shahram_Davari@pmc-sierra.com>
>                                                          To:        Julian Satran/Haifa/IBM@IBMIL
>   08/07/2002 06:43 PM
>                                                          cc:        ips@ece.cmu.edu,
>                                                  owner-ips@ece.cmu.edu
>                                                          Subject:        RE: iSCSI question
> 
> 
> 
> Julian,
> 
> To start a new session you need to start new connections and you need to support
> the PDU recovery. So how is that a subset of PDU and connection recovery?
> 
> 
> -Shahram
> 
> (I will explain the detailed clarity issues in another email)
> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Wednesday, August 07, 2002 11:34 AM
> To: Shahram Davari
> Cc: ips@ece.cmu.edu; owner-ips@ece.cmu.edu
> Subject: RE: iSCSI question
> 
> Session recovery is in fact leaving all recovery to SCSI - it drops everything and creates a new
> session.
> As for you comment on the clarity of chapter 5 at this stage it makes sense to be either specific
> or keep this type of comment out of this context.
> 
> Julo
> 
>    Shahram Davari <Shahram_Davari@pmc-sierra.com>
>                                                            To:        Julian
>    08/07/2002 06:09 PM                              Satran/Haifa/IBM@IBMIL
>                                                            cc:        ips@ece.cmu.edu,
>                                                     owner-ips@ece.cmu.edu
>                                                            Subject:        RE: iSCSI question
> 
> 
> 
> Julian,
> 
> Thanks. I have read that section but it is not very clear.
> 
> I also agree that Connection recovery requires everything in command recovery.
> But what about session recovery? isn't it a superset of both connection and command recovery?
> 
> Yours,
> -Shahram
> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Wednesday, August 07, 2002 11:03 AM
> To: Shahram Davari
> Cc: ips@ece.cmu.edu; owner-ips@ece.cmu.edu
> Subject: Re: iSCSI question
> 
> Sharam,
> 
> You may want to go over the recovery chapter.
> It has detailed answers to all your questions.
> The superset/subset is based on functions you need for the next level.
> 
> Session recovery drops real recovery to SCSI.
> Command recovery recovers from individual command errors without
> changing connection and the highest enable you to switch to a new connection and
> continue commands there.
> 
> 2 requires everything in 1.
> 
> Julo
>    Shahram Davari <Shahram_Davari@pmc-sierra.com>
>    Sent by: owner-ips@ece.cmu.edu                                      To:        ips@ece.cmu.edu
> 
>    08/07/2002 05:17 PM                                                 cc:
>                                                                        Subject:        iSCSI
>                                                                  question
> 
> 
> 
> Hi,
> 
> I have a question regarding the hierarchy of error recovery.
> Section 6.13 mentions the hierarchy as:
> 
> 2: Connection recovery
> 1: Digest failure recovery
> 0: Session recovery
> 
> And it states that the higher levels are a superset of the
> lower levels and that the level of complexity increases from 0->1->2.
> 
> Couple of questions:
> 
> 1) How is digest failure recovery done? by retransmission of PDUs?
> 2) Why is the connection recovery a superset of session recovery
> and more complex?
> 3) It seems to me the order should be:
> 
> 2: Session recovery
> 1: Connection recovery
> 0: Digest failure recovery
> 
> I appreciate any insight.
> 
> Thanks,
> -Shahram

-- 
Mark A. Bakke
Cisco Systems
mbakke@cisco.com
763.398.1054