RE: Requirements specification

[psarkar@almaden.ibm.com]

Mike,

I think we need some convergence here.

There is no denial that there are applications that can take
advantage of multiple connections. However, none of them
are particularly simple (they go way beyond round-robin and
a lot require router assistance as you say) and the point
being made here is that a reasonable place for managing
the complexity of multiple connections is above the transport layer.

This also does not mean that you need to change application
software - inserting SCSI filter drivers to handle
multiple connections can make this transparent to applications.

In summary, this is a layering issue - you can have one iSCSI
layer handling multiple connections, availability, etc. or you
could put the multiple connections layer on top of a simpler
iSCSI layer. Either way, the application is unaware of what is
going on. Furthermore, if you did not want multiple connections,
you could just remove the multiple connections layer.

I personally prefer the simpler iSCSI layer approach as it
would cater to the common case.

Regards,
Prasenjit


   Prasenjit Sarkar
   Research Staff Member
   IBM Almaden Research
   San Jose


Michael Krause <krause@cup.hp.com>@ece.cmu.edu on 08/04/2000 07:02:22 PM

Sent by:  owner-ips@ece.cmu.edu


To:   Prasenjit Sarkar/Almaden/IBM@IBMUS, ips@ece.cmu.edu
cc:
Subject:  RE: Requirements specification



At 10:58 AM 8/4/00 -0700, psarkar@almaden.ibm.com wrote:

>Two points:
>
>1.  The iSCSI presenters seem to imply that having 'n' connections through
>the IP fabric will automatically give you 'n' (or close to 'n') times the
>bandwidth if you round-robin your packets across the connections. This is
>probably true only in the very ideal case and in the real world, the
>situation is far less rosy to make such an assertion so confidently.

Not true that this has to be the ideal world - there are numerous
applications that maintain high-bandwidth through multiple connections on
the same path or across several paths.  Also, network routers often
implement RED / WRED schemes which may only impact a subset of the sessions
connections.


>2. Fault-tolerance currently is (and should be) layered above the SCSI
>transport layer. There are enough solutions from several vendors in the
>market which deal with this,
>so there is no use in reinventing the wheel.

Fault tolerance is a highest level of availability there are other levels
that allow one to recover from a variety of failure types without requiring
to implement to this level.  Most of these can be simply implemented below
the iSCSI level.  Also, dynamic changes in the network composition can be
dealt with without resorting to high-level changes to the
applications.  The objective of improving the availability is to not be
required to use this overly complex solutions but to create simple,
low-implementation costs solutions that will benefit the majority of the
customers.

>Based on the "Keep It Simple and Stupid" and "Optimize for the Common
Case"
>principles,I would appeal to the iSCSI "design team" to forgo multiple
>connections per
>session and use specialized solutions for remote mirroring and remote tape
>backup (the apps that require multiple connections).

Multiple connections can be done using a KISS principle and more robust
solutions can be built on top of this solution to expand the functional
capabilities without requiring the architecture to be radically permuted in
the process.

Mike