ISCSI: Why the use of the Urgent Pointer

[Matt Wakeley <matt_wakeley@agilent.com>]

Look, this whole discussion comes down to a simple question.  Do you want
iSCSI to succeed as an enterprise storage solution or not?  The intention of
the urgent pointer is to help meet the requirements specified in the iSCSI
Requirements document.

If we review the iSCSI Requirements document it requires:

- Low CPU utilization, equal to or better than current technology.

This requirement cannot be met using off the shelf TCP/IP stacks running in
the OS, especially at 10Gbps. iSCSI solutions will have to be implemented in
adapters that handle the transfer of data, similar to Fibre Channel adapters.
Otherwise, iSCSI will eat up all the CPU cycles performing TCP/IP processing,
and FC will blow iSCSI away.

- Cost competitive with alternative storage network technologies.

In order for iSCSI to succeed it must be able to be cost competitive with
other solutions (10Gig Fibre Channel, etc).  This means that it must be
implementable without putting gobs of (costly) memory on the adapter cards.
In order to avoid this memory requirement in the presence of lost frames, the
adapter will either have to place data in host memory were it belongs
*requiring a framing mechanism* or throw it away.  Fibre Channel adapters
today are able to operate in this manner today without a lot of memory because
they can place data that arrives out of order directly where it belongs in
host memory [because framing is provided].

The urgent pointer mechanism in the current draft provides the framing
mechanism so that data can be placed properly in the presence of dropped
frames.

Matt Wakeley
Agilent Technologies