RE: Framing Discussion

[julian_satran@il.ibm.com]

Dear Mr. Cheng,

You can't fit data to tags if you have lost an iSCSI data header.  However
you could (in theory) place them in anonymous buffers and tag them later
(once you have recovered the header). In this case you either copy them or
have the storage-end do some scather-gather on the fragments you have
placed in anonymous buffers.

Julo

"Y P Cheng" <ycheng@advansys.com> on 21/12/2000 18:21:12

Please respond to "Y P Cheng" <ycheng@advansys.com>

To:   "'Ips@Ece. Cmu. Edu'" <ips@ece.cmu.edu>
cc:
Subject:  RE: Framing Discussion




> I agree with David. For the sake of clarity however I would like to add
> that even for those cases in which buffers are anonymous (like
> some appliance fileservers or disk controllers) the data that comes
> from the wire are not necessarily entire storage data blocks and have
> to be placed somewhere within a storage buffer. That particular
> placement can be made itself anonymous only by "placing" a
> "scather-gather" burden on the storage end of the wire
> and that is either expensive or outright impossible.
>
> Julo
>

Julo,

You lost me on the very last statement.  For appliance fileservers or disk
controllers, there is typically an small RTOS running.  The anonymous
non-storage data are received into a small bucket, say 256 bytes, which is
passed back to application for processing.  These types of data are known
as
control data like mode-select and login, etc. The component interacting
with
wire is given a number of these small buckets. There is no scatter-gather
burden on the storage end.

In the context of TCP/iSCSI implementation, the wire interacting component
needs to sort out between solicited and unsolicited iSCSI PDUs. For
solicited PDU, it places the data based on target task tags.  For
unsolicited control data, it places them into these small buckets.  For
unsolicited disk block data -- like streamed writes -- it places the data
into anonymous buffers prepared by the application.  Scatter/gather
handling
is a part of the "wire interacting component" design.  No extra burden.  In
fact, the HBA for an iSCSI initiator or target is specialized in move data
to destination buffers quickly.  Its main task is parsing the TCP/IP
segments.

This is the way I understand how an appliance fileserver or disk controller
works.  Either big companies have total different designs or I must have
misunderstood your statement.

Y.P. Cheng, Connectcom Solutions.