RE: iSCSI: No Framing

["Douglas Otis" <dotis@sanlight.net>]

Rob,

Although it will be true large amounts of bandwidth will be exchanged
between the typical SAN, such systems also use cache due to mechanical
limitations with respect to rotation and linear access.  If it were just
storage to storage, Fibre-Channel or FC encapsulation would be preferable.
At least with that scheme, there are direct placement interfaces available.

Direct placement is needed with low latency high bandwidth to reduce
overhead lost during a subsequent memory copy.  SCTP offers a means of
implementing direct placement without kludging a portion of the application
beneath the transport.  In addition, this direct placement scheme can be
done in an general fashion to support thousands of higher level protocols.
As with most SAN, network related failures are not well tolerated, so the
added robustness of SCTP becomes highly beneficial.

In establishing a large remote mirror, the highest bandwidth means of
initialization would be physical transport of the image.  Once initialized,
only differential information is exchanged largely limited by the bandwidth
and error rate of the interconnect.

Doug

> De-lurking...
>
> Right now, I'd say disk-disk backup/restore and mirroring.  The individual
> disk devices of today can perform at ~70 MB/s.  One need only gang 14 of
> these together, streaming, to reach nearly 1000 MB/s, or 1 GB/s,
> or 8 Gb/s.
> The disk devices of tomorrow are going to perform at rates approaching 100
> MB/s per spindle.  Only ten devices are then necessary.
>
> This would be a single TCP stream (one initiator, one target, many LUs).
>
> If one has to mirror (/move) 10 TB in a reasonable amount of
> time, say, then
> a stream of 1 GB/s is not only necessary, but paramount.  Even at
> that rate,
> it would still take nearly three hours to move just 10 TB.  In a
> business-critical recovery situation, three hours is an eon.
>
> Don't think application (CPU/memory in a server) to storage.
> Think storage
> to storage.
>
> Rob
>
> Rob Peglar
> Corporate Architect
> XIOtech Corporation, a Seagate Company
> (314) 308-6983