On Thu, 25 May 2000, Jon William Toigo wrote:
I am seeking a few points of clarification:
1. Fibre Channel folks have attempted to explain to me why TCP/IP
could NEVER be a viable interconnect for block level storage
operations. They claim:
a. TCP is too CPU intensive and creates too much latency for storage
I/O operations.
b. The IP stack is too top heavy and processing packet headers is too
slow to support storage I/O operations.
c. The maximum throughput of a GE TCP/IP connection is 768 Mps, which
is too slow to support storage I/O operations.
This is not a theoretical limitation, but is in the ballpark reported by
many general-purpose operating systems with commodity hardware.
Is any of this true?
I don't believe that TCP/IP implementations couldn't be optimized to
support full link rate and low latency. If you're building a hardware
adapter that can do SCSI and RAID fast, adding TCP shouldn't be
prohibitively hard.
2. Adaptec has posited a replacement for TCP called STP for use as a
transport for storage. Does anyone know anything about this?
STP is the Scheduled Transfer protocol being standardized by the ANSI T11
folks. ST was designed to run on top of GSN (a.k.a. HIPPI-6400). In my
opinion, it is as heavy-weight as TCP with respect to most of the things
stated above. It does have the potential advantage of being designed from
scratch to support zero-copy access to user space using specialized
interface cards.
3. Current discussions of the SCSI over IP protocol seem to ignore
the issue of TCP or any other transport protocol. Does anyone know
definitively what transport is being suggested by the IBM/Cisco crowd?
I believe the assumption is that you will have a local network with no
packet loss or significant bit error rate. Basically, you assume that
your ethernet is as reliable as your SCSI cable or fiber-channel network.
For a well engineered, fully-switched LAN, that may be a reasonable
assumption.
-- Mike Fisk, RADIANT Team, Network Engineering Group, Los Alamos National
Lab See http://home.lanl.gov/mfisk/ for contact information