ghg@ei.ecn.purdue.edu (George Goble) (04/26/89)
For the last few weeks, I have played with a rather unusual 8mm tape
subsystem from TTI. It consists of the usual enclosure, EXB-8200 Exabyte
tape drive, and a power supply. It also contains a proprietary board,
about the size of a playing card, which monitors the SCSI bus read-only
and drives a front panel display.
The display consists 8 bright green LEDS which decode the SCSI function
being executed. (REW, WEOF, BSF, BSR, FSR, FSF, RD, WRT). Also, there are
two bright green seven-segment display windows. Tape remaining to logical
EOT (LEOT) is displayed in units of 1000's of Kbytes (close to Megabytes
left, but they divide by 1000 instead of 1024, so it is slightly
different). A real time error retry rate is computed and displayed in the
bottom display in the form XX.XX percent.
The data for both these displays originates in the Exabyte drive and is
contained in the sense buffer after a sense command is executed.
The error retry rate on writes are the number of blocks which were
rewritten later down the tape, after a one or more bit error was detected
on the read-after-write. On reads, this number is the number of blocks
which had the ECC correction invoked. The ECC can correct a 263 byte
burst error in a 1Kbyte block as well as 60-80 one and two bit errors in
the same block. Over 400 bytes of ECC are added to each 1Kbyte block.
We have seen error rates as high as 25% on writes (due to a dirty drive)
and everything still "worked" ok -- no unrecovered errors! This drive had
not been cleaned in 9 months and got used everyday for several hours. If
your drive is working well and is reasonably clean, you should see 1% or
less on both read and write retries. 0.2 - 0.5% are common here.
This device needs the device driver to issue sense commands in order to
generate the tape left and errorlog displays. With the source, it is easy
to change the SunOS "st.c" (about 10 lines) to do this. TTI should be
able to give you a driver. Our driver (modified st.c/si.c for SunOS 4.0)
is also available (source and binary) to those with the appropriate Sun
licenses.
For more info:
Mark Jaffe
Transistional Technology Incorporated.
1411 N. Batavia, Suite 203
Orange, CA, 92667
(714) 744-1030
George Goble, Engineering Computer Network, Purdue U, W. Lafayette IN 47907
(317) 494-3545 Arpa: ghg@purdue.edu uucp: {backbone}!pur-ee!ghg