name@portia.Stanford.EDU (tony cooper) (10/10/89)
I have a replacement for the A/UX tcb command that speeds up tape backups to
the 3M MCD-40/SCSI tape drive (otherwise known as the Apple Tape Backup 40SC).
It is a combination of tcb and dd so that you can use it in the form
tar cvf - . | tbcd >/dev/rmt/tcx
or in the form
tbcd if=/dev/rmt/tcx ibs=20x8K |tar xvf -
where tbcd is the name of the program. The letters of the name are a mixture
of tcb, dd, and db which stands for double buffering. The A/UX tcb does not
use double buffering, tbcd does. The result is that reads from disk and writes
to tape can be done simultaneously (in a sense). This means that the tape
transfer is speeded up by 50% in the worst case. In the best case the speedup
can be 600%. This occurs when the tape drive does not normally stream and
tbcd allows it to stream. For example I can pipe the tar through compress to
save space but the tape drive does not stream. Using tbcd allows streaming and
the backup goes six times faster.
The program is available now. The catch is that the program is not free.
You can get a free copy if you contribute to the program. Otherwise the cost
is two million dollars plus tax (educational discounts available).
Here's how to contribute: I want to find out in what circumstances the program
performs best; what are the optimal buffer sizes to use; can the program be
improved over its performance now; are there various double buffering
algorithms some of which are better than others; can I do better by aligning
the buffers on page boundaries etc etc etc. All you have to do is contribute
to my knowledge in any way and you get a free copy.
Other questions: what are the operating parameters of the tape drive eg what
is the maximum rate at which data can be saved? 1Mbyte per minute, or 2? The
specs in the Apple SC40 owners guide say the maximum burst rate is 500Kb per
second. This is half a megabyte per second. The drive transfer rate is 400Kb
per second (non interleaved). What do these figures mean? They are extra-
ordinarily fast. For continuous tape motion 21.3Kb/s. Ie just over 1Mb per
second. Is this the fastest we can go? You can't go any faster than continous
motion - or can you. The R/W speed is 60 ips. Inches per second? Then a tape
205 ft long will take 16 minutes to write; ie 2.4Mbyte per minute. The
fastest I can get under A/UX is 1.08 Mb per minute with tbcd and 0.71 Mb per
minute without tbcd. Why so slow? Why so many contradictory speeds anyway?
Have you got SCSI programming info for the drive? I don't. Why does dump.bsd
abort dumps with tape write errors on good tapes? Is tar faster than cpio?
Is afio faster than cpio? How does GNU tar compare with tar? How does BRU
compare in speed? What about non Apple tape drives?
A simple way to contribute is to tell me your configuration, tell me what
commands you use, and time a backup or two to get speed info. Then I can get
an idea of how people are doing without tbcd. Eg do this:
timex tar cf - <directory> | timex tcb > /dev/rmt/tc1
(note the two timex commands - one for each side of the pipe). So it's easy to
contribute and save the cash price. I will collate the results, timings, etc
and make them available to all users of tcbd in the final version of the
program.
Note that tcbd is safe and does not involve configuring the kernel, making
SCSI calls, or rewriting drivers etc. It is a simple UNIX filter that copies
stdin to stdout. It works with any UNIX command that can use filters eg
cat file.txt | tbcd is the same as cat file.txt.
Send contributions to
Tony Cooper
name@portia.stanford.edu
tony@popserver.stanford.edu
Deposit large cash sums in Swiss bank account Number .... well I'll tell you
the number if you want to buy it, but wouldn't you prefer to get it for free?
PS Do you have a tape drive that you want to get working under A/UX but have
no driver for? I can write drivers for most SCSI drives (for a price).
PPS Do you have a SyQuest 44Mb removable drive you want to get working under
A/UX? I have some software that will get it working (for free).
My prediction: The backup medium of the future will be DAT tape drives; These
will be used on micros up to mainframes. The drives and the tapes will be
cheaper and smaller than optical devices and tape drives using video technology
(such as the Exabyte drives).