SUMMARY: Exabyte 8500 w/add-on compression

From: Dave Sill (
Date: Mon Sep 13 1993 - 12:32:17 CDT

I originally stated:
> I've got an R-Squared Exabyte 8500 with an add-on compression unit (it
> has a nice LCD display showing the mode, status, compression ratio,
> error rate, etc.). I'm trying to dump a *large* filesystem (2.7 GB,
> mostly gzipped data files). The problem is that I'm hitting the end
> of the tape before the dump completes. Since I'm dumping in 8500
> mode, I should be getting closer to 5GB on the tape.

And followed up with:
> 1) SunOS has a file size limit of 2GB, but I really am hitting the end
> of the tape. The LCD display counts down the remaining tape
> capacity, and when it hits zero, the dump fails with something like
> "error writing to tape". Subsequent attempts to write to the tape
> fail with the same error.
> 2) I'm using the command:
> dump 0bsdf 200 12000 54000 /dev/nrst0 /dev/md1a
> I think 200 is about the largest block size I could get to work.
> The size and density multiply out to better than 7GB.
> Although I'm writing to rst0, the drive is in "8500, compressed"
> mode--it says so on the LCD panel. The bit about adding 8 to get
> 8500 mode is only necessary when the drive doesn't default to 8500
> mode.
> 3) Most of the files I'm dumping are gzipped, so I don't expect the
> built-in compression to do much. In fact, it reports a compression
> ratio of 1.1 before the dump dies. Without compression, though, I
> should get nearly 5GB on the tape. I'm only getting half that.
> 4) This isn't an 8500C, it's an 8500 with an add-on compression unit.
> The kernel mod for the 8500C won't help, and only implements
> selection of density and compression by specifying the appropriate
> device name.
> It seems to me that there are only two possibilities: the unit is
> defective (not really using 8500 mode, for example) or dump is wasting
> half the tape. The throughput--reported by the LCD panel, again--was
> an average of 300 KB/s. Is streaming a problem with these drives? Is
> this slow enough to waste half the tape?

I received *many* replies, most suggesting:

    -turn off the compression
    -use a smaller block size; 126 is the maximum supported
    -use /dev/rst8 instead of /dev/rst0

Turning off compression doesn't help--it just causes the dump to use
more tape.

Dropping the block size from 200 to 126 doesn't help--it just slows
down the dump considerably. I've been using 200 for years, and have
had no problems dumping or restoring.

Switching from /dev/rst0 to /dev/rst8 did the trick. Even though the
drive was claiming it was writting in 8500 mode, it wasn't. Even
though this is documented in the "st" man page, I consider this a bug
since the drive incorrectly reported the mode it was in.

My dumps are now achieving a sustained transfer rate of over 500 KB/s!

Thanks for all the help.

Dave Sill (                 Computers should work the way beginners
Martin Marietta Energy Systems           expect them to, and one day they will.
Workstation Support                                                -- Ted Nelson

This archive was generated by hypermail 2.1.2 : Fri Sep 28 2001 - 23:08:10 CDT