Why isn't dump maximally efficient with TK70 tapes?

[Ian! D. Allen [CGL]]

>> Dumps are of major importance; I would have thought that dump would be
>> the most clever user of the tape drive.  I can't believe this.  Am I
>> missing something?  I must be missing something.
>
>Mostly that dump doesn't document the -b switch to let you specify a more
>efficient block size and restore doesn't support it, which makes it a
>pain to restore tapes with non-standard block sizes.

Even using -b to set a 32K block size (the maximum the drive seems to
allow without "write: Error 0" messages), it's still faster to dump to
stdout and pipe into dd with wbuf=2 to write the tape.  Is this the
performance problem mentioned in the 3.1C release notes (item 1.1.12/13)?

And you should see how long all this takes using rdump!  Since Ultrix
dd is supposed to handle multi-volumes, I'd recommend this as an
alternative to rdump:

     % rsh host dump <options>f - <what> | dd wbuf=2 bs=32k of=<tape>

Actually, bs=32k doesn't do quite what I want -- it doesn't assemble 32k
of output data before writing it.  As partial records come through the
pipe, dd writes them to tape, wasting space.  "ibs=32k obs=32k" works,
but makes dump waste *much* time copyng the data.  I wrote a simple
program that reads and reads until it gets 32k worth of stuff, then does
a 32k write using Ultrix nbuf I/O and two output buffers.  Using dump to
stdout into that program to write the tape is three times faster than
using dump with 32k buffers:

    # time dump 0b 32k /
      DUMP: 345 tape blocks were dumped on 1 tape(s)
      DUMP: Dump is done
    0% real=7:37 usr=0.3 sys=1.6 rd=1 wr=4 mem=92 pg=2 rec=18 sw=0 sig=0 cs=2037

    # time sh -c "dump 0f - / | ./a.out >/dev/rmt0h"
      DUMP: 11295744 bytes were dumped to Standard Output
      DUMP: Dump is done
    4% real=2:39 usr=0.6 sys=7.2 rd=0 wr=9 mem=45 pg=2 rec=17 sw=0 sig=0 cs=7636

Think how much faster this would be if dump did nbuf I/O most efficiently.
-- 
-IAN! (Ian! D. Allen) idallen at watcgl.uwaterloo.ca idallen at watcgl.waterloo.edu
 [129.97.128.64]  Computer Graphics Lab/University of Waterloo/Ontario/Canada