I'm also running amdump every weeknight, with a single tape, with
runtapes set to 2, and considering raising it to 3.

I have autoflush set to on, and I am using a patch so that the maximum
flushed plus dumps stays below runtapes * length.  The last step isn't
completely necessary, but it limits the total amount dumped when
things get behind.

Each night I perform a tape's worth of dumps, which are written to the
holding disk.  In the meantime, last night's dump gets written to
dump.  Sometimes, one night's dump runs over the limit, and is added
to the next night's dump.  Sometimes, one night's dump is a bit short,
and the next night's dump gets added to the flush.

Here are the advantages:

When there are leftover dumps, the tape drive is busier from the start
of amdump until the tape is full.  There is no waiting for a dump
before a write is possible.  In my case, writing the entire tape takes
about 12 hours. The tape drive usually available for restores before I
leave for the day.  With runtapes set to 1, a complete amdump,
including the write, often takes more than 20 hours, leaving little
time for restores, increasing the risk of not swapping in the correct
tape.

Inaccuracy of estimates doesn't matter as much.  When starting new
DLEs, the estimates are often way off.  The dumped files are usually
not written until the following day.  By that time, the true size of
the file is known.  With runtapes set to 1, when a large file is
finally dumped, it is usually the only one available for taping.  The
tape is usually mostly full at this point.  It is common for the large
file to be too large for the remainder of the tape, leaving perhaps
30% of the tape unused.  With runtapes set to 2, the large files are
written at the beginning of the flush, where there is room for them,
with smaller files filling out the end of the tape, as it should be.

If the estimate for the large dump is too small, this extra room can
be filled up with the current days dumps.  There will be many files to
choose from by the time yesterday taping is finished.

Using exclude for samba is not as painful.  The estimate may be far
larger than the real size, but the backup is often saved until the
next day, and the extra space can be used with the current day's
dumps.  With runtapes set to 1, the extra space goes unused, backing
up a small amount of the balanced DLEs if accurate information were
known.

The most recent backups can be restored quickly.  Most of the most
recent backup is on disk, so the restore of the most recent backup is
quick.


Holidays are easier to handle.  The backup goes to disk, and moves to
tape on the next day when I swap tapes.

This methods handles undependable machines better.  With my setup, it
is common for a machine to go offline for a few days, perhaps skipping
level 0 backups.  When the machine comes back online, a lot of DLEs
get done at once.

Disadvantages

The primary disadvantage is the need for a holding disk which can
handle roughly twice the size of the tape, though I can get away with
slightly less space than this.

-- 
Kevin Dalley
kevin AT kelphead DOT org