
MULTITAPE SUPPORT IN AMANDA 2.2

Draft 1 - jds 3/29/94

1. INTRODUCTION

The goal of this enhancement is to make Amanda independent of the number of
tapes used per run or even per dump cycle.  Specifically, I would like
amanda to handle the following:

	- output of amdump run goes to more than one tape
	- a single dump file can straddle two tapes
	- more than one amdump run can be done in a single day
	- planner should not care how many runs per cycle occur

And later:

	- multiple runs of amdump can go onto one tape (eg an append mode)
	- any dump files from a previous run that are on the holding disk
	  are written to tape in this run (eg eliminate amflush)
	- taper write to multiple tape drives simultaneously


2. NEW PLANNER ALGORITHM

2.1 TIME

Previously, planner marked time by the number of amdump runs, which it
equated with number of tapes, and number of days.  In Amanda 2.2, amanda
keeps track of the real passage of time, and doesn't generally care about
the number of runs or tapes between any two events.

While Amanda 2.2 doesn't care about spacing between runs, dump cycles are
still in terms of days, to make things easy to understand for the user.
So, time differences are rounded to the nearest 24 hours:

	days_diff(A,B) = (<B> - <A> + 86400/2) / 86400

Where the times A and B are in seconds since the Unix epoch, and 86400 is
the number of seconds per day.  This rounds a 2.49 day difference down to 2
days, and a 2.5 day difference up to 3 days.  No, Olafur, Unix time does
not handle leap seconds.  Give me a break. :-)

2.2 FULL BACKUPS

The first thing planner does is calculate when each filesystem is due for a
full backup.  This is trivial for normal backups:

	full_is_due = days_diff(<time of last full>, <curtime>) >= dumpcycle

There is a complication for "skip-full" filesystems.  Under 2.2, these will
be skipped on any runs that occur on the day the full is due, but we have
to do the right thing if multiple runs are done that day, and if no runs
are done that day (in which case we should be doing an incremental).  Also,
the time of last full dump is a fiction maintained by the planner -- Amanda
has no way to tell whether the full backup was actually done or when it was
done:

	if(skip-full) {
		if(full_is_due)
			<time of last full> += dumpcycle;
		if(days_diff(<time of last full>, <curtime>) == 0)
			skip the filesystem on this run;
		else
			do an incremental dump of this filesystem;
	}


2.3 SCHEDULE BALANCING

The "runtapes" parameter tells planner how many tapes it should plan to use
each run.  It multiplies this by the tape length to get the size available
for the run.  (NOTE: later amend this size if appending to tapes, or if
there are dumps on the holding disk waiting to be flushed).  Other than the
size calculation, planner doesn't really care how many tapes will be
written to.

The fundamental problem with attempting to balance the schedule is that we
no longer know how many amdump runs will be done in a full cycle.  The
number may change from cycle to cycle if there are extenuating
circumstances.

So, planner must guess at how many runs will be done in one cycle, by
looking at the information for the last cycle, or, if this is the first
cycle, assuming one run for each day in the dump cycle.

2.4 OVERWRITE DETECTION

When can a tape be overwritten, considering that it might have old dumps on
it?  We want to be able to warn when full dumps are going to be
overwritten, but given the possibility of old files on the tape, how can we
know when the tape is no longer needed?  I think we can get this when going
through the info file, considering each full dump and what tape it is on.
Make sure we correctly handle stale information.


3. TAPER ALGORITHM

3.1 CHOOSING A TAPE

Taper must now handle writing to multiple tapes in one night, but choosing
the tapes from the tape rack is done one at a time as needed, re-applying
the same algorithm each time (see TAPE.CHANGERS).

3.2 END OF TAPE HANDLING

As in earlier versions of Amanda, Taper itself does not try to restrict
writing to the tape size given in the config file.  It relied on planner
having correctly estimated backup sizes and limiting itself to what would
fit on one tape.

Now, Taper needs to switch to a new tape when the current tape has filled
up.  The tape is considered full when taper gets a write error.  This will
most likely occur in the middle of writing a (potentially large) backup
file, perhaps even from a direct-to-tape socket, so there is no possibility
of starting the backup file over again on the next tape, it must start from
where it left off, rewriting the block that got the error on the next tape.

To insure correct operation, the file header of the continued file should
contain an indication that it is a continuation, and at what offset.
amrestore of course needs to be aware of this scheme and handle it
correctly, perhaps by double-buffering internally.  XXX provide more alg
details here, or just leave it with the general idea?

3.3 TAPE FORMAT CHANGES

We need to specify the sequence number of the tape in the run,  in the tape
header file.  The file header block specifies whether it is a continuation
file or not.

3.4 TAPELIST FILE CHANGES

The lines in the tapelist file should contain the sequence number of the
tape in its run, as well as the amount of data written on the tape, and
perhaps whether or not the end of tape was reached.
