
Chapter 31. Multitape support in AMANDA 2.2
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Chapter 31. Multitape support in AMANDA 2.2


James da Silva

Original text
AMANDA Core Team
<jds@amanda.org>

Stefan G. Weichinger

XML-conversion;Updates
AMANDA Core Team
<sgw@amanda.org>
Draft 1 - jds 3/29/94
Table of Contents


  Introduction

  New_Planner_Algorithm


        Time

        Full_Backups

        Schedule_Balancing

        Overwrite_Detection


  Taper_Algorithm


        Choosing_a_tape

        End_of_tape_handling

        Tape_Format_Changes

        Tapelist_File_Changes



Note

Refer to http://www.amanda.org/docs/multitape.html for the current version of
this document.

 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


 New Planner Algorithm


 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. :-)

 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;
  	}
  	


 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.

 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.

 Taper Algorithm


 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 AMANDA_Tape_Changer_Support).

 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?

 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.

 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.
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