can Amanda append to a tape that's not full?

[Matt Morgan]

On 12/16/05, Les Mikesell <lesmikesell at gmail.com> wrote:
> On Fri, 2005-12-16 at 14:08, Matt Morgan wrote:
> > I'm about to do my first Amanda install next week (or so I thought),
> > so I pulled out my O'Reilly "Unix Backup and Recovery" book. It's from
> > 1999, and refers to Amanda version 2.4.2, but it says "Amanda
> > currently starts a new tape for each run and does not provide a
> > mechanism to append a new run to the same tape as a previous run ...".
> >
> > So I thought, well, that's probably out of date and I went to check
> > the docs at Amanda.org, which appear to have been written for v. 2.4.2
> > as well and not updated (although the version I got through yum a
> > couple days ago is 2.4.5).
> >
> > Can anyone tell me if this is still true? Also, it looks like Bacula
> > works the other way--it keeps writing to a tape until it's full
> > (although I can't tell yet if that's true over multiple jobs). Can
> > anyone confirm that?
>
> Yes, a 'run' is a daily set that may include many filesystems and
> it can automatically schedule a mix of fulls and incrementals
> among those filesystems to make them fit on the available
> tape.  But, it does want a new tape for every run.
>
> > This is for a very small business, with employees who aren't in the
> > office a lot. The idea was to change the tape weekly, but write
> > differentials to it 6 nights a week, and a full backup once a week (or
> > something like that). The tapes are easily big enough to hold that
> > much, and changing them more often would be very expensive.
>
> > Maybe I should just write a script and use tar.
>
> You can give amanda a large amount of holding disk space where
> it will spool the dumps before streaming to tape.  It checks the
> tape label and if the tape it wants is not available, it leaves
> the disk copy until you do an 'amflush'.  If you aren't going to
> take the tapes offsite, you might as well let them sit on the
> holding disk until someone feels like putting in a new tape
> and flushing a week or so.
>
> However, you normally don't use new tapes - you decide how
> long a rotation you need, label that many tapes, and recycle
> them.  10 tapes would probably be good for a rolling set
> changed every weekday, and you can do a separate tar dump to
> different tapes if you want an occasional long-term archive.
> Actually, you'd probably make that 2 tape 'sets' of 5 in
> the amanda config, and then amanda would make sure that
> you get at least one full backup of every filesystem within
> 5 runs along with at least an incremental of each in every
> run.
>

Thanks to Les, Gene, and Charles for the tips, which make good sense.
I've read a lot more documentation and it's sort of making sense to me
now. Two more questions:

1) is there a way to tell Amanda to wait to flush from the holding
disks until there's enough to be worth writing to the tape? Or is the
only way to do this to manually keep the tape out of the drive until a
certain day?

The problem I see with this is, how do you know what day to put it in
so that you are sure to get a full backup on each tape? I realize you
can set dumpcycle to seven days, and runsprecycle to one; even so,
that doesn't mean Amanda does exactly one full backup every seven
days, correct?

2) What is the smart way to give the amanda user read access to all
the files on disk so they can be backed up? It looks like the FC4
packages started this job, by making amanda's group = "disk," but that
doesn't seem to help, really:

[root at matt ~]# su - amanda
-bash-3.00$ cd /home/matt
-bash: cd: /home/matt: Permission denied

Thanks a lot,
Matt