Last week I asked a few questions about virtual tape size. Well, I
wouldn't say I resolved them but I think I have one clue. One of the
things I asked about is the meaning of the "length" parameter for a tape
definition. I had set it to 25G for my virtual tapes but amanda was
writing 38G to each vtape. This was going to be a problem because I
have a quota on my virtual tape file system of 2Tb and I had therefore
calculated that I could create 80 25Gb tapes. Here is what the
amanda.conf man page has to say about the tape length parameter:
> length int
> Default: 2000 kbytes. How much data will fit on a tape, expressed in
> kbytes. Note that this value is only used by Amanda to schedule which
> backupswill be run. Once the backups start, Amanda will continue to
> write to a tape until it gets an error, regardless of what value is
> entered for length (but see amanda-devices(7) for exceptions).
The device for virtual tapes is vfs-disk or something like that. But I
couldn't find anything in the docs for how it determines how much to
write to a vtape. If it keeps writing until it got an error, in my case,
it would write over 2Tb until it hit the hard quota on the filesystem.
So that couldn't be.
Then it occured to me that 25 * 150% = 38G. . Coincidence? I recreated
the vtapes with a size of 16Gb. Now amanda is writing 25Gb to each
tape. My first backup wrote to 11 tapes. Slots 1-10 have 25Gb on them
with 19G on the last. The amount of stuff on tapes 1-10 is not exactly
the same on each tape but it's a little over 24G each.
IMO, this borders on a bug. The wiki strongly implies that you can take
your disk file system size and divide by the number of tapes to get the
tape size. While the amanda developers aren't responsibe for the wiki
being misleading, it is natural to assume the length parameter for a
virtual tape is the actual size of the tape. I'm not the only one who
made that assumption, so did whoever wrote the wiki entry on virtual tapes.
Anyway, I'd still like to know how amanda determines when to stop
writing to a vtape. I can put comments in my amanda.conf to say that the
tape length is set to 16G so amanda writes 24G to 25G to each tape. But
that's hardly an ideal solution.
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John G. Heim, 608-263-4189, [email protected]
