Hi All,
Let's suppose I want to do a weekly full backups and daily
incrementals. So I've got:
dumpcycle 1 week
runspercycle 7
A full backup is going to take 2 tapes. I expect that the incrementals
will be very small and will take 1 tape, but that's not guaranteed.
Regarding runtapes, the wiki says:
how many tapes can be used for a single execution of amdump.
I presume that means *any* single execution of amdump, regardless of
whether it is for a full or an incremental backup? In that case, I set:
runtapes 2
Regarding tapecycle the wiki says:
The ideal scenario is for the tapecycle to be at least
(runspercycle + 1) * runtapes.
(I presume that '1' is so to ensure that a full backup is not
overwritten with another full backup that happens to get corrupted
as it is written at the same time as the backup client's disks get
corrupted.)
Fine, then I'll set:
tapecycle 16
Add, say, 2 tapes to that to allow for tape breakages etc, then I
need 18 tapes.
This is more than I will normally need. Normally, a tape cycle of
6 single tapes for incrementals, plus 1 pair of tapes for the full
backup, plus another pair to avoid overwriting the full backup with a
corrupted full backup, i.e. 10 tapes, would suffice. So I *could* set:
tapecycle 10
Again, add, say, 2 tapes for tape breakages etc, then I would
*normally* need 12 tapes.
In the tapecycle wiki page it says:
It is possible to configure Amanda with fewer tapes than
runspercycle * runtapes but this scenario will not allow
Amanda to function properly ...
Okay. I understand that. Then the same wiki page goes on to say:
... and will create at least the possibility of overwriting
data that is still needed for a full restore.
But this I don't understand. If those incrementals were all
unexpectedly large and used 2 tapes each then the fifth incmental
backup would overwrite the full backup... but Amanda *knows* that it
is the only remaining full backup on tape? Err ... ?
I can see that just adhering to the rule:
set tapecycle to be at least (runspercycle + 1) * runtapes
means that Amanda never need to even *consider* that it might be
about to overwrite the full backup, but shouldn't/doesn't Amanda
have a higher priority, namely:
Never overwrite any of the tapes containing the only remaining
full backup
?
Have I missed something? Any advice appreciated! Thanks!
Alexis
