On Feb 3, 2009, at 8:01 AM, Elizabeth Schwartz wrote:
>> -full backup every 56 days (8 weeks)
>> two rotations of:
>> - 1,2,3,4,2,3,4 ; 2,3,4,5,3,4,5 ; 3,4,5,6,4,5,6 ; 4,5,6,7,5,6,7
> so anyway, what do you think of this schema?
That doesn't look like every 8 weeks, it looks like every
four. What happens on the first day of the second
4 weeks if it's not another 1?
There are three things to look at when you evaluate a backup
schedule; cost to back up, cost to restore, and risk. Without
knowing when you're recycling tapes (is every 2 onto the same
tapes, or are you keeping all of those tapes?) risk is
hard to evaluate. However, it's pretty high, because you can't
directly restore an incremental without the level it depends on.
So any time you have 6 tapes to restore from, you are at the very least
risking time-consuming nightmares 3 times as often as when you
have 2 levels. And you are also risking data loss 3 times as often.
Failure (loss, destruction) of a tape is in my experience much more
common than failure of a disk.
Cost to restore is easier. If you have indexes, cost to restore an
individual file is relatively invariant no matter what you do.
If you don't have an index, trying to figure out what tape the
file is on is going to really hurt. Cost to restore a whole file
system? Ouch, ouch, and more ouch. On average across those 4 weeks,
you have to mount 4 tape sets to do the restore. That's a significant
cost in both elapsed time and people time. As in, you're probably
upping the time to do a full restore by a day. And you're maximizing
your chances of getting something wrong in the process. (For instance,
if you keep multiple level 2s -- and it would be awfully risky not to --
you get the chance of restoring the wrong one, and having to go back and
restore the correct one before you can continue. Rinse and repeat for
each level.)
I would really try to get the number of levels down. And, for a
university setting, I would look at patterns of usage around the
term and plan for those (for instance, it would be really,
really good to have fast, easy restores near the time dissertations
are due if you serve graduate students -- otherwise, there will be
tears and shouting and people crossing the street to avoid you,
something
I personally would rather never do again).
Elizabeth
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