Thanks to Kevin and Paul for responses.
We use a modified Tower of Hanoi scheme (on top of rsync and --link-dest
and recycling) for deciding which backups to keep.
Here is a sample of our holdings for one area:
home.20111124.seq.0 set 0
home.20130512.seq.512 set 10
home.20140203.seq.768 set 9
home.20140414.seq.832 set 7
home.20140708.seq.896 set 8
home.20140815.seq.928 set 6
home.20140831.seq.944 set 5
home.20140904.seq.948 set 3
home.20140908.seq.950 set 2
home.20140909.seq.951 set 1
home.20140910.seq.952 set 4
home.20140911.seq.953 set 1
home.20140912.seq.954 set 2
Found 13 backups as expected up to sequence number 954
>>>>Marking for recycling home.20140908.seq.950, set number 2
The coverage matches the likelihood of restorations being required - the
coverage tails off exponentially over time.
I found the key to running a Tower of Hanoi scheme is to assign a
sequence number to each backup, from which you can derive a set number.
The dates are for humans - scripts don't have to deal with days of week,
days of months, etc....
We keep one of each set number, except we keep two set ones, and a
'set 0' as a base set.
With daily backups, any file that has existed for a period of more than
about 1.5*n days in the last 2*n days will be covered, with better
coverage than that for recent files.
A strategy to keep two of every set will easily provide cover for every
file that existed for n days in the last 2*n days.
Kevin: you could use Tower of Hanoi for managing your snapshots... :-)
Regards
Rob.
Dr Robert C. Bell
HPC National Partnerships | Scientific Computing
Information Management and Technology
CSIRO
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