Autzoo.717
hacknews
utzoo!henry
Mon Jun  8 22:42:40 1981
backup tapes
After some contemplation, the rotation scheme for backup tapes has been
settled.  The V6 scheme (a modified towers-of-hanoi algorithm) has
two defects:  it thins out too rapidly as one moves into the past, and
it tends to overwork the tapes in the short cycles.  The approach that
I have come up with is a triple-cycle scheme, working as follows:

The primary cycle is 5 tape sets, numbered (accidentally) 12 through 16,
providing 4-5 weeks of weekly backups.  They are simply used
in rotation, one each week.  Every FOUR weeks, however, the tape set used
is not the one predicted by the rotation but a new one;  the predicted
set is retired into the secondary cycle, acquiring a number in the 20's.

The secondary cycle is 7 tape sets, numbered 20 through 26, providing
6-7 months of monthly backups.  Every four weeks, a set from the primary
cycle is retired to the secondary cycle, displacing the oldest member of
the secondary cycle.  Most of the displaced sets are recycled into the
"tapes for sale" bin;  every sixth displaced set, however, acquires a
number in the 30's and goes into the tertiary cycle.

The tertiary cycle, whose length is somewhat undecided at this time,
provides N years of semi-annual backups.

All this has been automated nicely and will be documented shortly.