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Parameters
You can set up the following parameters of a Tower of Hanoi scheme.
Schedule
Set up a daily (p. 159), weekly (p. 161), or monthly (p. 163) schedule. Setting up
schedule parameters allows creating simple schedules (example of a simple daily
schedule: a backup task will be run every 1 day at 10 AM) as well as more complex
schedules (example of a complex daily schedule: a task will be run every 3 days, starting
from January 15. During the specified days the task will be repeated every 2 hours from
10 AM to 10 PM). Thus, complex schedules specify the sessions on which the scheme
should run. In the discussion below, "days" can be replaced with "scheduled sessions".
Number of levels
Select from 2 to 16 backup levels. See the example stated below for details.
Roll-back period
The guaranteed number of sessions that one can go back in the archive at any time.
Calculated automatically, depending on the schedule parameters and the numbers of
levels you select. See the example below for details.
Example
Schedule
parameters are set as follows
Recur: Every 1 day
Frequency: Once at 6 PM
Number of levels
: 4
This is how the first 14 days (or 14 sessions) of this scheme's schedule look. Shaded numbers denote
backup levels.
1
2
3
4
5
6
7
8
9
10 11 12 13 14
4
1
2
1
3
1
2
1
4
1
2
1
3
1
Backups of different levels have different types:
Last-level
(in this case, level 4) backups are full;
Backups of
intermediate levels
(2, 3) are differential;
First-level
(1) backups are incremental.
A cleanup mechanism ensures that only the most recent backups of each level are kept. Here is how
the archive looks on day 8, a day before creating a new full backup.
1
2
3
4
5
6
7
8
4
1
2
1
3
1
2
1
The scheme allows for efficient data storage: more backups accumulate toward the current time.
Having four backups, we could recover data as of today, yesterday, half a week, or a week ago.
Roll-back period
The number of days we can go back in the archive is different on different days. The minimum
number of days we are guaranteed to have is called the roll-back period.
The following table shows full backup and roll-back periods for schemes of various levels.
Number of
levels
Full backup
every
On different
days, can go
back
Roll-back
period
