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Copyright © Acronis, Inc., 2000-2010
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
2
2 days
1 to 2 days
1 day
3
4 days
2 to 5 days
2 days
4
8 days
4 to 11 days
4 days
5
16 days
8 to 23 days
8 days
6
32 days
16 to 47 days
16 days
Adding a level doubles the full backup and roll-back periods.
To see why the number of recovery days varies, let us return to the previous example.
Here are the backups we have on day 12 (numbers in gray denote deleted backups).
1
2
3
4
5
6
7
8
9
10 11 12
4
1
2
1
3
1
2
1
4
1
2
1
