Kjell Rilbe wrote:

> First day of year: N=0, initial complete backup.
> First day of each month: N=1, will contain all pages changed since first
> day of year.
> First day of each week: N=2, will contain all pages changed since first
> day of month.
> Each day: N=3, will contain all pages changed since first day of week.
>
> If two such days coincide, you still need to run both "colliding" levels
> (lower N first, higher N directly afterwards), or the sequence will be
> broken next day.

That's not necessary, and the higher level backup will add nothing on
that moment.

Suppose you make a level-2 backup every Sunday. Then it makes sense to
schedule the level-3 backups daily from Mon-Sat.

If you want to make a point-in-time restore later, it will involve 4 files
if that point in time is on Mon-Sat, and 3 files if it is on a Sunday.
(And 2 files if it is the first day of the month.)

All you have to do is determine the most recent backup before the chosen
point in time. If that is a level N, you need N+1 files for the restore
(levels 0-N, each one being the most recent file of that level before
time 'T').


Cheers,
Paul Vinkenoog