When the optimized blanking strategy has not been configured, Cedar Backup uses a simplistic approach: rewritable media is blanked at the beginning of every week, period.

Since rewritable media can be blanked only a finite number of times before becoming unusable, some users — especially users of rewritable DVD media with its large capacity — may prefer to blank the media less often.

If the optimized blanking strategy is configured, Cedar Backup will use a blanking factor and attempt to determine whether future backups will fit on the current media. If it looks like backups will fit, then the media will not be blanked.

This feature will only be useful (assuming single disc is used for the whole week's backups) if the estimated total size of the weekly backup is considerably smaller than the capacity of the media (no more than 50% of the total media capacity), and only if the size of the backup can be expected to remain fairly constant over time (no frequent rapid growth expected).

There are two blanking modes: daily and weekly. If the weekly blanking mode is set, Cedar Backup will only estimate future capacity (and potentially blank the disc) once per week, on the starting day of the week. If the daily blanking mode is set, Cedar Backup will estimate future capacity (and potentially blank the disc) every time it is run. You should only use the daily blanking mode in conjunction with daily collect configuration, otherwise you will risk losing data.

If you are using the daily blanking mode, you can typically set the blanking value to 1.0. This will cause Cedar Backup to blank the media whenever there is not enough space to store the current day's backup.

If you are using the weekly blanking mode, then finding the correct blanking factor will require some experimentation. Cedar Backup estimates future capacity based on the configured blanking factor. The disc will be blanked if the following relationship is true:

bytes available / (1 + bytes required) ≤ blanking factor
      

Another way to look at this is to consider the blanking factor as a sort of (upper) backup growth estimate:

Total size of weekly backup / Full backup size at the start of the week
      

This ratio can be estimated using a week or two of previous backups. For instance, take this example, where March 10 is the start of the week and March 4 through March 9 represent the incremental backups from the previous week:

/opt/backup/staging# du -s 2007/03/*
3040    2007/03/01
3044    2007/03/02
6812    2007/03/03
3044    2007/03/04
3152    2007/03/05
3056    2007/03/06
3060    2007/03/07
3056    2007/03/08
4776    2007/03/09
6812    2007/03/10
11824   2007/03/11
      

In this case, the ratio is approximately 4:

6812 + (3044 + 3152 + 3056 + 3060 + 3056 + 4776) / 6812 = 3.9571
      

To be safe, you might choose to configure a factor of 5.0.

Setting a higher value reduces the risk of exceeding media capacity mid-week but might result in blanking the media more often than is necessary.

If you run out of space mid-week, then the solution is to run the rebuild action. If this happens frequently, a higher blanking factor value should be used.