About media overwrite protection
Each media is associated with a media set, which is a set of rules that manage media.
These rules include the following:
Table: Rules specified in the media set
The amount of time that data can be appended (added) to media. It is measured from the time the media was first allocated. It can be specified in hours, days, weeks, or years.
Overwrite protection period
The amount of time that media is protected from being overwritten. It is measured from the time of the last write to the media, that is, at the end of the last append or overwrite job. It can be specified in hours, days, weeks, or years. When the overwrite protection period is over, the media becomes recyclable and can be overwritten.
The overwrite protection period begins when the backup job is completed. If there is an append period, the overwrite protection period begins again each time an append job completes. Because the overwrite protection period does not begin until the job completes, the amount of time that the job takes to complete affects the amount of time until the media can be overwritten. You may shorten the overwrite protection period to take into account the amount of time a job may run.
For example, setting the overwrite protection period for seven days and the append period for four days ensures that data will not be overwritten for at least seven days, and that data can be appended to the media for the next four days. The last data appended to this media is retained for at least seven days.
Your media rotation strategy must balance between your need to save useful data as long as possible, and the fact that media are not in infinite supply. The compromise between the longevity of stored backup data and the cost of more media is controlled in Backup Exec by the rules specified in the media set, which allows Backup Exec to identify which media can be written to and which media is overwrite-protected.
The following graphic shows the relationship between the append period and overwrite protection periods.
Figure: Append periods and overwrite protection periods
The append and overwrite protection periods that you specify apply to all the data on the media.
Each time data is written to a media, the time remaining in the overwrite protection period is reset and the countdown restarted.
Figure: How overwrite protection periods are reset
Because the overwrite protection period does not begin until the job completes, the amount of time that the job takes to complete affects the amount of time until the media can be overwritten.
For example, suppose that you create a media set named Weekly with an overwrite protection period of seven days, and an append period of 0 days, and you schedule a full backup job to run each Friday at 20:00. When it is time for the full backup to run at 20:00 the following Friday, the job cannot run because the first backup job that ran the previous Friday did not complete until 21:10 p.m. The overwrite protection period for the Weekly media set still has 70 minutes remaining.
Typically, to prevent this situation, you would shorten the overwrite protection period to account for the amount of time a job may run. For this example, the scheduled job recurring at 20:00 can run if the overwrite protection period is set to 6 days instead of 7 days.