If your cell is like most cells, you have a limited amount of room for storing backup tapes and a limited budget for new tapes. The easiest solution is to recycle tapes by overwriting them when you no longer need the backup data on them. The Backup System helps you implement a recycling schedule by enabling you to associate an expiration date with each dump level. The expiration date defines when a dump created at that level expires. Until that time the Backup System refuses to overwrite a tape that contains the dump. Thus, assigning expiration dates automatically determines how you recycle tapes.

When designing a tape-recycling schedule, you must decide how far in the past and to what level of precision you want to guarantee access to backed up data. For instance, if you decide to guarantee that you can restore a user's home volume to its state on any given day in the last two weeks, you cannot recycle the tape that contains a given daily dump for at least two weeks after you create it. Similarly, if you decide to guarantee that you can restore home volumes to their state at the beginning of any given week in the last month, you cannot recycle the tapes in a dump set containing a weekly dump for at least four weeks. The following example dump hierarchy implements this recycling schedule by setting the expiration date for each daily incremental dump to 13 days and the expiration date of the weekly full dumps to 27 days.

The tapes used to store dumps created at the daily incremental levels in the /sunday1 hierarchy expire just in time to be recycled for daily dumps in the /sunday3 hierarchy (and vice versa), and there is a similar relationship between the /sunday2 and /sunday4 hierarchies. Similarly, the tape that houses a full dump at the /sunday1 level expires just in time to be used for a full dump on the first Sunday of the following month.

   /sunday1   expires in 27d
           /monday1  expires in 13d
           /tuesday1  expires in 13d
           /wednesday1  expires in 13d
           /thursday1  expires in 13d
           /friday1  expires in 13d
   /sunday2   expires in 27d
           /monday2  expires in 13d
           /tuesday2  expires in 13d
           /wednesday2  expires in 13d
           /thursday2  expires in 13d
           /friday2  expires in 13d
   /sunday3   expires in 27d
           /monday1  expires in 13d
           /tuesday1  expires in 13d
           /wednesday1  expires in 13d
           /thursday1  expires in 13d
           /friday1  expires in 13d
   /sunday4   expires in 27d
           /monday2  expires in 13d
           /tuesday2  expires in 13d
           /wednesday2  expires in 13d
           /thursday2  expires in 13d
           /friday2  expires in 13d

If you use appended dumps in your cell, keep in mind that all dumps in a dump set are subject to the latest (furthest into the future) expiration date associated with any of the constituent dumps. You cannot recycle any of the tapes that contain a dump set until all of the dumps have reached their expiration date. See also Appending Dumps to an Existing Dump Set.

Most tape manufacturers recommend that you write to a tape a limited number of times, and it is best not to exceed this limit when recycling tapes. To help you track tape usage, the Backup System records a useCount counter on the tape's label. It increments the counter each time the tape's label is rewritten (each time you use the backup labeltape or backup dump command). To display the useCount counter, use the backup readlabel or backup scantape command or include the -id and -verbose options when you issue the backup dumpinfo command. For instructions see Writing and Reading Tape Labels or Displaying Backup Dump Records.

Even if you make extensive use of tape recycling, there is probably some backup data that you need to archive for a long (or even an indefinite) period of time. You can use the Backup System to archive data on a regular schedule, and you can also choose to archive data on tapes that you previously expected to recycle.

If you want to archive data on a regular basis, you can create date-specific dump levels in the dump hierarchy. For example, if you decide to archive a full dump of all data in your cell at the beginning of each quarter in the year 2000, you can define the following levels in the dump hierarchy:

   /1Q2000
   /2Q2000
   /3Q2000
   /4Q2000

If you decide to archive data that is on tapes you previously planned to recycle, you must gather all of the tapes that contain the relevant dumps, both full and incremental. To avoid accidental erasure, it is best to set the switch on the tapes that makes them read-only, before placing them in your archive storage area. If the tapes also contain a large amount of extraneous data that you do not want to archive, you can restore just the relevant data into a new temporary volume, and back up that volume to the smallest number of tapes possible. One reason to keep a dump set small is to minimize the amount of irrelevant data in a dump set you end up needing to archive.

If you do not expect to restore archived data to the file system, you can consider using the backup deletedump command to remove the associated dump records from the Backup Database, which helps keep it to an efficient size. If you ever need to restore the data, you can use the -dbadd flag to the backup scantape command to reinsert the dump records into the database. For instructions, see To scan the contents of a tape.

To associate an expiration date with a dump level as you create it, use the -expires argument to the backup adddump command. To change an existing dump level's expiration date, use the -expires argument to the backup setexp command. (Note that it is not possible to change the expiration date of an actual dump that has already been created at that level). With both commands, you can define an expiration date either in absolute terms (for example, 13 January 2000) or relative terms (for example, 30 days from when the dump is created).

If you omit the -expires argument to the backup adddump command, then the expiration date is set to UNIX time zero (00:00 hours on 1 January 1970). The Backup System considers dumps created at such a dump level to expire at their creation time. If no dumps in a dump set have an expiration date, then the Backup System does not impose any restriction on recycling the tapes that contain the dump set. If you need to prevent premature recycling of the tapes that contain the dump set, you must use a manual tracking system.