When selecting expiration of a NetVault backup you have 3 options
1/ ‘Discard After Full Backup Count’: this means keep X number of full backups at all times. It should be noted that your backup media will need space for X + 1 savesets as the backup that is due to expire will not be removed until the current backup has completed so for a short period of time there will be X + 1 backups on media. This is done to make sure that X number of backups are always available and if the backup that is running fails for any reason we still have X number of backups to restore from.
When creating incremental backups from a full backup it is best to select the same number of generation’s expiry count and ‘Don’t Discard based on Time’.
You could however select both ‘don’t Discard After Full Backup Count’ and ‘Don't Discard based on Time’ and the incremental backup will expire when the tied full on which it is based expires.
However if you are duplicating the full with a different expiry which is a time based expiry you must state the number of generations you wish the incremental backup to expire or the incrementals will tie to the duplicated full backup and will not expire until the duplicated backup expires.
2/ ‘Discard After’ time based expiration date. The full backup will expire after X weeks, days, months.
When setting the incremental retirement, the retirement should be set to the same as the full backup (on which it is based) it is tied to. If the retirement period of any incremental is less than that of the full it is tied to, then, when it expires all incremental backups will retire too. This is because if an incremental backup in the chain retires before the full it is tied to then the sequence of full-incremental-incremental backups, that form a chain, is broken and the backups became effectively of no use and therefore must be retired.
You could however select ‘Discard after Full Backup Count’ and ‘Don't Discard based on Time’ and the incremental backup will expire when the tied full expires. However if you are duplicating the full, with a different expiry with time based expiry, you must state the expiry date you wish the incremental backup to expire or the incrementals will be tied to the duplicated full backup and will not expire until the duplicated backup expires.
3/ it is possible to select both options ‘Discard After Full Backup Count’ and ‘Discard After’ time based expiration date; the rules above still apply but this works on an “OR” basis which means whichever occurs first i.e. the full backup count or the expiration time - will expire the backups.