There is an interaction between differential backups and other incremental or differential backups that have been taken since the last full backup (regular or synthetic). It is helpful to keep this interaction in mind when creating a new differential backup.
When producing a differential backup, the Commvault software compares the current contents of the client with the contents of the last full backup for that client. The resulting backup contains all data that has changed or been added since the last full backup. If other incremental or differential backups have been produced after the last full backup, but before the current differential backup, it is possible to lose some backed-up data.
Consider this diagram:
The incremental backup (number 2) backs up a new version of file A because it has changed since the last full backup. The first differential backup (number 3) backs up file D because it has been created since the last full backup was taken. When the second differential backup (backup number 4) is created, it carries forward file A version 2, but does not carry forward file D because it was deleted before backup number 4 was taken. The synthetic full backup (number 5) contains versions 0 and 2 of file A (but not version 1) and does not contain file D even though it existed on the client with backup number 3 was taken. File A version 1 can still be restored from backup 2, and file D can still be restored from backup 3, unless the Do not retain intermediate incremental and differentials before a new differential option is selected.
Important
Although version 1 of file A and file D can be restored from backups number 2 and 3, selecting the Do not retain intermediate incremental and differentials before a new differential option would render them unrestorable. This option saves media by aging intermediate incremental and differential backups once a succeeding differential backup has been created, but without those intermediate backups, version 1 of file A and file D cannot be restored.