This Article explains how Netvault Incremental NDMP backups work and how the DUMP levels determine what data is backed up.
To understand how NDMP Incremental backups work we need to explain how NDMP Full backups work and what happens on the filer we are backing up, as they work almost exactly the same way. When we backup from a filer using NDMP, we use a local filer DUMP command, and the functionality will be restricted to the limitation of the DUMP program on the filer operating system.
When we do a full backup we actually issue a dump level 0 command that will backup all files on the desired volume or directory structure selected in the NDMP GUI backup window. Before this command can be executed a snapshot of the desired source Volume or directory structure is created, enabling us to backup any open or active files as the backup will be from the point in time the snapshot is created.
Incremental backups use the remaining Dump levels 1- 9, and a list of volumes and top level directories that have been backed up with the date, time and dump level is kept in the /etc/dumpdates file. Using these dump levels we can create daily Incremental and Differential incremental backups.
Note: We cannot consolidate incremental NDMP backups
Example 1 :
To run a daily incremental backup for a week, run a full backup (dump level 0) on the Sunday. When running the incremental backup, use dump level 1 on Monday, dump level 2 on Tuesday, dump level 3 on Wednesday and so on until Sunday comes round again when you run the full dump level 0 again .
This backup sequence will perform a full backup on Sunday, a daily incremental on Monday ( files that changes since last backup), then you peform daily incrementals until the next full backup.
To run a differential backup for a week, run a full backup (dump level 0) on the Sunday then for the rest of the week run differential incremental backups (dump level 1) every day. Every day, this differential incremental backup will backup all data modified since last full.
You may want to perform a mixture of the two. Run a full backup (level 0 dump) on Sunday, followed by a level 4 dump on Tuesday, which will be a daily incremental. Then run a dump level 5 as another daily incremental and so on until Friday. This means you would have run dump levels 4 - 8 as daily incrementals. On the Saturday you the run a dump level 3. This dump would then contain all files modified or added to the filesystem since the "level 0" (Sunday) backup.
For any complete restore, you would start with the restore of the lowest dump level and run seperate jobs for all subsequent dump levels. Once the dump level 0 restore has run, a file called restore_symboltable will be created by the filer in the root of the subtree that is being restored. This file contains information necessary for the restore of the subsequent incremental dump levels. If it is not present, these restores will fail.