Comparing incremental-by-date, journal-based, and NetApp snapshot difference to full incremental and partial incremental backups

Incremental-by-date, journal-based, and NetApp snapshot difference are alternatives to full incremental and partial incremental back methods.

Incremental-by-date backup

An incremental-by-date backup takes less time to process than a full incremental backup and requires less memory.

An incremental-by-date backup might not place exactly the same backup files into server storage because the incremental-by-date backup:

• Does not expire backup versions of files that you delete from the workstation.
• Does not rebind backup versions to a new management class if you change the management class.
• Does not back up files with attributes that change, unless the modification dates and times also change.
• Ignores the copy group frequency attribute of management classes (Journal-based backups also ignore this attribute).

Journal-based backup

The memory requirements for an initial journaling environment are the same as the memory requirements for a full file space incremental, because journal-based backups must complete the full file space incremental in order to set the journal database as valid, and to establish the baseline for journaling.

The memory requirements for subsequent journal-based backups are much less. Journal backup sessions run in parallel and are governed by the resourceutilization client option in the same manner as normal backup sessions. The size of the journal database file reverts to a minimal size (less than 1 KB) when the last entry has been deleted from the journal. Since entries are deleted from the journal as they are processed by the client, the disk size occupied by the journal should be minimal after a complete journal backup. A full incremental backup with journaling active takes less time to process than an incremental-by-date backup.

On AIX and Linux, journal-based backup does have some limitations. See Journal-based backup on AIX and Linux for information.

NetApp snapshot difference

For NAS and N-Series file servers that are running ONTAP 7.3.0, or later, you can use the snapdiff option to invoke the snapshot difference backup from NetApp when running a full-volume incremental backup. Using this option reduces memory usage and is faster.

Consider the following restrictions when running a full-volume incremental backup using the snapdiff option, to ensure that data is backed up when it should be.

• A file is excluded due to an exclude rule in the include-exclude file. Tivoli Storage Manager runs a backup of the current snapshot with that exclude rule in effect. This happens when you have not made changes to the file, but you have removed the rule that excluded the file. NetApp will not detect this include-exclude change because it only detects file changes between two snapshots.
• If you added an include statement to the option file, that include option does not take effect unless NetApp detects that the file has changed. Tivoli Storage Manager does not inspect every file on the volume during backup.
• If you used the dsmc delete backup command to explicitly delete a file from the Tivoli Storage Manager inventory, NetApp cannot detect that a file was manually deleted from Tivoli Storage Manager. Therefore, the file remains unprotected in Tivoli Storage Manager storage until it is changed on the volume and the change is detected by NetApp, signaling Tivoli Storage Manager to back it up again.
• Policy changes such as changing the policy from mode=modified to mode=absolute are not detected.
• The entire file space is deleted from the Tivoli Storage Manager inventory. This action causes the snapdiff option to create a new snapshot to use as the source, and a full incremental backup to be run.

The NetApp software determines what is a changed object, not Tivoli Storage Manager.

If you run a full volume backup of an NFS-mounted NetApp or N-Series volume, all the snapshots under the snapshot directory might also be backed up.

If you run a full volume backup of a CIFS-mapped NetApp or N-Series volume, all the snapshots under the snapshot directory might also be backed up.

To avoid backing up all snapshots under the snapshot directory, do one of the following actions:

• Run NDMP backups
• Run backups using the snapshotroot option
• Run incremental backups using the snapdiff option
  Tip: If you run an incremental backup using the snapdiff option and you schedule periodic incremental backups, use the createnewbase=yes option with the snapdiff option to create a base snapshot and use it as a source to run an incremental backup.
• Exclude the snapshot directory from backups.
  On AIX® and Linux systems, the snapshot directory is in .snapshot.

  Note: The .snapshot directory is not backed up for some versions of Red Hat Linux, so you are not required to exclude it.

  On Windows systems, the snapshot directory is in ~snapshot.