Friday, March 13, 2009

Progressive Incremental Backup

First, I apologize for the delay in getting a post on here. I will try to be a little quicker on updates. I have had a hard week with long hours. Haven't even been on TSM stuff. It has been all DB2/400 and Oracle related. But enough of that....

When TSM says it is doing an incremental backup it does not do a normal incremental. Most incremental backups store any file that has changed since the last full backup. Then each incremental that follows backs up that file again. With this method, each incremental backup gets a little larger than the previous and takes a little longer to complete. As an example, take four files file1, file2, file3, file4.
  • Full backup - file,1, file2, file3, file4
  • File1 changes
  • First incremental - File1
  • File2 changes
  • Second incremental - File1 and File2
  • File3 and File4 change
  • Third incremental and all subsequent - File1, File2, Fle3, File4

So at this point you might as well be doing full backups on each pass. This will conitue until the next full backup. When Tivoli Storage Manager does a backup it goes like this:

  • First backup - File1, File2, File3, File4
  • File1 changes
  • First incremental - File1 (original copy of file1 marked inactive)
  • File2 changes
  • Second incremental - File2 (original copy of file2 marked inactive)
  • File3 and File4 change
  • Third incremental - File3, File4 (original copies of file3 and File4 marked inactive)
  • File1 changes
  • Fourth incremental - File1 (Second copy of File1 is marked inactive)

As you can see, the Progressive Incremental that TSM uses is much more effecient than the than a true incremental. In fact there is never a need for a full backup. You can force one, but the incrementals keep a full set of the current files on the system along with a configurable number of inactive files.

How does TSM keep track of what is new and what has been backed up before? Through it's database. It actually checks every file on the system during an incremental backup and only copies the ones to it's storage pool volumes that have changed. This reduces the amount of traffic on the network while keeping a complete set of the computers data available for restore in the event of loss.


  1. The example for incremental backup that you explained is differential backup, isn't it?