How Backup Exec stores catalog information on a tape

Article:TECH5261  |  Created: 2005-01-09  |  Updated: 2013-04-09  |  Article URL http://www.symantec.com/docs/TECH5261
Article Type
Technical Solution

Product(s)

Issue



How does Backup Exec store catalog information on a tape


Solution



When backing up, Backup Exec stores information to media in the following sequence:

In this example, the backup is of drives C:, D: and E. The backup operation is an overwrite:

Tape Header
Filemark
Data Set #1: C:
Filemark
Catalog for Data Set#1, w/ Set Map which contains only location of Set#1
Filemark
Data Set #2: D:
Filemark
Catalog for Data Set#2, w/ Set Map which contains locations of Sets # 1 and 2
Filemark
Data Set#3: E:
Filemark
Catalog for Data Set#3, w/ Set Map which contains locations of Sets#1, 2, and 3
Filemark
EndOfData

So, the catalog for each set is stored immediately after the set. As part of that block of information, Backup Exec also stores a Set Map which tells us, by going to the last one on the tape, how many sets are on the tape, and where they are located.


What happens during an append operation?

In this example, a backup of drive F: and G: is appended to the original media.

Tape Header
Filemark
Data Set #1: C:
Filemark
Catalog for Data Set#1, w/ Set Map which contains only location of Set#1
Filemark
Data Set #2: D:
Filemark
Catalog for Data Set#2, w/ Set Map which contains locations of Sets # 1 and 2
Filemark
Data Set#3: E:
Filemark
Catalog for Data Set#3, w/ Set Map which contains locations of Sets#1, 2, and 3
Filemark
Data Set#4: F:
Filemark
Catalog for Data Set#4, w/ Set Map which contains locations of Sets#1, 2, 3, and 4
Filemark
Data Set#5: G:
Filemark
Catalog for Data Set#5, w/ Set Map which contains locations of Sets#1, 2, 3, 4 and 5
Filemark
EndOfData

The Set Map is always cumulative. When performing an append operation, Backup Exec goes to EndOfData (EOD), skips back a little to read the last Set Map on tape, moves forward past the last Filemark, proceeds with the new backup, drops FM, writes the set's catalog, appends the Set Map information read at the beginning of the operation, and of course, adds an entry for the Set just completed, drops another FM, and it's done.

If another volume is being backed-up as part of the operation, Backup Exec immediately just starts writing the new set's data.

One other point of potential interest: if a tape family consists of three tapes, the Set Maps include all the sets residing on the prior tapes as well.

Just remember that the Set Map after each dataset will contain information on sets 1 through x, x being the set after which the Set Map appears, regardless of how many tapes are in the family.

 



Legacy ID



191654


Article URL http://www.symantec.com/docs/TECH5261


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