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03 Sep 2010

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	Calculating the size of backup indices

Solution Organisation basics

PresSTORE maintains for each file in the backup the tape it is stored on and the time stamp of that file. This storage is done in the backup index.
The backup index consists of multiple file sets, each one holding the data for one backup cycle.
The files where the backup index data is stored in subdirectories below the PresSTORE home directory.

The index data is kept as long as files from the according cycle can be recovered from the tape, that is, as long as the tape exists. The index data file is kept as long as there exists as least one volume of that index cycle.

Calculating the backup index size

An index entry requires approximately 1 KB of data. Example:
For 100.000 files over 4 backup cycles, the total amount of space is approximately 100000 * 4 * 1 KB = 400 MB.
The number of 100.000 files represents approximately "all files of a workstation computer".
Note that the index size depends on the number of files, not on the amount of data.
To figure out, how many files are on the disk, execute the following command from a terminal window in the root directory of the volume to be backed up
On Mac, Linux and Solaris:
ls -R | wc -l
On Windows (regard the last two output lines):
dir /s

Pitfalls

In case hughe disks are to be backed up, the disk space in the PresSTORE directory must be sufficient for the indices.
In case a backup is missused as archive and the backup tapes are kept forever, note that the index files are kept forever, too.
In case old tapes of the media pool are recycled (when the cycle is no longer required) the according indices are cleared when the last tape of the particular cycle is recycled.
In case tapes are taken off the system and kept elsewhere, the according index files are kept automatically.

Note: When erasing the tapes outside of PresSTORE, make sure the according volume is cleared from the volume manager.

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05 Jul 2007 02:12 PM