Question : Problem: Calculating space for backup to LTO-2 using EMC Retrospect

Hello.
I have a Dell PowerVault 132T with LTO-2 drive. The main objective is to backup four 1TB hard drives on the host computer. I'm using EMC Retrospect 7.5 for the task.

I have created four Backup Sets, one set per HDD. Each set is three LTO-2 tapes with hardware compression enabled. According to Retrospect I have 1172GB available in each set. Backup script is set to clear the tapes on each backup and backup whatever is on the hard drive. All hard drives are usually pretty much full (max per drive is 931GB).

When the backup script is run, almost by the end of it, when ~60GB is left to backup I get a message that one more tape is required. If I supply that tape, the backup is completed successfully.

I want to know however, why is 3 tapes with compression (400GB per tape) not enough. After all, Retrospect shows a total of 1172GB per set.

Any ideas?
Thanks in advance!

Answer : Problem: Calculating space for backup to LTO-2 using EMC Retrospect

Depending on what you backup, you will hardly ever get a compression ratio of 2 (ie 2x200 = 400GB).
For example, at the moment our two sites have two LTO2 drives with tapes in them, here is what compression ratio they have:
1.72 (344 GB)
1.48 (296 GB)
1.44 (288 GB)
1.78 (356 GB)

The ones at 1.7 are normal word docs, images, excel documents etc.
The ones at 1.4 are database files.

I have also found that the older the tapes get, the lower the compression ratio I get. Some as low as 1.2.