We help IT Professionals succeed at work.

We've partnered with Certified Experts, Carl Webster and Richard Faulkner, to bring you a podcast all about Citrix Workspace, moving to the cloud, and analytics & intelligence. Episode 2 coming soon!Listen Now


Calculating space for backup to LTO-2 using EMC Retrospect

alexkink asked
Medium Priority
Last Modified: 2013-12-01
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!
Watch Question

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.

Not the solution you were looking for? Getting a personalized solution is easy.

Ask the Experts


I see. I didn't realize that the compression on LTO is not guaranteed x 2 but rather varies. Thank you very much!
Access more of Experts Exchange with a free account
Thanks for using Experts Exchange.

Create a free account to continue.

Limited access with a free account allows you to:

  • View three pieces of content (articles, solutions, posts, and videos)
  • Ask the experts questions (counted toward content limit)
  • Customize your dashboard and profile

*This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.


Please enter a first name

Please enter a last name

8+ characters (letters, numbers, and a symbol)

By clicking, you agree to the Terms of Use and Privacy Policy.