AIT 35GB drive

[Gerry Sullivan]

I am using an AIT 35/70GB tape drive to back up one of our servers. When compressed, only about 51GB is written on the tape. After that, we need to insert a 2nd tape. I realize that it is not always possible to achieve 2 times the data even with compression, but is there anything I can do to increase the amount of data backed up on a single tape to about 60GB?

[Brian M Rawlings · Accepted solution]

Back up different data.

Sorry, that's a little flippant, but that's really the answer. Compression is dependant on the type and mix of data being backed up.

You are only ever guaranteed to get the native capacity, in this case, 35GB, on any given tape. Anything more than that is good, but nothing more is for sure.

You can try it yourself on your PC, using PKZIP (or winzip). Zip a .exe file (binary executable), and you'll generally get 1-3% compression. Zip an HTML or text file -- forms and legal docs are best... you'll get 3:1, 5:1, sometimes 10:1 (depending on if there is white space or repeating characters in the text).

Try zipping a JPEG or MPEG file (which are already compressed), and you may get a few percent, no compression, or (rarely) the "compressed" version may actually be bigger than the original.

Now imagine a system-wide backup. It is hitting all these data types as it trawls through all the different directories. Some binary, some text, some compressable, some not. If you back up multiple systems, each one will have a different overall compression ratio, since they each have different data in different amounts.

Your case, where you are getting ~ 1.5:1 compression, is much more common (in my experience) than the semi-mystical 2:1 compression that is commonly stated as the "compressed capacity". That is marketing BS, plain and simple, nobody can predict what compression you will get. This is a "try it and see" deal.

Sorry, I'm rambling. But there you have it, be happy with what you've got, 1.5:1 ain't bad at all.

Oh, having said that, there are different compression algorithms out there, some compress more but take longer to do it. It is not out of the question that you could turn off the HW compression in your tape drive (using a different special device file in HP-UX, or a selection in Windoze), and run a command line utility that would compress directories with some killer compression algorithm, then pipe them into your backup command line, or something. But it would be semi-supported, a challenge to implement and keep up, and would probably slow down your backup to a crawl. I wouldn't suggest it.

Lastly, with Commvault Galaxy, you can do compression in the backup agent, and turn off compression in the tape drive. I have no idea if this would give you better than 1.5:1 compression, the same "try it and see" situation exists (your mileage will vary from mine, or from any test lab's). But it is the only other option I can think of to attempt to improve compression. They say it is "very good compression". I say, "Show Me".

Regards, --bmr