can you explain compression to me please!!!! confused! 1

[StressedTechie]

Hi guys

Just need a little bit of advise really. I am confused about Tape compression.

Our It support firm has setup a drive pool in Symantec Backup Exec 10d. We have a requirement to backup 70 Gig a night. We are using Compaq DLT8000 drives and 40/80 DLT IV tapes. So this means we can backup 40 gigs uncompressed per tape and 80 compressed or so I thought.

Why is the backup job changing to drive 2 at the 45 gig compressed mark and then continuing on drive 2. Its not a problem because it working and I can successfuly restore. The problem lies with the number of tapes required for a successful rotation. I used to have a two week rotation of dailys and 4 week rotation of End of week tapes and 4 End of Month tapes. This has now been gobbled up and I now only have enough tapes for one week rotation plus a two weeks EOW.

I am currently accessing whether or not it is worth purchasing another 14 or so tapes or biting the bullet and going for a whole new backup solution like Ultrium drives and tapes.

I am not fully understanding how the compression on these drives is working. If I can only fit 45 compressed gig on each tape I am gonna fill that allowance pretty quickly and spending cash on new tapes for an obsolete tape drive is starting to sound a little stupid.

Anyway just wanted some advice and maybe your views.

Thanks

 

[LWComputingMVP]

From my web page on backup:

http://www.lwcomputing.com/tips/static/backup.asp#software

Do NOT base your capacity and tape requirements on the advertised compressed capacity of a tape. Most people will find the supposed 800 GB LTO3 tape only holds 550-650 GB compressed. Your mileage WILL vary based on the type of data being backed up. I have backed up a wide variety of data over the years and NEVER seen a tape come CLOSE to a 2:1 compression ratio as advertised. The amount of compression attained will be highly dependant upon what you are backing up. If you are backing up MPEG, JPG, MP3 and ZIP files, they are already highly compressed and will not compress much if at all when backing up. It is even possible that such highly compressed files will take more space on a backup medium than they do normally. Files such as large text files, databases, and uncompressed graphics will compress much better and if that is all you are backing up, you might see compression ratios much closer to the advertised 2:1.

 

[StressedTechie]

Thanks LW
Very useful information there and has cleared my backup confusion.

Thanks
ST