Compression on LTO3 tapes

[Mark_Casemore]

We have just upgraded to LTO3 tape drives, and as far as I am aware the tapes have a native capacity of 400Gb while having a compressed capacity of 800Gb. Unless I am mis-understanding how compression works, I ran the nbemmcmd to see how much data has been written to the tapes and 400Gb is being written, why cannot 800Gb be written to tape? We are writing our client backups to a SAN, then streaming the images to tape (if that makes a difference), the policies have compression ticked. Is it that once the initial backup from the client is compressed, it then will not compress the images when streaming from SAN to tape.

[Stumpr2]

> the policies have compression ticked

kindly remove the tick

I never enable software compression. I let the tape drives do the work. You want to get the data as fast as possible to the tape drives in order to get them streaming. Software compression slows the throughput and may also be counter productive by creating negative compression on some files due to the compressing of compressed files factor :)

BTW: I like Dennis' answer. You may consider giving him some points. He's very close to becoming a master.Message was edited by:
Bob Stump

[Mark_Casemore]

Will this then utilise the full 800Gb instead of 400Gb ? Thanks.

[Dennis_Strom]

800 is optimal or an average. It depends on the data. You will get better hardware compression if you do not do software compression. Hardware compression is a bit faster and takes some load off your clients. It will however move some of that load to the network since there will be additional data going across it.
Different data compresses differently. If you have databases especially large ones you will see better than 2:1 compression. jpegs, gifs, zips, and tar files can actually get a bit larger (not always) if compressed a second time. text files will compress down about 2:1, here again it depends on the size of the file. So to answer your question yes will see 800 gig. But then again you may see less and you may see more.

[The_Director]

I have some tapes that I see 1.4TB on and other that barely get 400GB.... It all depends on the data being backed up to the tape. 1.4TB tapes are deceiving because its all raw oracle db stuff that is a lot of empty space. The 800GB is optomal hardware compression. If you are backing up millions of image files with a large folder structure you will be lucky to get over 5-600GB.

Dont use the software compression you can select within NetBackup. The hardware compression of 800GB will be done automatically at the tape drive level.

[Chia_Tan_Beng]

wow, I almost confused that the one who posted answered the question.

Hi Mark Casemore, do you find their reply helpful or relevant? Points are always welcome :)

Just another 0.02 cents worth of input. Check your version of device driver. In some case correct/later version may improve drive performance as well.