Red Hat Enterprise Linux AS release 4 and HP SureStore DAT40 Tape Drive

[Jonathan Southwick]

It doesn't appear compression is running on our drive. We are dumping 3 volumes (~6MB, ~6GB, and ~21GB) to a DDS4 tape. Native storage on a DDS4 is 20GB so with 100% compression we should get 40GB. We are between that with about 27GB.

How do we know if compression is running on the drive?

I did try 'mt -f /dev/nst0 compression on' before the dump started and made sure the tape was rewound but we are still tunning out of tape when the last volume is getting dumped:

DUMP: 79.26% done at 3111 kB/s, finished in 0:24

Any ideas?

Jonathan

[Marino Meloni_1 · Accepted solution]

The Native capacity of a DDS4 is in fact 20GB as you said, but with compression 1:1 (we do not use %) you can get only 20 GB. the 40GB can be reached if your data can be compressed 2:1, nowadays it is difficult to reach this value.

You can test the compression capability with LTT, in alternative, you can try to backup the 21GB data, if you can store more than 20 GB, this mean the drive is compressing.