DDS tape size

[Kapil_2]

Is it possible to find out the capacity of a DDS tape through any command?
Also is it possible to find the size of the data stored on the tape?

Kapil

[Justo Exposito]

Hi Kapil,

The capacity depends on the DDS type I,II,III and the compress ratio of your data. This apears in the cover of the tape.

I you have information in the tape stored by a tar you can do a tar tvf in order to extract the directory and the tape blocks by file.

Regards,

Justo.

[Stefan Farrelly]

To find the size of the data on the tape you can try with dd;

dd if=/dev/rmt/ of=/dev/null bs=1024k

This will read the tape in 1 MB blocks and at the end will say;
xxxx blocks in
xxxx blocks out

The xxxx is how many MB on the tape.

As for estimating the capacity of the tape its best to stick to the uncompressed limits first; eg. DDS2 with 120M tape = 4GB, but with 2:1 compression you can get 8GB. The only way to see how much your data will compress down to (eg. ascii text files will compress over 10:1 but movies or jpegs will not even compress at 2:1) is to do a test write to tape and see how many blocks get written before it fails (fbackup will tell you).

[A. Clay Stephenson]

First of all, it is only possible to determine the native capacity of a tape; the capacity of the tape when compression is enabled is data dependent. Secondly, because of slight differences in manufacturing, you are not going to be able to assume that just because 1 DDS3-125m tape has a capacity of exactly 12GB (native) that another DDS3-125m will have exactly that capacity; it will be very close.

You can use the dd command if=/dev/null bs=64k
to determine the capacity if you disable compression.

cpio, frestore, & tar all have commands to list the contents of a tape so that you can see how much data is stored. You would need a bit of awk or perl to add up the file totals.

[Pete Randall]

Published capacities are as follows:

DDS 2: 4GB
DDS 3: 12GB
DDS 4: 20GB

These figures are for uncompressed data.

Pete

[Rodney Hills]

If you go to the HP software archiving centre, you can find a utility called "ddsinfo". For more modern DDS drives it can interogate the hardware and determine actual bytes transfered to a tape, compression ratio, and other useful info.

Hope this helps.

-- Rod Hills