hardware compression does not work

Question

How do I get hardware compression to work against my PowerVault 100T under Linux? I have a directory taking up 37 GB of disk space. I own a PowerVault 100T DAT 72 which (I think) is suppose to be able to save 36 GB of data uncompressed to the tape and 72 GB of data compressed. I use the Linux mt command to turn on compression: mt -f /dev/st0 compression 1 I then use tar to save to tape: tar cvf /dev/st0 /path/to/data The data saves, saves, and saves, but at the end it says 'device full'. I find that very hard to believe, especially if the data is suppose to be compressed. I only have 1 GB of data over the size of the tape (Dell DAT72-170M). My data is plain text. No binaries. No images. No compressed files. How can I tell whether or not compression is really turned on? What am I doing wrong?

at5147 · Answer

General rule is if it is over the native capacity of the media, then compression is working. I am not aware of any tools for Linux to check the status of hardware compression, however you can check the dip switch on the back of the tape drive to confirm it is on as well. Also, if the backup software allows you to set the job to use hardware compression only, no software, and you are still going over 1GB of the native capacity, then that is a good sign compression is working.

ericleasemorgan · Answer

Thank you for a prompt reply.

Yes, hardware compression does seem to be turned on by default. Thus, when I tar 37GB of data to a 36GB tape the end results returns no errors and all the data is intact. On the other hand, if it turn on compress (mt -f /dev/st0 compression 1), the compression+compression seems to happen, and the tape fills up.

While I consider this problem resolved, I feel as if the hardware was doing me a favor I didn't ask for, and/or I was making the process more difficult than it needed to be.

Again, thank you very much.

PowerVault

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