I'm looking to check the compression ratio applied by hardware to work out how much free space we have on tapes.
mt -f /dev/rmt0 status shows:
If I add up the output of the du command and what we backup from the script, it gives me the total space we tape up. So the plan was to find the compression ratio applied and apply that to the size I have noted for each backup. Then simply subtract this from the size of the tape. Although, perhaps there is an easier way to see the free space on a tape?
I found the reason for the backup sizes changing, was due to /proc - but still trying to find out what compression is being applied, and if we apply it at all. It seems like it is applied (2:1), but I want to make 100% sure because we are going to be backing up more to one tape and I need to make sure we don't try to backup more than what we have space on the tape for.
compression ratio can change for type of data..
maybe you can try to add new files while mt fails and determine the space.
(2:1) ratio is can be avarage ratio not general ratio..
In my experience "hardware compression" is a load of bull, mostly used to bloat the advertised capacity of a tape drive. If your data isn't trivially compressible you'll get no dependable gain from it.