the problem with compression

[pkb]

So I understand that compression is a bad thing, so I'm trying to disable it. On CentOS 5.4, and using an HP MSL2024 Tape Library, I'm trying:

# mt -f /dev/nst0 compression 0
/dev/nst0: Input/output error


And also trying to disable the default compression setting using:

# mt -f /dev/nst0 defcompression 0


Any advice on the first error?

Yet the same commands return no error when used against a single Dell PowerVault (Quantum LTO4 ULTRIUM) drive.

More food for thought, I also created /dev/stinit.def as follows:

# cat /etc/stinit.def
manufacturer=HP model = "Ultrium 4-SCSI" {
mode2 compression=0 blocksize=0
}
manufacturer=QUANTUM model = "ULTRIUM 4" {
mode2 compression=0 blocksize=0
}


and ran:

# stinit -f /etc/stinit.def -v /dev/st1
# stinit -f /etc/stinit.def -v /dev/st0

in an attempt to create devices with compression disabled and blocksize=0

Hoping this would work, when I tried to create a new amtapetype, I got the following warning that compression is still enabled:

# amtapetype -c -t HPLTO4 /dev/nst0l
Applying heuristic check for compression.
Wrote random (uncompressible) data at 45361118.4262295 bytes/sec
Wrote fixed (compressible) data at 70949441.6410256 bytes/sec
Compression: enabled
Turn off compression or run amtapetype with the -f option


My version of mt doesn't allow me to view the current device settings:

# mt -v
mt-st v. 0.9b


And it appears that it's not really changing them the way I expected it to.

I'm confused by the whole compression thing, and feel like a need the big picture. Any advice appreciated!

-pkb

[pyeatman]

I am most familiar with the device name used being what turns on/off compression. What do you get if you drop the "l" and run:

# amtapetype -c -t HPLTO4 /dev/nst0

[pkb]

I tried that first and got the same response about compression still being enabled - even after I issued the command: "mt -f /dev/nst0 compression 0"

Here's the output:

# amtapetype -c -t HPLTO4 /dev/nst0
Applying heuristic check for compression.
Wrote random (uncompressible) data at 43080250.7540984 bytes/sec
Wrote fixed (compressible) data at 69155139.368421 bytes/sec
Compression: enabled
Turn off compression or run amtapetype with the -f option


Since I got this output regarding compression still being enabled, *after* trying to force the compression off, that's when I tried creating the other devices with stinit...

I'm probably missing something basic, so feel free to offer any and all thoughts!

-pkb