How can I tell which tape device uses compression?

[RobertCarback]

Just purchased a new DAT72 tape drive and see the following devices --
/dev/rmt/0m
/dev/rmt/0mnb
/dev/rmt/c3t3d0BESTn
/dev/rmt/c3t3d0DDSb
/dev/rmt/0mb
/dev/rmt/c3t3d0BEST
/dev/rmt/c3t3d0BESTnb
/dev/rmt/c3t3d0DDSn
/dev/rmt/0mn
/dev/rmt/c3t3d0BESTb
/dev/rmt/c3t3d0DDS
/dev/rmt/c3t3d0DDSnb

Which is/are used for compression?

[Court Campbell]

The ones with the word BEST.

[James R. Ferguson]

Hi Robert:

The manpages for 'mt(7)' offer the insights you want.

Regards!

...JRF...

[OldSchool]

and the 0m / Omn should be ok as well.

try lssf /dev/rmt/0m, it should say "best available" for density

[RobertCarback]

Thanks,
I would imagine that the 'n' after BEST is no-rewind. What is 'BESTb' ?

[Court Campbell · Accepted solution]

It's berkeley compression vs. at&t. b = berkeley.

[RobertCarback]

Thanks!

[A. Clay Stephenson]

Note: There is no such thing as Berkeley or AT&T compression. The 'b' in no way indicates compression but is rather the "style" attribute and determines the way the tape is positioned after a read of a file on a norewind device. Do a man 7 mt and look under the "Tape Behavioral Characteristics" section for a detailed explanation. When given a choice, choose the Berkeley style because the AT&T style to my mind is insane.

[Court Campbell]

Thanks, Clay. You learn something new everyday.

[Raju_S]

Hello Robert,

The word "BEST" indicates that with this device file, the highest possible capacity format will be used. This will also include data compression if it is supported by the device. The character "n" shows that the tape will not be rewound at the end of the operation. The "b" character shows that the tape drive will follow the Berkley style instead of AT&T.

Tape Drive Device File Options


Density Use one of the four: BEST, NOMOD, DDS, or D. The BEST option uses the highest capacity, including compression. The NOMOD option can be used on DDS and 8-mm devices only, and it maintains the density of previously written data. DDS shows the number of the DDS standard used, i.e., DDS2 or DDS3. The D option followed by a number defines the density of the tape as a numeric value.

C Compress. The optional number following C shows the compression algorithm.

n No rewind at the end of the operation.

b Berkley-style tape behavior.

w Wait for the physical completion of the operation before returning the status.

Regards