Veritas-bu

[Veritas-bu] Hardware compression not working

2006-04-18 13:44:51
Subject: [Veritas-bu] Hardware compression not working
From: MarkP AT spectralogic DOT com (Mark Pinder)
Date: Tue, 18 Apr 2006 11:44:51 -0600
One other thing to keep in mind is that once a tape is written to with 
compression off, the drive will default to compression off when that tape is 
loaded. It will require a format of a tape (I.E. !! at BOT with a "cbn" device 
driver) to clear this condition and allow the tape to use compression. All of 
the tapes that have been formatted with the SW and Compression off will 
"forever" write in an uncompressed mode regardless of the device driver used. 
The drive does this on it's own by sensing/reading the tape. 

I agree that you should do some formal and systematic testing. A couple of ways 
to do this are 1) Create a text file that repeats some data pattern and make it 
large. Then count the number of times that file can be written to tape. 
Alternatively, 2) dd from /dev/zero to the tape device file and see how much 
"data" you can get on a tape or write anything more than the capacity of the 
tape's worth of '0's.  For both of these tests you should use a fresh tape and 
use a compression enabled driver. 


Mark Pinder : 
Systems Engineer:
Spectra Logic :