How to check hardware data compression


 
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# 1  
Old 08-09-2010
Question How to check hardware data compression

Hi,

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:
Quote:
rmt0 Available 04-08-00-0,0 LVD SCSI 1/2-inch Cartridge Tape Drive
attribute value description user_settable

block_size 0 BLOCK size (0=variable length) True
compress yes Use data COMPRESSION True
density_set_1 66 DENSITY setting #1 True
density_set_2 64 DENSITY setting #2 True
extfm yes Use EXTENDED file marks True
mode yes Use DEVICE BUFFERS during writes True
ret no RETENSION on tape change or reset True
ret_error no RETURN error on tape change or reset True
size_in_mb 200000 Size in Megabytes False
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?

Cheers,
Rab

Last edited by rab; 11-09-2010 at 10:50 AM..
# 2  
Old 11-09-2010
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.

Any ideas?

Cheers,
Rab
# 3  
Old 11-09-2010
The compression ratio will greatly depend on what type of files are being backuped...
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# 4  
Old 11-09-2010
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..
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# 5  
Old 11-09-2010
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.
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