Subject: Re: A few mt(1) questions
To: None <>
From: Jorgen Lundman <>
List: netbsd-users
Date: 03/18/2002 15:03:55
Just like to throw in 2p here. Two companies back, they were backing up 
on a Novell system, using the wrong version of the software, which means 
it flatly refused to restore anything when the raid had died.

I plugged the DAT into NetBSD, read the tapes raw, and sat down to work 
out the layout the software had used on tape, the stream switch tags, 
start and stops etc.

Had software compression been turned on, I would have had no chance in 
finding what particular compression algorithm they had used etc, and it 
would have gone from a 2 day task to a X weeks.

Rather a special case, but it did save us (and yes, the Novell admin had 
quit some months before).


David Laight wrote:
> On Mon, Mar 18, 2002 at 05:55:55AM -0800, wrote:
>>>4) Does it make more sence to use tar(1) to pass through gzip(1) or
>>>bzip2(1) to do compresstion, or use hardware built-in compression?  Just
>>>looking for opinions speaking from an experience standpoint.
>>It depends.  For examples:
>>	1) software compression will get you better compression
>>	2) software compression puts more load on your cpu
>>	3) software compression makes it more difficult to recover
>>	   data from a tape that has damage/errors
>>	4) software compression makes it so you can't use GNU tar's
>>	   feature of multiple-volume sets
>>	5) if you use hardware compression to back up data that
>>	   has already been compressed with software, the "compressed"
>>	   data may take more space than if you were not using hardware
>>	   compression
> 	6) if you use software compression you know whether the archive
> 	   will fit on the tape.
> 	7) software compression will (probably) let you read the tape
> 	   one other drives.
> 	8) hardware compression means you don't have to remember which
> 	   compression algorithm you chose.
> 	David

Jorgen "Lord" Lundman <>
Technology Manager, Unix Administrator
Phone: +44 (0)20-86591860  Mobile: +44 (0)79-58642918
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