On Sun, Jan 25, 2004 at 02:40:24AM -0500, Jon LaBadie (jon AT jgcomp DOT com) 
wrote:
> On Sun, Jan 25, 2004 at 04:35:17PM +1100, Jobst Schmalenbach wrote:
> > 
> > Folks,
> > I am not clear about compression within amanda.

[snip]

> I say guess because I just looked at my recent numbers,
> some DLE's only compressed a couple of percent.  Others
> compressed to just 1/5th of their original size.  What
> will be the compressibility of the data on any particular
> nights run will vary greatly with which DLE's are dumped
> at level 0.

the problem is that this is a new drive, so I havent go
any numbers ....

> 
> jl
> 
> BTW your reasons are all valid for considering HW compression.
> I think more amanda sites use HW compression than most of us
> realize.  Just be aware that there is no way for amanda, or
> any other software, to know how much data will fit on a tape
> using HW compresssion.  Nor, to my knowledge, to know how much
> tape is used/available after a tape file has been written.

I just had a problem getting data of a tape which was software
compressed (amanda tape). I think I decided the last time to
use software compression for just those reasons ... and
turned hardware compression of. 

The only problem (and I guess you could call it a trade off)
is that gzip throws an error at you if the tape was faulty
and you cannot recover from that.

Maybe currently I am bitten, so twice shy .... ;-)




I learned something more.
In the manual for dd it is mentioned if dd should continue
after a read erro to specify "noerror". This is NOT enough
and it took me a while (an a lot of webpages and couple of 
days  later) that you MUST specify "notrunc" as well or
gzip will throw so many errors at you that you think the
file/archive is completly stuffed.

I got away this time, so I am going to change a few 
things in my backup setup ...



jobst





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