--On Wednesday, August 31, 2005 10:17:54 -0400 Mason Loring Bliss <mason AT 
blisses DOT org> wrote:

> Hey, all.
> 
> I've got an inherited Amanda system I'm managing, and we're using an AIT-2
> tape drive / changer. I see the following:
> 
> define tapetype AIT-2 {
>           comment "Generic AIT 2 Drive -- real world numbers"
>           length 41000 mbytes
>           filemark 1000 kbytes
>           speed 2920 kps
> 
> }
> 
> Now, we've got data compression turned on on the tape drive, which should
> theoretically make more space, and we also have clients doing compression.

Generally not good to do both.

> Looking at the Faq-O-Matic, I see this:
> 
>       http://amanda.sourceforge.net/fom-serve/cache/439.html
> 
> In short, the person entering that data got less space and speed with
> hardware compression enabled. They were using the same drive we use here.
> 
> My questions are profligate, but I'll limit them to these:
> 
> 1. Why would the "length" parameter end up being so much shorter with
> hardware compression enabled on that drive? This confuses me.

Trying to compress already compressed data makes it larger.  Some
tape drives are smart enough to not compress the data again but AIT
isn't one of them (at least up through AIT-3, not certain about -4).

> 2. Am I to take it from the derived speed parameter that dumps will go
> quicker without hardware compression enabled? It seems strange that
> hardware compression would create that sort of impact, even when it's
> dealing with already-compressed data.

A tape drive can only write so many MB/sec.  If the data is getting
larger it has to wtrite more (i.e. your 41GB is turning into maybe 46GB).

> I'll try to schedule some time to run amtapetype at work, but we're in the
> middle of a move, so I might not get the chance for a while, and if I can
> safely get more speed and space out of my tapes with such a small change,
> I'd like to do so.

Pick either H/W or S/W compression, but don't do both.  If you decide
to disable H/W compression on your drive, you probably need to look
for Gene's recurring postings on the subject in the archives, as many
drives detect that a tape was previously used compressed and will
re-enable compression even if you think you've disabled it.  With
H/W compression off, yopu should be able to get close to 50GB on your
AIT-2 drive and not 41GB.

Frank

> Thanks kindly and heart-feltedly in advance!
> 
> -- 
>     Mason Loring Bliss     mason AT blisses DOT org       They also surf who
> awake ? sleep : dream;     http://blisses.org/     only stand on waves.



-- 
Frank Smith                                      fsmith AT hoovers DOT com
Sr. Systems Administrator                       Voice: 512-374-4673
Hoover's Online                                   Fax: 512-374-4501