Re: [SLE] PROBLEM WITH TAPE HARDWARE COMPRESSION

On Fri, 30 Dec 2005, Ken Schneider wrote:

> On Fri, 2005-12-30 at 13:18 +0000, KRAMO FIACRE wrote:
> > Hi!!!
> > I've got an HP SURESTORE DAT 40 on my SUSE LINUX ENTERPRISE SERVER 8 BOX
> > SP3.
> > The tapes I use are marked 20/40GBYTE. But I can only make backup up to 20G,
> > whereas on my ALPHA server with tru64 unix, I can save up to near 40 Gbytes
> > using the dev file "/dev/rmt0h" (for tru64 V 4F) or "/dev/tape/tape0_d1"
> > (for tru64 V 5.1B).
> > I try to enable compression using the command "mt -f /dev/st0 datcompression
> > 10", this command achieved succefully but nothing. I try to use the
> > /dev/st0m or /dev/st0a, but these devices files seem not to be associated to
> > physical device.
> > So how can I perform backup with hardware compression?
> > Please help
> >
> >
> You may need to turn on compression after every reset of the tape drive.

Indeed. Try using mt -f /dev/st0 datcompression 1 <<- not '10'

> Also remember that you will never get 40G of data on the 20G tape using
> compression. Binary files never compress 50% and actually compress very

Neither of those statements are strictly true. I've seen ratios of 10:1
or better. It all depends on the nature of the data that you are backing
up. Now, I don't know if hardware compression can do quite that well,
but gzip certainly can, and bzip is even better (although *far* more
expensive).

> little and some even get larger when being compressed.

> Also I don't recommend using software compression and hardware
> compression together.

That's good advice.

--
Carpe diem - Seize the day.
Carp in denim - There's a fish in my pants!

Jon Nelson <jnelson-suse@xxxxxxxxxxx>

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