On Thu, 24 Apr 2003, Jon LaBadie wrote:

> On Thu, Apr 24, 2003 at 09:26:12AM -0500, Russell Adams wrote:
> > > Of course hardware compression has one big advantage: speed.  If you
> > > have to fit lots of data in a small backup window at night, and your
> > > cpu's are not fast enough, or interfere with the nightly cpu-intensive
> > > jobs, then hardware compression is the way to go.
> >
> >
> > I have to take issue with this. My experience with hardware
> > compression on DDS and DLT drives is that when enabled it makes things
> > incredibly slow.
> >
> > My DLT IV (er, 7000?) drives can do 4.9 MB/s uncompressed, and only
> > 800KB/s in compressed mode.
> >
>
> This is of course counter to advertising claims :))
>
> Are you sure there was not some other factor involved.  A possibility
> that comes to mind is that the no-hw-compression came from data already
> in a single file or local to the server while the with-hw-compression
> test data had to be "found" and that procedure caused the slow down?

Yes, I agree.  This sounds suspicious.  The slowdown described sounds
like exactly what happens with DLT4000 and DLT7000 when you can't feed
them data fast enough to keep them streaming.  Supposedly this was fixed
somewhat in DLT8000 but I never bought one to verify that claim.  I've
never heard of anyone claiming this also happens with compression.

John Jackson was (is?) running several DLT drives and said he eventually
made the switch from software to hardware compression just for the
reasons already stated here.  The bottom line is you have to get the
data onto the tapes and he was able to do it much faster with h/w
compression.

-Mitch