Using the -z optiont to tar may cause the data to be written to
	the drive in variable sized blocks.


		David/absolute
				       -- www.netbsd.org: No hype required --

On 4 Jun 2000, Eric Smith wrote:

> > does it work if you remove the z option in tar. I have experience some
> > problems with my dds2 and dds3 when trying to apply gnuzip to the tar
> > archive. I have a ultrix with a tk50 i can do some research in this area
> > later and tell how it reacts to the option z in tar.
> 
> I'm not disputing that there might be a problem, since I haven't tried
> it myself, but the very concept that a TK50 might have problems with
> the gnutar -z option amazes me.  How would the TK50 even "know"?
> 
> By default, the DDS2 and DDS3 drives attempt to do their own compression,
> so if you use the gnutar -z option you will send already comressed data to
> them, which they don't expect.  The net result is that the drive actually
> *expands* the data somewhat, although typically not back to its original
> size.  This is because the drive generally can't detect that the data is not
> compressible until it's already been processed, by which time it's too late
> to do anything about it.  However, I've never seen this cause any actual
> "problems".
> 
> If for some reason you *must* use gnutar with "-z" on a DDS/DC or DDS 2-4
> drive, for best performance you should configure the drive to disable
> its built-in compression.  This can usually be done through either jumper
> settings or SCSI commands.
>