How do I configure a tape drive for both hardware compression and non-compression?

[David Trusty]

Hi,

I have a Sony SDX-500C tape drive, and I want to configure
the device special files
to show both compression
and non-compression.

Here is what I have now:
stape card instance 3 SCSI target 1 SCSI LUN 0 at&t best density available at address 10/0/15/1.1.0 1m
stape card instance 3 SCSI target 1 SCSI LUN 0 berkeley best density available at address 10/0/15/1.1.0 1mb
stape card instance 3 SCSI target 1 SCSI LUN 0 at&t no rewind best density available at address 10/0/15/1.1.0 1mn
stape card instance 3 SCSI target 1 SCSI LUN 0 berkeley no rewind best density available at address 10/0/15/1.1.0 1mnb
stape card instance 3 SCSI target 1 SCSI LUN 0 at&t best density available at address 10/0/15/1.1.0 c3t1d0BEST
stape card instance 3 SCSI target 1 SCSI LUN 0 berkeley best density available at address 10/0/15/1.1.0 c3t1d0BESTb
stape card instance 3 SCSI target 1 SCSI LUN 0 at&t no rewind best density available at address 10/0/15/1.1.0 c3t1d0BESTn
stape card instance 3 SCSI target 1 SCSI LUN 0 berkeley no rewind best density available at address 10/0/15/1.1.0 c3t1d0BESTnb


Also, although the description
shows "best density", it seems
that there is no hardware
compression occurring.

Any ideas?

Thanks,

David

[Craig Rants]

That's why they call it best density. I can't say that I've used your particular drive before so I can't comment on the hardware compression. However, software compression is usually a configurable option in most backup software packages. I don't think that there is a lot that you have to do.

C

[Jeff Schussele]

Hi David,

Without knowing just what SW or command you're using to backup & not being familiar with this Sony drive, it's hard to determine just why you're not getting noticeable HW compression.
But I would offer the guess that if you're using backup SW such as OB II or Veritas or such that they me be compressing fairly well on they're own & only negligible HW compression can be applied further.

Rgds,
Jeff

[David Trusty]

Thanks for the feedback.

Here is some more info on
the problem.

I've used an open source
utility which determines
the tape capacity, and
I get approximately 50GB as
the results.

According to the specs for
the drive/tapes, the
uncompressed capacity is 50GB,
and the compressed capacity
is 100GB. From these results
and specs, I believe that
the hardware compression
is not enabled.

I'm not sure how hardware
compression is controlled
on tape drives. Is it some
special SCSI command?

Also, I expected to see a
set of device special files
with and without the "best"
shown, but did not.

Any ideas?

Thanks!!

David

[Jeff Schussele · Accepted solution]

Hi David,

It may be that the utility knows what "switch" to send to turn off compression & was just giving you a total uncompressed capacity.

As I'm sure you know some files compress much better than others & the 2X figure is basically an educated guess as to what the max compressed capacity would be.

A test I would do is to backup a dir or set of files that is mostly ASCII text - like the /etc dir

First do
du -sk /etc
to get the size of the dir in Kb.
Then back it up with tar & compare the size of the backup with your du results.
If they are the same then I would break out the manual on the tape drive because there are some tape drives out there that can have HW compression disabled via a DIP switch & that may be the case here.

HTH,
Jeff

[Jeff Schussele]

Hi David,

I found the manual on-line at

http://www.storagebysony.com/support/consumer.asp

It's a pdf file & it states that HW compression is enabled when DIP SW 7 is ON & control by host can be disabled when SW 8 is ON.

So check those DIPs.

HTH,
Jeff

[Jeff Schussele]

Oh & to answer your other questions - Yes there is a SCSI command I believe to turn on/off compression - as you can see by DIP SW 8 it can be overridden.

And by default HP-UX only creates the "BEST" device files - I think for fairly obvious reasons - if you can get the extra capacity - might as well use it.

Besides BEST there are others than can be manually created via mksf & mknod:

NOMOD -> used on DDS & 8mm only & maintains the density of previously written data.
DDS -> shows the number of the DDS standard used i.e. DDS2 or DDS3.
D -> followed by a numeric to define the density of the tape directly.

Rgds,
Jeff