HP Response - Data Compression

[CA756694]

It looks like there is a lot of confusion over compression and exactly how much data can be expected to be stored on a data cartridge.

There is a good document available on our website that talks about how compression works:

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg50244&locale=en_US&docId=99541

Most problems with compression are not a result of a problem with the hardware, but a result of a misunderstanding of how compression actually works and what your expectations are. For example, the type of data being compressed or using both software and hardware compression at the same time.

If you are still certain that there is still a problem, the document listed above gives some advice but it???s worth repeating:

1. Check the type of data being backed up. The document above lists some of the types and what the compression or each is.

2. Check the DIP switches on the bottom of the drive to make certain that the hardware compression is enabled. If you download library and tape tools:

http://www.hp.com/support/tapetools

and run the compression test the support ticket will tell you whether compression is enabled.

3. Check that both software and hardware compression aren???t enabled at the same time. Here is a document on hardware and software compression:

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg50030&locale=en_US&docId=75327

I am afraid that we don???t list how to disable the software compression in backup applications are there are just so many variants!

4. Streaming problems can also cause issues with DAT drives, as some empty frames will be present. This is not a huge problem but it can amount to a perhaps 5% or so of the total capacity. Here is a document on performance:

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg51160&locale=en_US&docId=40883



Problems with the backup application asking for another piece of media on DDS drives, before the native capacity has been reached, can be down the media. There are problems with different types of manufacturers media, not because they aren???t compatible but because the format specifications are wide enough to allow a little variation.


Hopefully, this should provide a starting point for compression problems.

Remember that you can always contact one of our call centres for assistance:

http://www.hp.com/support/callcenters


cheers,

John

[CA756694]

I'd like to emphasis that already compressed data (like most multimedia files) will actually expand with further compression (like hardware compression). It may expand significantly.

This will mean that overhead will start to claim space on your tape and reduce the native capacity. This difference could be as much as 10-20% (or more) in some cases.

The compression rate generally indicates the space lost. i.e. a 90% compression rate would equal around 2GB 'lost' on a native 20GB tape and it would be full at 18GB.


cheers