topic Re: 9840B compression in Operating System - HP-UX

9840B compression

Ted Ogiela_2 — Wed, 15 Dec 2004 14:58:45 GMT

Is there a way to determine tape device predefined compression without actually performing a backup?

Re: 9840B compression

Sanjay_6 — Wed, 15 Dec 2004 15:07:37 GMT

Hi Ted,

I don't think there is a way to find the compression beforehand. It depends on the type of media used, the type of data you are copying and the device file you are using. Use of /dev/rmt/*BEST* devices normally give you the best compression.

Hope this helps.

Regds

Re: 9840B compression

Leif Halvarsson_2 — Wed, 15 Dec 2004 15:07:45 GMT

Hi,
No, this is not possible. Compression ratio is highly data dependant and it is not possible to predict the ratio before the data is wtitten to the media.

I don't know which compression algoritm your drive use but, if LZW you can get an idea about compression ratio if you try compress or gzip on some of the data files.

Re: 9840B compression

Sanjay_6 — Wed, 15 Dec 2004 15:10:21 GMT

Hi Ted,

Maybe this doc will help,

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg50244

Hope this helps.

Regds

Re: 9840B compression

A. Clay Stephenson — Wed, 15 Dec 2004 15:13:27 GMT

No, because compression depends upon the data. In fact, you might very well find that the same files, even if the same native size of each and every file remains constant, will compress differently between backup runs -- if the contents of the files changes.
For example, suppose a 100MB file named "mydata" consisted entirely of "X's"; that would compress greatly. Now suppose that this very same file were updated with random data; the same 100MB file would not compress at all. Yet both are 100MB files.

In short, the only value that you can absolutely depend upon is the native capacity of the medium less any formatting/metadata overhead required by your backup software.

Re: 9840B compression

Ted Ogiela_2 — Wed, 15 Dec 2004 15:21:26 GMT

Thank you all for the replies. What I want to find out is device driver compression if there is such a parameter when device is created irrespective of data being backed up.

Re: 9840B compression

A. Clay Stephenson — Wed, 15 Dec 2004 15:25:58 GMT

lssf /dev/rmt/* will tell you about as much as you can know. Man lssf for details.

For some tape devices
mt -f /dev/rmt/xxx status will also help.

Re: 9840B compression

Ted Ogiela_2 — Wed, 15 Dec 2004 15:30:55 GMT

I already tried lssf command and it only reports "best density available" that does not tell me much.

I am trying to compare 9840B driver performance to the stated performance. Per spec this drive is rated at 19MB/sec native and upto 70MB/sec compressed. I am getting around 30MB/sec with NB.