Only 12 gig fits on DDS3 with SureStore DAT24

[John Downs_1]

Greetings, I have a SureStore DAT24 connected to my server running HP-UX 11.00. I'm using Maxell HS-4/125 DDS 3 tapes. The problem is: I can only fit (about) 12 gig on these tapes. I thought I was able to put 24 gig on these tapes. My tape device is: /dev/rmt/c1t1d0BESTn. Is this a compression problem or a limitation of my setup?

-john

[Christopher McCray_1]

Hello,

I have the same problem on one of my servers with oracle backups. The filesystem is about 14Gb filled and the files there are already compressed, so only 12Gb can be used on the tape. Luckily, we have an autochanger and have a script to change the tape once it is filled.

Hope this helps

Chris

[A. Clay Stephenson]

It could be both. First of all, the 24GB figure assumes a 2:1 compression ratio; not all data can be compressed. The smart play when sizing media requirement is to assume that no compression takes place. There are no surprises that way.

I don't have one of these guys to examine but it is also possible that compression has been disabled by jumper or DIP switch setting.

[harry d brown jr]

If a tape is labeled as being able to store 24Gigs, it means that the tape can ONLY hold 12GB's, because they like to screw with our heads and claim that if you "get" compression, then 24GB should be compressable (2:1) down to 12GB's.

It's MARKETING math - fuzzy math - enron math - worldcom accounting - ....


live free or die
harry

[MANOJ SRIVASTAVA]

Also John you ahve to use a DDS III media as the same will be abl;e to hold 24 GB if the compression dip swtich is turned on adn you use the best option.


Manoj Srivastava

[S.K. Chan]

You're using the correct device file (with "BEST") to achieve the highest capacity possible provided the tape drive supports compression. The reason you're not getting 24GB as expected is because the data you're backing up could not be compressed. What's the nature of your data ?

[John Downs_1]

Most(75%) of the data is CAD data(cadence, ads, libra, sonnet). The remainder is split between model files, libraries, scripts, html, and various other user created files.

As far as the jumper setting on the drives, are the set to compression by default? Where can I find information on the jumper settings?

Thanks!
Enron marketing, I like that!

-john

[Christopher McCray_1]

Hello again,

I was talking this over with a collegue and here is an interesting point. Yes, it is my impression that unless you are using an older drive, compression should be the default. anoter interesting point; We use 0m as our backup device, but when we look at the /dev/rmt directory, it has the same tmestamp as its corresponding BEST device. So it is my impression that in order for you to take full advantage of the available compression of the tape relies on the data.

Hope this helps

Chris

[A. Clay Stephenson]

This link should explain your switch settings:
http://h200004.www2.hp.com/bc/docs/support/SupportManual/lpg28477/lpg28477.pdf

[Leif Halvarsson_2]

Hi
The compression ratio depends on the type of data, 2-1 is what you can expect with plain text files. With other data I have get anything between 1.2-1 to 5-1 or more. In some cases with already compressed files the ratio can be negative, the data increase instead of decrease.
But, if yor files is not already compressed you should get at least some compression. You can try with another device file, I always use /dev/rmt/0m (or which scsi adress you use).

Another simple test is to use the HP-UX "compress" command on some of the files. As "compress" uses the same algortitm as the drive (LZW) you can expect the same ratio with the drive.