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Only 12 gig fits on DDS3 with SureStore DAT24

John Downs_1
Frequent Advisor

Only 12 gig fits on DDS3 with SureStore DAT24


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
I'd rather be fly fishing for trout!
16 REPLIES
Christopher McCray_1
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

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
It wasn't me!!!!
A. Clay Stephenson
Acclaimed Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

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.

If it ain't broke, I can fix that.
harry d brown jr
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24


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
Live Free or Die
MANOJ SRIVASTAVA
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

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
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

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
Frequent Advisor

Re: Only 12 gig fits on DDS3 with SureStore DAT24



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
I'd rather be fly fishing for trout!
Christopher McCray_1
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

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
It wasn't me!!!!
A. Clay Stephenson
Acclaimed Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

This link should explain your switch settings:
http://h200004.www2.hp.com/bc/docs/support/SupportManual/lpg28477/lpg28477.pdf
If it ain't broke, I can fix that.
Leif Halvarsson_2
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

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.
Bill Hassell
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

As mentioned, compression depends on 2 factors: device file and the data itself. For HP-UX, *always* use the lssf command to decode device files. (related commands: insf, not mknod, and rmsf, not rm). The word: BEST density indicates that when the this device file is used, the driver will set the compression option on the drive.

To test compression, use the prealloc command to create a very large (multi-meg) file. It will be extremely compressible (perhaps 10:1 up to 100:1). However, to easily see that the data is compressed, get a copy of the tapeinfo program from:

ftp://contrib:9unsupp8@hprc.external.hp.com/sysadmin/programs/tapeinfo/


Bill Hassell, sysadmin
Frank Slootweg
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

Which backup utility do you use?

If fbackup, then run it with the "-v" option. That will report how much data fbackup writes to the tape drive, i.e. including fbackup 'overhead'. If that number is slightly over 12GB (the Native Mode (= uncompressed) capacity of a DDS3 tape) (or more), then apparently hardware data compression is working and all is well.
John Downs_1
Frequent Advisor

Re: Only 12 gig fits on DDS3 with SureStore DAT24


Hi again, I'm using dump and the tape drive is configured to do compression. This is the lssf output of the device file that I am using:

stape card instance 1 SCSI target 1 SCSI LUN 0 at&t no rewind best density available at address 8/16/5.1.0 c1t1d0BESTn

Thanks!
-john
I'd rather be fly fishing for trout!
John Downs_1
Frequent Advisor

Re: Only 12 gig fits on DDS3 with SureStore DAT24



Still no solution. I suspect that I am using the wrong arguments in the dump command. The dump command is : rdump 0udsf 60000 30000 la:/dev/rmt/c1t1d0BESTn filesystem.

I'm using HP dds-3 cartridges C5708a

How do I now or determine the density and length of a tape?

-john
I'd rather be fly fishing for trout!
Vincent Farrugia
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

Hello,

On the back of all DDS tapes, there should be 7 holes.

DDS3 tapes have the holes setup as follows:

From left to right:

1st Hole: open
2nd Hole: closed
3rd Hole: open
7th Hole: closed

All DDS3 tapes are 125m long.

HTH,
Vince
Tape Drives RULE!!!
Frank Slootweg
Honored Contributor

Re: Only 12 gig fits on DDS3 with SureStore DAT24

> How do I now or determine the density and length of a tape?
[for dump(1M)]

dump(1M) and friends are for reel-to-reel magtapes. DDS is totally different, so you have to 'fool' dump(1M) that it is talking to some special magtape drive.

As far as I know, this subject is (still) covered in the "comp.sys.hp.hpux FAQ". You should be able to find this FAQ on Google Groups. Let me know if you can not locate it and I will look for you.

John Downs_1
Frequent Advisor

Re: Only 12 gig fits on DDS3 with SureStore DAT24



Hi, the tape is as you said. But I'm still not sure if I am giving the correct options on
the dump command line.

I couldn't find comp.sys.hp.hpux. I tried http://www.comp.sys.hp.hpux, comp.sys.hp.hpux with the same results. Can't find server or server is down.

Is google group a different location?

-john
I'd rather be fly fishing for trout!