Adjusting compression for DLT 8000 Tape Drive - OmniBack - 20gb max

[Jack C. Mahaffey]

How do I adjust compression for a DLT 8000 tape drive in a SureStore E 10/20 Tape Library. I only get 20gb on a DLT IV tape. OmniBack defaults to 20gb when tapes are formatted. Compression ration from xstm shows 1: 1.0

Thanks... jack

[Alan Riggs]

Don't get hung up on teh "capacity" setting. It is an aproximation for human use only. You can overide it when tapes are initialized, but it has no effect on compression ration.

In you backup definiions, you can select to have software compression turned on or off. Turning it on slows the backup process for local devices, but can speed performance and lower network load if backing up to a remote device.

Hardware compression can be turned on or off withthe buttons/menus on the DLT. Default is hardware compression on.

To see how much data you are actually writing to tape, run a backup of more than 20GB, then check the statistics on the media for that backup. You should see that all data was written to the tape despite the 20GB "capacity".

Compression ratio will vary depending upon teh data. 4:1 is an "expected" ratio for a DLT8000 allowing 80GB o be put on a tape. I use DLT7000s and have written more than 100GB to a tape (expected capacity 70GB). Database files compress particularly well.

[Jack C. Mahaffey]

Thank-you for your response. The tape drive for the SureStore E tape autoloader is internal and I did not think of taking the device out to check for buttons for setting the compression settings. Can be unmounted from the rear of the unit.

I didn't realize that the device wasn't allowing more than 22gb until I got a mount request for anther tape. Up to this point, less than 22 gb was on the tape. I then started investigating to determine what the current setting consisted of. Running xstm showed that there wasn't any compression going on. OmniBack defaulted to 22gb. I assumed OmniBack derived this value by looking at the hardware setting. I was looking for a command to change the hardware setting without opening the library.

What I will do, is create to an OmniBack pool device to be larger than the default and then see if I can get more than 22gb on the tape.

As for accessing the compression buttons. What specific steps should I follow to take the tape drive out of the rear of the auto loader? Are there any potential alighment problems that I may encounter when re-inserting the tape drive?

e.g.

1 - Power down tape array
2 - Remove tape drive
3 - Locate and set compression setting button(s). I was hoping I could do this from a command prompt.
4 - Return tape drive.
5 - Power-on tape array.
6 - Run xstm to verify setting.

jack... Thanks once again...

[Alan Riggs]

Jack -- before doing any of that please sheck teh settings on your backups to see whether software compression is turned on. If software compression is activated, then teh 22GB that you are seeing might be already compressed data.

[Jack C. Mahaffey]

Will do.

I also found a couple of patches that deal with compression and DLT tape drives. Patch deals with at least two problems:
1-Recognizes DLT 8000 tape drives.
2-Fixes problem with compression not being set correctly.

Article identifies patches PHKL_19805 and PHKL_18543

[Donald Kok]

I have been testing with the compression possibilities of omniback the last couple of weeks. I found somewhere that to enable/disable hardware compression is just a matter of using the right devicefile. In my case :
/dev/rmt/c2t1d0BEST is the devicefile with compression ON
/dev/rmt/c2t1d0 is the devicefile with compression OFF.
I use this from xomni -> devices & media management -> 'drive' -> exchanger datadevice. But I have a 2/20 DLT robot. In your case the menu items may vary.

Would this suggestion help you?

[Jack C. Mahaffey]

Thanks for the reply. I'm already pointing to c#t#d#BEST. I'll try the other device file.

[Darrel Louis · Accepted solution]

Hi,

If you check in the following file:
/etc/opt/omni/options/global for the MC_X
parameter.
The default for DLT is
DLT | 10 | 1 | 36 | 250 | 20 GB | 4
What you should have in the file should look like the following example
MC_10=1 36 250 4096000 4

Can be set in pool configuration.
# DefSize is the default size of media (in KBytes) of that
# class. This is used to set to medium size if size cannot be (detected by the device itself.)

[Darrel Louis]

I've calculated the size in Kb

40 * 1024 * 1024 = 41943040
or you can try the non marketing size:
40 * 1000 * 1000 = 40000000

So what you should add to the global file is:
MC_10=1 36 250 41943040 4
Stop and restart OBII.

[Tom Danzig]

When initializing the tapes, choose "specify" instead of "determine" for tape capacity. If you are using the compression device file on a DLT7000 drive, you can try specifying 71650 in the capacity field.

I have found that OmniBack always defaults to the lowest capacity for the tape type selected regardless of compression used.

[Jack C. Mahaffey]

Thanks to all for your responses.

[Alan Riggs]

One more thing not to get hung up on -- the "capacity" omniback reports for a media. This "limit" is for human convenience only. It gives you an idea how much data you may still be able to write to a media (if the capacity was initially set correctly AND you have been using the expected compression/no compression at the software level).

Omniback will not stop writing to tape based upon the recorded capacity. It will write until the media is full.