Low Compression Rate - DLT 40/80

[Alain Deslex]

Since few days, compression rate has dropped to 1.23:1 instead of 1.4:1 usualy. Now, I need a second tape to finish backup. File types have not changed.
LTT test shows 2.0:1 rate.

DLT 40/80 inside a HP ML370 under Win2K and BackupExec with the Veritas driver.

What can be the cause : drive ? Media ? something else ?
I've tried power off/on -> same

Thanks for your help.

Alain

[Alain Deslex]

Hi,
I also notice that I have more soft write errors since compression ratio is bad :
Usualy less than 2000, now more than 20000.

Hope this help.
Thanks again.

Alain

[David Ruska]

Alain,

A high soft error rate will definately affect your tape capacity, as the drive will rewrite any blocks that fail to verify.

You can run the LTT drive assessment test on a few tapes, and see if the problem is only one certain tapes, or on all. I believe the test checks the soft error rates.

[Alain Deslex]

Thank David for your reply.

After I changed block and buffer size (old : 32K, new : 64K) the Friday's job was pretty good. Transfert rate = 358Mb/min and compression ratio was 1.34:1
But the same job has run yesterday with a transfert rate of 66Mb/min and Comp ratio of 1.10:1 with a another tape.
So, maybe you're right saying that it's depend of the tape.

The server is on another site. So it's difficult to us to change tapes without the need of the client. But we will try to test with LTT.


Thanks again for your help.
Alain

[Alain Deslex]

Hi,
I have tried with new tapes but the problem still remains the same. Compression Ratio below 1.20:1
Can I optmize this by tuning device configuration (block, buffer size, ..) ?
I have more than 1k files of more than 10Mb in size (tif, ppt, zip, ...).
Tomorrow, I will put the cleaning cartridge.

Thanks,

Alain

[David Ruska]

Where/how are you measuring your compression rates?

Are these reported back from the drive's data compression log pages?

At the drive level, a particular set of data (backed up in the same order) should always have the same compression rate, regardless of the tape drive's error rate, performance, etc. It's a digital alogorithm, and it will produce exactly the same output given exactly the same input.

> I have more than 1k files of more than 10Mb in size (tif, ppt, zip, ...).

Any file format that already includes compression (e.g. zip files, compressed image/movie formats) typcially will not compress at all (.e. 1.0:1).

With LTT (in the support ticket), you should be able to read the compression data and the soft error counts, to see what is contributing to the capacity loss, on the currently loaded tape.

[Alain Deslex]

I read the compression ratio value that BackupExec returns in a "media properties" log after a backup.

With LTT v3.5SR3, I am not sure on what it is reported.
In Support Ticket / Detailed Device Information / Device Analisys it shows me "There was insufficient data transferred to calculate read/write compression ratio"
Errors=0, Warnings=0
No problem found

I did a "Data compression test" with LTT and it reported me a Read/Write compression ratio of 2.28:1 for 600Mb

The person who changes tape every day told me yesterday that she puts a cleaning cartridge once per month (even if it is not needed by the device) and that the cartridge is a DLT III type. Maybe it is not very good for device heads.
The device has been already changed in october 2005 for an another problem.

Hope this help.

Alain