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Help with compression?!

Christer Zulawinski
Occasional Advisor

Help with compression?!

Hi everyone!

I've had this problem for over 1 year, no one seems to have any answeres for me, please have a look at my problem.

I have a HP Surestore DAT24i (C1537a) tape device it has the latest bios version L005. It?s connected to an Adaptec 2940u2w SCSI card with the latest bios (July 2001). I have all the dip switches set to standard witch is all the switches set to on except the 3rd one witch is off. The OS is w2k with ms drivers for the devices.

Now we come to my problem. When testing it with HP library and tape tools I get no compression, none what so ever, both on reading and writing I get the compression 1:1.0. This is consistent; I?ve tried about 1 million times. I get no compression from the built in backup software in w2k. I can only get software compression from Veritas backup exec. All this makes me believe that I have a faulty tape drive, well, here?s the killer: If I change the 2nd dipswitch to off, I get hardware compression using HP library & tools but not using any of the 2 mentioned softwares?

I have read that some special settings for the SCSI card is in order so I have changed it to:

Send Start Unit Command: No/Disabled
Enabled Disconnect: Yes/Enabled
Initiate Wide Support: No/Disabled

I?ve read that I should disable sync negotiation, I can?t find that anywhere. I?ve also read that the transfer rate should be 10MB or lower. So what should I set it to, ASYNC or 10MB?
9 REPLIES
paul courry
Honored Contributor

Re: Help with compression?!

You are correct in that you have faulty settings. The tape drive itself appears to be working fine from what you described.

Can anybody give this person the right settings?
Christer Zulawinski
Occasional Advisor

Re: Help with compression?!

Oh, comon! This started so good! I thought that was going to solve this 1 year old problem...... Please help!
Eric de Lange_2
Respected Contributor

Re: Help with compression?!

Hi Christer,

The dip switches for compression are set to 'on at power on with host control' which means that when booting the system compression will be on but allowing Windows 2000 to change this setting.

By setting switch 2 to off, you basically 'lock' the compression of the DAT24 drive to be always on.

The catch here is the following: if you backup a lot of compressed data like zip, mp3, and cab files, there is little that the compression chip on the drive can do with it. In fact the amount of data that will be written on tape will be more than the initial amount of data that you wanted to backup. This will reveal itself buy only being able to store 10GB or so on a DDS-3 tape. Somthing to be aware of. When backing up highly compressed data you might be better of not using hardware compression.

I hope this helps you solve the issue that you are seeing.

the test is simple, do a backup with hardware hardware compression enabled and see how much data the tape will hold. Then, do a backup with hardware compression disabled and again check how much data will fit on a tape. If there is a significant difference, your hardware compression is working.

best regards,

-Cal.
Eric de Lange_2
Respected Contributor

Re: Help with compression?!

more about the switch settings of the drive can be found at http://www.hp.com/cposupport/information_storage/support_doc/lpg51068.html

-Cal.
Christer Zulawinski
Occasional Advisor

Re: Help with compression?!

Ok, this gives me something to go on, thanks alot. Beacuse it is true that my tapes usually don't take more than 10-11 gig, but it is not true that I backup compressed data. I'm only backing up like the winnt dir and program files...

I will try to choose no compression in veritas and see how much the data the tape will use...

Thanks again for your answer, I really apreciate it!
Christer Zulawinski
Occasional Advisor

Re: Help with compression?!

Ok, this problem is getting worse and worse. I disabled hardware compression in Veritas backup exec 8.6 and set it "none" compression. The log still reported that hardware compression was used? And this time it used more of the tape than last time??? I can't remeber exactly how much data it was the last time I did a backup but it was around 10 gig, this time it used 11.8GB.

Also, when I had that successfull hardware compression test with Hp tape tools I had a differant dip setting. That time I had it set to start the device with hardware compression on and software has no control over it. Since this wasn't standard and I had problems I set it back to standard settings and that how it is not, but then hardware compression doesn't work in HP tape tools anymore??

The transfer rates was also quite low wich makes me beleave that hardware compression wasn't used.. The rate was around 50-52 mb...

Maybe I'm just missing something here? Please shed some light on me and my lost computer.....

Christer Zulawinski
Occasional Advisor

Re: Help with compression?!

I guess this problem will never be solved? Well, there goes my trust in HP out the window...
D Wong
Regular Advisor

Re: Help with compression?!

Hi Christer:

Hardware compression on HP tape drives is on by default. The firmware inside the tape drive determines when it is best to use compression. If the data stream from the disk drive is too slow, compression is turned off.

The backup software compression can make the performance of the tape drive even worse. Since the backup software compresses the data and if the data is streaming fast enough, the hardware will try to compress the data again. This is why your backup with software compression can be worse and take more tape space.

To get a demonstration of compression you need a continuous fast data stream going to the tape drive preferably with large files. Large files are required because small files like the WINNT directory take many disk drive seeks to find each file before sequentially writing to tape. So a real demonstration would be a raid disk set and large file that are not already highly compressed.

Hope that helps
Eric de Lange_2
Respected Contributor

Re: Help with compression?!

Christer,

Based on your description of August 2, I think your tapedrive is working just fine, the compression of the drive included.

Also based on your description that you are able to store 11.8GB on a tape after disabling HW compression (yes, I know, Veritas told you something different) I think you are in pretty good shape. The throughput could be a tad higher but that could be achieved by tweaking BackupExec parameters.

Best regards,

-Cal.