Compaq 4/16 Gb TurboDAT question

[Ernest Ford]

I have a 4/16-Gigabyte TurboDAT Drive in a Prosignia server running Server 2000.

I'm using 120m DDS-2 cartridges, which is the longest supported by this drive (also says 4.0 GB Native Capacity on the label), and the Windows backup, the most I can get on a single tape is 4Gb, even though I have checked the box in the backup software - advanced settings to use compression if available , and the drive has been jumpered to allow compression.

I don't expect to get 16 Gb, but somewhere in the vicinity of 8Gb should be possible and definitely more then 4Gb.

It's obvious that compression is not occuring, even though the drive is correctly jumpered and the appropriate box in the software checked - any suggestions?

[Brian_Murdoch]

Hi Ernest,

You say that the unit is jumpered for compression. By default there should not be a jumper across pins 9-10. This is actually compression disable according to the specs.

Please have a look at the attached diagram.
There were two models of the 4/16 TurboDAT, I suspect your one is the Conner CDT-8000.

You will probably never ever achieve the theoretical maximum of 4:1 compression but I agree you should get something better than 4Gb total. It depends entirely on the type of data being backed up and some type simply won't compress. Please check this jumper as the original wording on the installation might have been confusing.

I hope this helps,

Brian

[Ernest Ford]

I have the Archive 4326XX and the 9-10 jumper is NOT in place.

[Brian_Murdoch]

Hi Ernest,

The 4326NP should have a switch pack, maybe older models have the jumpers but the positions and number of pins should hopefully be the same. Please check the positions etc. with the attached diagram for the Archive 4326NP.

Regards,

Brian

[Ernest Ford]

The 4326XX has jumpers - identical to those in the Conner diagram and according to the manual that shipped with the drive, compression is enabled by default, jumpering 9-10 will disable it, exactly as you pointed out for the Conner product.

[Gus Kwong]

Looks like people on Google say if you backup already-heavily-compressed files while the hardware compression is switched on, it'll add roughly extra 5% to its size before it puts it on tape... which is logical really. Imagine, a batch of data that cannot shrink any further, but extra headers keep adding on for each compression process.

[Ernest Ford]

I've heard this "data compressibility" issue before - in fact when I first got the 4/16Gb TurboDAT, I asked Compaq about the 16Gb figure, because ALL the other DDS2 drives I had worked with claimed a theoretical maximum of 8Gb. Compaq at first attempted to BS me with "data compressibility" and when I pushed the issue, acknowledged that claiming 16Gb was an overly optimistic estimate that probably could not be reached in a real world environment.

I recognise that some types of data file compress better than others, and that trying to compress an already compressed file can cause it to grow. My data is a fairly normal mix of word processing and spreadsheet documents, with the occasional small database and should compress.

To prove the point - I installed WinZip and compressed the data I'm trying to backup - from 4.5Gb to 3.4Gb.

[Gus Kwong]

I've tried a quick search on "Lempel-Ziv compression" (I believe this is what the DAT/DLT drive uses), and apparently at best the algorithm can actually do 4:1 sometimes 8:1 on Unix system, and I guess salesmen at Compaq took the chance and tried to make it look super-impressive.

There is still an old Compaq document about what kind of ratio for file to be compressed on HP site http://h18004.www1.hp.com/support/techpubs/whitepapers/244a0996_ch4.html

It's interesting you've raised the point on tape compression. Did that 4.5GB database file sucessfully put on tape? What about the zipped up database?

I also find that DAT tape is very stingy in terms of spare space --- you only get what it is stated. DLT is a little more generous and normally gives you a little extra.

[Brian_Murdoch]

Ernest,

Can you post your drive firmware revision level here and I'll see if there were any improvements from the revision you have.

Regards,

Brian

[Ernest Ford]

Brian - I'll get back to you later on the firmware level - I'd have to restart the server to get that and that's not convenient right now.

Gus - let's call it a "dataset" rather than a database, and yes the compressed file does fit on the tape.

I'm currently running a test backup using an evaluation edition of Veritas Backup Exec that allows "hardware compression if available otherwise software compression".

What's immediately obvious is that the transfer rate has jumped by about 50%, so I'd say that I am seeing some sort of compression occuring.