Data does not compress on 4/16Gb TurboDAT

[CA893598]

I'm using Server2K's backup program to backup to a Compaq 4/16Gb DDS2 drive. The drive is jumpered to enable data compression and the box in the software checked to allow data compression, but I cannot get more than 4Gb of data onto a single 120m cartridge.

I don't expect to get 16Gb but I should get more than 4Gb - any suggestions?

[meloni]

Conpression of data may be due to a lot of factors, data compressibility, but also tape quality, streaming of data, i/o errors or retry, software scsi commands...
I would suggest you to run StorageWorksLibrary&TapeTools, compression test. There is also the possibility to create a fixed amount of know compressible data and the backup those to a tape and see the amount on the tape (for this use createdata) http://h20000.www2.hp.com/bizsupport/TechSupport/SoftwareDescription.jsp?locale=en_US〈=English+%28US%29&pnameOID=63998&prodSeriesId=42883&prodTypeId=12169&basePartNum=COL1945&locBasepartNum=co-9515-2&os=Microsoft+Windows+2000&tech=Utility

[CA893598]

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

[meloni]

Hi Ernest,
you are right, the 16 Mb is a litle bit optimistic, but may be achieved depending of the file type, years ago, when the files to backups came from main farme or unix system, and everything was caracter based, it may be some documents reach compression also higher than 4:1. In hour days, it si difficult to reach 1.2:1 because os already optimise files and directories.
The test with Winzip was a good test until four or five years ago, now the compression algorithm of Winzip have been updated and optimized, the algorithm for DDS Drivers never changed to keep compatibility with legacy systems.
marino

[CA893598]

I successfully backed up that zipped file to the tape, so I think we've pretty much eliminated the tape capacity AND "data compressibility" from the equation - in my mind, it's now a question of how do we enable compression on this drive - given that it has been "jumpered" according to the manual - 9-10 open - and that doesn't do it.

I'm also running a test backup using an evaluation edition of Veritas BackupExec which does software compression and have seen an increase in transfer rate to the order of 50% which indicates that compression IS occuring - unfortunately this is not a cost effective option and I will have to find an alternative solution within the next 60 days