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Different compression ratios

Different compression ratios

(OP)
HI,
We use a Quantum tape library and Sony DLT IV tapes to back up our data. I've noticed that say nearly all the tapes will have a different compression ratio. Even on our full backup jobs, which span multiple tapes, the compression ratio varies. Eg, for our full Netware backup, the first tape might have 45GB on it, the next might have 60GB on it and the next may have up to 90GB on it. Is there a reason for this?

Thanks

RE: Different compression ratios

jasonsas

Jason, I suspect the issue is that the files on the server is dynamic.  You may have the server set to compress files - probably yes, perhaps after a week on inactivity.

Now, let say you backup an active area - the files will be uncompressed, and the compression rate will be high.

Then the tape hits large sections of the volume that is very static - read-only documents, program files, etc.  These files have been compressed already -- you will not be able to compress then further - compression ration will be high.

Another situation is backup up encrypted files, or an email database - these files can also not be compressed.  If your backup hits a large ccMail or Lotus Notes database, it will chug along at a very slow rate with no savings.

Perhaps the worst situation I encounter is when I backup a huge number of very small files (FTP copy program) - half the time of a our backup is spent archiving this section - small encrypted PDF files have a relatively poor compression, and require a zillion open / close I/O activity which is deadly.

Richard

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