Tape backup "Used Capacity" consistently higher than "Data"

[shnrub]

Server 2003 (SBS) 32bit, using Backup Exec for Windows Servers (problem first seen in 11d, same problem exists in 2010 R2) with an LTO5 tape drive, LTO4 tape media.

Backups are consistently using significantly more space on the tape than the data written (eg a backup fills a blank LTO4 tape, 780Gb of 781.5Gb capacity, job hangs waiting for overwritable media to be inserted) with only 460Gb of data written to tape.

Have tried all combinations of hardware software compression - compression does appear to succesfully compress data (compression might get 580Gb of data on a 780Gb tape)

Diagnostic tools suggest the tape drive is OK, have tried latest vendor drivers and latest symantec supplied drivers.  Have tried adjusting the selection list (exclude large portion of data, problem remains, exclude everything and include only what was previously excluded, still same issue).

The same amount of data was previously just at capacity on an LTO3 tape which achieved around 1.4:1 compression.

I've never seen anything like this before, and can't find anyone else with similar problems - will test with NTBackup and possibly LTO5 media, but I have no intention of using either of these for actual backups at this stage.

[CraigV]

Hi,

I have seen this a couple of times on the forums, and it appears as if the data that is written gets "uncompressed" slightly during the write process. For what reason, I am not sure.

Have you upgraded all the latest patches in BE 2010 R2?

Can you also confirm what type of data is being backed up?

[Hemant_Jain]

Please provide more details on kind of data being backed up, like platform, agents etc.

Thanks

[shnrub]

"Slightly" I could live with (though I wouldn't be happy, as the data is compressible), but an extra 200Gb of overhead from 580Gb of data is obviously not normal.

The data being backed up is a pretty typical fileserver data.  Word docs, cad drawings, pdfs and images mainly.  The same data used to average 1:1.3 or better compression with the old tape drive.   It's an SBS server, so has Exchange and SQL agents, though even without including this data in the backup set the problem remains.

[pkh]

Have you tried using a LTO5 tape with your LTO5 drive?  Does that makes a difference?

When you use software compression, the hardware does not come into play.  So if you still get bad compressoin then it is your data.  Images compresses badly.  You can read about this in my article below.

https://www-secure.symantec.com/connect/articles/compression-short-explanation

To trouble-shoot your problem, examine each resouce that backed up and you should be able to see the compression ratio for each of them.

[shnrub · Accepted solution]

Damn, turns out it was actually a faulty tape drive.  While diagnostics completed succesfully on LTO4 media and no errors were reported during backup, LTO5 media resulted in write failures, diagnostics failed, drive was replaced and all is well.