Compression issues with DAT72

[David_Blowes]

Hi guys,

I'm a new entrant into the world of server administration, having only just finished installing a SBS 2003 server for our company. We have a IBM DD5 gen5 SCSI tape drive installed, and Symantec Backup Exec 10.0 (build 5520). The tape drive currently reports through the device manager that it is using Symantec drivers.

I've been running the first backup run, and the compression ratio is crap. In fact, there doesn't seem to be any compression! (the job is still running, so I can't check the actual ratio, but give that it's needed two tapes for 43GB, it can't be too good)

I've searched through the forums and seen a few other people with this issue, with a few suggestions for things to check.

Hardware compression is enabled (with software compression set as a backup). I haven't tested just doing the windows SBS backup yet, which I'm thinking will be the next step (I won't get a chance to do that until Monday though).

Does anyone have any further suggestions? As I mentioned, I'm fairly server noobie, so guided help is much appreciated!

[Keith_Langmead]

Well it's difficult to judge until you've finished the backup job and confirm how well the compression has worked, but realistically you shouldn't expect too much in terms of compression ratio, certainly nothing like what the manufacturers would have you expect.

Remember that a DAT72 tape is 36GB native, 72GB compressed, and reality most people find they get a compression ratio of around 1.1:1 to 1.2:1, so you shouldn't really expect the tape to backup more than between 39GB to 43GB. Also consider that being an SBS server you're probably going to have a larger proportion of files which can't be compressed a lot, eg binary files, databases, exchange datastore etc, unlike something like a web server which contains lots of text files, logs etc.

Let us know how you get on when you've finished the backup job.

Keith

[Sascha_H]

Let's say there are many different reasons why the ratio is "not good"...

It can be
- media related (old media tends to lose capacity)
- cleaning issues (DDS drives should be cleaned every 24hs, AIT every 100hs, other refer to manual)
- cleaning media that simply does not work (wrong brand, used up, broken)
- transfer speed of the SCSI controller (too low or too high) preventing stream of data
- already compressed data, like mpeg, jpg, zip, pst, etc. does
- sometimes software compression gets better results than hardware one
- the ratio listed during backup does not always tell the truth (there is a Symantec TechNote about it!)
- incorrect SCSI configuration, termination (connecting tape drives to RAID controllers is not advisable)
- old drivers / firmware of the backup unit / controller

What is the native capacity of the media? What drive is it exactly as I do not know the "BM DD5 gen5 SCSI"? Tell me more about the configuration of your system.

Thanks

[Rucha_Abhyankar]

Hi David,

Refer to this:

Backups fail to get double the amount of the native capacity recorded to a media, even when hardware compression is being utilized.


http://seer.support.veritas.com/docs/199542.htm

Compression Ratio does not display or displays incorrectly when using software compression.

http://seer.support.veritas.com/docs/273089.htm


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