8 Krypton

Tape Usage

Hi to all,

I have an IBM Tape Library TS3200, both drives are LTO4 so the capacity of LTO4 tapes is 800GB, and with hardware compression is 1600 GB.
Insight into the media/volumes section to see the tapes usage I see that 32GB data took 4% of the total amount of tape. 32GB is 2% and not 4% of 1600GB.
Is it means that Networker can't see hardware compression of tape Library or what?

Do I need to define on both drives "Volume Default Capacity" with value 1600 GB or I have to configure something else?

The hardware compression on Tape Library is turned on.
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8 Replies
8 Krypton

Re: Tape Usage

You may not have compression enabled on your drive. Also the percentage figure is an estimate of the amount of space used; it might be higher than you expect because you are backing up data that does not compress much.
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8 Krypton

Re: Tape Usage

Yes, I checked twice, compression is enabled on both drives.

We talk about hardware compression and it does not matter what type of data stored. IBM says that it is a ratio of 1:2, so in our case 800GB of Native Raw Space must be 1600 GB at least.

Thanks for your reply.
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sgt1
6 Indium

Re: Tape Usage

Usage rate in NetWorker is just a bit of "guesswork".

There is an entry "Volume default capacity" on Device Properties --> Configuration Tab. NetWorker shows the usage rate for the capacity YOU have specified here.
If not specified, NSR assumes uncompressed capacity by default, because for compression rate the milage varies.
The value is in effect only for freshly labeld volumes in that drive, not for cartridges you have labeled before setting the value.

Hope this helped -sgt-
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8 Krypton

Re: Tape Usage

"We talk about hardware compression and it does not matter what type of data stored"

I don't know who told you that but 10 gets you 20 it was probably a salesman who didn't know what he is talking about. Whether it is software or hardware compression it works by applying an algorythm to your data where it looks for common or repetitive data which it replaces with something that takes less space to represent it; it is not a case that it does something which fits 2 bytes into 1.

Some data can be heavily compressed - for example a newly created Oracle database may have a large amount of blank table space and can compress significantly more than 2:1. Conversely, other data is already compressed and applying compression again has minimal effect and may even cause the file to get bigger - some examples of compressed files - zip, tar, jpg, mp3.

You need to look at what data you are backing up and assess how compressible it is likely to be.

Look at the tapes you have already filled up and see how much data is being stored there - the percentage is an inaccurate estimate and not a definite figure - use the command:

mminfo -q %used=full -r volume,capacity,written
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8 Krypton

Re: Tape Usage

Thanks a lot guys,

I only set "Volume Default Capacity" at the 1600 GB and after relabeling this workarround solved my problem. Everything is fine now, 32GB of data shows 2% of tape usage.

My backup data are MSSQL databases backed up with Networker module for MSSQL and with software compression turned on (-C switch).
Is it possible that the compression will be greater when it is located on the tape?
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8 Krypton

Re: Tape Usage

Hardware and software compression may use different compression algorithms so may result in slight variations in compressed size but it is unlikely to vary significantly.
Applying compression multiple times on data is unlikely to result in further compression and may result in data getting larger so generally isn't something you wish to do.
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8 Krypton

Re: Tape Usage

Thanks David

This case can be closed
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8 Krypton

Re: Tape Usage

Wicked Dave thanks for the command,
I gave it a run and am getting some really cool compression across my media.

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