Media Set Capacity Wrong

[danieln]

Hi there

 

I'm using Retrospect for mac 10.5 with a Quantum Scalar i40. Im using 2,9TB LTO-5 Tapes.

Within a Media Set I have 9 Tapes. By my calculations 9 X 2,9TB = 26,1TB. However Retrospect seems to be doing different mathamatics? (12,9TB)

What am I missing?

 

Please see screenshots attached:

 

 

[Lennart_T]

http://kb.retrospect.com/articles/en_US/Retrospect_Article/Tape-Capacity-FAQ/

[danieln]

* Starts looking for alternative backup software....

[Lennart_T]

* Starts looking for alternative backup software....

Care to explain why?

[danieln]

I have created a new media Set, with brand new tapes and this what I find:

 

Brand new tapes are being used. So there is no excuse for Condition of Tapes or Stopping and Starting of Backups.

I am using SAS connected locally to Active Storage RAID 6/mbps, so Source Speed is not applicable.

The capacity given is before any backup has completed. So No Compression issues are applicable.

Have not run a backup yet, so NO Media failures have occurred, so it cannot be a problem.

 

I understand its an estimate, but a 10TB loss is one hell of loose margin.

 

I've been using Retrospect successfully for many years and find it very good, but this is the first time using it with so much data. Maybe I'm doing something wrong?

[Lennart_T]

Short answer: Your data is not 2:1 compressible.

 

Long answer: Please read the article again.

[Mayoff]

Retrospect has no control over hardware compression. It is controlled by the hardware. If a new tape is required, it is because the hardware needed a new tape to continue. Retrospect uses the tape until it is physically full.

[twickland]

To chime in, the tape capacity that Retrospect displays is only an estimate, and is not controlling in any way. A tape will be used until it is either full or until a fatal write error causes the drive to request a new tape.

 

The capacity shown on the summary page is the actual amount of space used for data in the media set plus the estimated space remaining on the last member. It has nothing to do with the advertised capacity of the tape—which is based on a 50% compression rate in the tape drive, and which is often optimistic—nor with the previous estimates of tape capacity which, as your screenshot notes, are still listed in the Members window. (It would be nice if Retrospect would replace the estimated total capacity for earlier members with the actual amount used, so that this window would match the Summary window.)

[oleksiak]

I understand its an estimate, but a 10TB loss is one hell of loose margin.

 

I've been using Retrospect successfully for many years and find it very good, but this is the first time using it with so much data. Maybe I'm doing something wrong?

 

There is no 10 TB lost.

You are not doing anything wrong.

 

The only thing you misunderstand is the real physical capacity of LTO5 media.

LTO5 tape physically fits 1,5 TB of data, not 3 TB.

It can fit up to 3 TB ("advertised capacity" ) by compressing "easy to compress" data on-the-fly internally, but as fellow members pointed out - the data you are backing up is already compressed (or too hard to compress).