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Sony Tape Compression

So I have Backup Exec 9.1 and a Sony AIT drive.  The tapes say that they will do 260 GB compressed, but only 100 GB native.  I have compression enabled in the backup job, I also have right clicked the drive and enabled compression there.  For whatever reason in the media tab the available capacity is still only 100 GB???  Why?
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Afer some research it looks like everyone recommends installing the Veritas version of the tape drive driver.  Where do I download this?  I have been all over the Symantec site.
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It's reporting the native capacity because that's all that's guaranteed to be written to the tape. If you were backing up compressed TIFs, for instance, you'd only get 100GB's worth.

But it doesn't actually matter what is being reported as the capacity, as the software will write as much as it can to the tape, until the drive reports LEOT (Logical End of Tape) at which point Backup Exec will write a trailer before it hits PEOT (Physical End Of Tape) and then request another tape.

So you will be getting the maximum capacity the tape can hold given the compresabillity of the data, not what is actually being reported as the capacity.
Not exactly sure I follow.  Obviously not all the data I am backing is already compressed, so it should go at least a few GB above the native.  How do I figure this out aside from looking at the data I am backing up and trying to figure out what the original size is?
And you saying that even though the tape says 260 GB it will never report this much as a capacity in Backup Exec?  It will simply compress what it can and fit the compressed data into the 100 GB capacity.  In effect being 100 GB of compressed data that would equal much more uncompressed?
You have to find out the capability of the tape drive(s)

Look at the documentation for your tape library machine and the tape drives installed in it.  DLT, SDLT, SDLT 1 tapes all have different compression capabilities and will only compress to the max capability of the Tape DRIVE used.

If you have an SDLT1 160/320Gb tape, its theoretically possible to have 320GB data written, but if your drive is only rated for DLT 110/220, the 100GB is all you are going to get.  After compression you could expect to see about 140-160 gb of data on the tape.

If you are only getting 100Gb, chances are you have a DLT drive
Available capacity on a blank tape will always show the native capacity i.e 100GB in your case.
Once you have run a backup check the devices tab in BEWS and the last column will show you the compression ratio
if it is anything but 1:1 then compression is working
Do not expect to get 260 GB on the tape cause that is purely theretical and almost impossible to achive that kind of compression ratio
Please remeber that the amount of information that can be written on any tape can never be more  than the NATIVE Capacity

If you can compress X amount of DATA to anything less than the NATIVE CAPACITY of any TAPE MEDIA then you can write X GB of data on that tape.
Remeber that no matter what kind of compression you use it uses an ALGORITHM in to search the block of data for similar long re-occurring strings of 0s and 1s and the more number of strings and the longer the string better the compression ratio.Hence the compression ration depends entirely on the kind of data being backed up
For more information refer to  the following documents
How Backup Exec for Windows Servers controls hardware compression on tape devices. 

Backups fail to get double the amount of the native capacity recorded to a media, even when hardware compression is being utilized, or the total byte count in the job log is higher than the maximum possible compression rate of the tape media. 
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Once the tape is written to by way of a backup job it will show you the USED CAPACITY ,AVAILABLE CAPACITY which will depend on the compression ratio acheived by that job.
New tapes will show the native capacity always as they do not have any data written
The available capacity of a tape is not always the TOTAL CAPCITY - USED CAPACITY the USED CAPACITY will not be the true size of your backup job and will vary with the compression ratio
I have seen 1.2 TB of data on a 160 GB tape : compression ratio 26:1 
I even tried a restore from the tape and it worked 

What kind of data was on that tape?