Tape Size

[H_Sharma]

Hi Experts,

We are using LTO 6 tapes. I found one starange thing today however their size is 2.5 Native capacity. Some tapes are still assinged with 4 GB capacity.

We didnot configure compression as far as i know. So pls suggest what is happening. Where should i check via command whether its been configured with

compression?

Thanks,

[Marianne]

Compression is enabled by default on the tape drives.

The amount of data that fits onto a tape depends on type of data. 
Some types of data compress better than others.
e.g. text compress well, media files like (.jpg and .mp4) do not.

[RiaanBadenhorst]

LTO 6 should be capable of 2.5:1 compression so (if you're lucky) (and the data is very compressable) you might get around 6TB on a tape.

http://en.wikipedia.org/wiki/Linear_Tape-Open

We assume you mean 4TB and not 4GB :)

[H_Sharma]

Hi Experts,

My mistake its 4 TB.

So if its compressing images by default it means it will take more time to backup?

[RiaanBadenhorst]

No, compression is done on the tape drive by a special chip. It has no impact on your performance.
 

[Marianne]

I have never seen compression at hardware level having any impact on performance.

Hardware compression has been around for as long as I've known NBU.

Factors that cause slow performance are normally slow client disk and network. Hardly ever tape drive.

[jim_dalton]

Hardware compression is a long way from being the performance bottleneck in your environment. Mine too.

Jim

[Dilip_Choudhry]

Hi Hargyan Sharma , Check the below link , Will be helpfull to you .To Understand how compression works and to confirm whether Hardware is compressing the files or not . http://www.symantec.com/docs/TECH78389 http://www.symantec.com/docs/TECH50960

[mph999 · Accepted solution]

And to answer your other point ...

The tapes are not configured with a capacity, there is no command to check because it does not exist.

NBU has no understanding of tape capacity.  the different densities, hcart / hcart2 / 4mm etc ... have no technical meaning, they are just a label to allow a certain tape to be used in drives with the same density (eg. hcart tape will only go in an hcart drive).

I can set my LTO drive on my test server, and my tapes as 4mm.  It will (and does ) work exactly the same as if I configured it as hcart, or hcart3 - hell, I could even configure it as 'Micky Mouse', NBU wouldn't care - ok I'm joking on that last part, but you get the idea ...

Tape capacity is determined by the tape drive itself.  At some point near the physical end of the tape there is a 'special mark' that is detected by the drive firmware, and this sets a 'flag' in the tape driver.  When NBU tries to send another block of data the tape driver refuses to accept it (well, the OS does ...) and it passes a message back to NBU that the tape is full, and we need a new one.  So, without this message, NBU would try and write to the same tape for ever.

Reasons for low capacity are usually that the data isn't as compressible as you think it is.  Other causes are bad firmware and even faulty hardware.

[jim_dalton]

If he's got 4Tb on a 2.5Tb native capacity tape then he's compressing.

If you have a management gui for your drives/robot then this may also tell you the same deal as it write out data realtime, Then again it may not.

I wouldnt spend much more time thinking about it.

Jim