Enabling multiplexing on VTL

[Sachii]

In my environment backup is going to VTL but multiplexing was not enable, now i am planning to implement multiplexing, please share your view on this, what would be the advantage/disadvantage on enabling multiplexing on VTL.

[watsons · Accepted solution]

We know that VTL is essentially disk-based, and multiplexing does not apply to disk.

However, there is a very good whitepaper about this question, worth a read and I believe gives you something to think about - not just the multiplexing, but what to do with I/O setting:

http://eval.symantec.com/mktginfo/enterprise/white_papers/b-whitepaper_using_virtual_tape_libraries_with_netbackup_04-2009.en-us.pdf

[Nicolai · Accepted solution]

I don't recommend to  use multiplexing on VTL's - instead deploy more virtual tape drives. Since they are virtual it don't cost you anything.

The reason to multiplex on tape, is to obtain the full writing speed on a tape drive, and prevent the shoeshine effect (many start/stop). Cost of slower restore is the price you pay.

But with virtual tape drives, you don't have to pay the restore penalty. There is no "max/min writing speed" or shoeshine effect on virtual tapes.

If the device do deduplication like Data Domain don't even think about it, because each MPX backup will look like new unique data the box has to save.

[mph999]

I might be missing something here but I don't agree with that white paper.

Take the simple example of two clients and one tape

If the two backups from each client are  1000 MB

The drive max speed is 100 MB/sec

Client 1 supply data at 100 MB/sec so would take 10 sec to complete

Client 2 can only manage 50 MB /sec will take 20 sec to complete

Run the backups separately takes 30 sec in total

Max them, still takes 30 sec ... as the limit is the speed of data transfer from the client, max does not affect this. 

As vtl has no issue with a minimum I'm streaming speed, where is the benefit of mpx to vtl.

Please do correct me if I'm wrong ....

[INT_RND]

If your goal is to increase performance I would recommend not using a VTL at all. It adds an additional software layer that provides no benefits.

Just write straight to disk. 

[Genericus]

Unfortunately, at one time VTL was recommended, Gartner upper right quadrant and everything. At that time NetBackup had issues clearing fragments from disk based storage.

Our company invested heavily in Fiber channel architecture from server to VTL, so all our servers, media servers, storage and tapes are fiber channel based, to get off VTL we would need to invest in fairly expensive 10G ethernet cards as well as all the 10G networking infrastructure.

Building new from scratch is not always the same as working with existing infrastructure.

Coming into a job where the VTL and FC is already in place and recommending they scrap it and rebuild at great expense is NOT appropriate.

When asking for help with VTL, and getting told not to use VTL, IMHO equals no help at all.

I have the same issues as Sachii, and I am looking to move off VTL, but I can't. 

Having said that - Nicholai is correct - DO NOT use multiplexing, just make more virtual drives. DO NOT make them shared though - that causes issues. The other thing to work towards is increasing the speed from the sources - checking buffer settings on the clients, optimizing the storage/networking, etc.