One of the nifty features of the EMC's EDL is it's ability to run a NetBackup Media Server on the DL Engine. Just to back up a second: an EDL has 3 logical components: the DL Engine, the Clariion, and the disk. The DL Engine is where the good stuff happens that virtualizes disk as tape. It is also where data compression happens, system management occurs here, and so on. A couple of years ago, we enabled this server to run a full copy of either a NetBackup Media Server, or a NetWorker Storage Node.
Why? Because this would permit the EDL appliance to make a second copy of a virtual tape (either local or remote) and it would update the backup application catalog to show that second copy. No more did you need to worry about creating a replica with the appliance that was invisible to the backup application, and shared the same bar code (this is bad).
So life is good, right? Well, pretty much, but there is one little trick that we need to do to fully take advantage of the capabilities of the Embedded Media Server (EMS). And that is to utilize a feature in NBU called the "alternate read server".
Again, lets look at what happens when this isn't enabled: an external media server backs up a client to virtual tape A. I want to create a second copy or a physical copy of A, using the EMS. The EMS asks the external media server to mount the virtual tape, the external media server reads the virtual tape, transmits the contents to the EMS over IP, and the EMS writes to physical tape.
It works, right? You bet. But it is slow. Based on observation, I would say that it is typical for this operation to run in the 25-35 MB/s range (even with an LTO3 or LTO4 drive). This has a lot to do with the data traversing the IP network, and some shoeshining behavior on the tape drive as a result.
So, how can we get much better performance? Use the "alternate read server" feature. The alternate read server can be accessed and set in a few different places, but most often we are interesting in defining it as a storage destination in lifecycle policy. The manual says: an "alternate read server can be specified for both backup and duplication destinations. However, to use an Alternate Read Server as part of a duplication operation, the name of the alternate server must be specified in the backup destination configuration."
Basically we are enabling the EMS as an alternate read server. This allows it to directly read virtual tape A over the internal FC connections of the EDL. It can then write to the physical tape, over FC too. No more traversing an IP network. Much better speed. One client recently reporting duplications speeds in the range of 90 MB/s using this approach.
So, although you can create the second copy either with or without an alternate read server, you will see much better performance by enabling the EMS as an alternate read server.