Optimize for concurrent lan backups is a more a efficient way of transferring data. I think it was introduced around the V6/V7 timeframe. However there is one caveat.
When you use a firewall configuration, it will not make use of the 'additional ports'. This means that your data transfer IS going over the tunnel port, and the tunnel port only. Bundling up the data with the HTTP/HTTPS protocol will slow down the data transfer, as there is a lot of overhead when pushing this via the tunnel.
When you turn off 'optimize for concurrent lan backups' we fall back to the legacy transfer method, which is less efficient on MA resouces, but it will actually use the additional ports you specify in the firewall config.
This means the data transfer portion of the job is going directly to the CVD process on the remote media agent, which is why your dash copies fly.
You can actually get the transfer to use one port that is NOT the tunnel port when you have the optimize option turned on. To do this, add the CVD port to the additional ports (in 99% of cases this is port 8400). Make sure that 8400 is open on your firewall between your dash media agents for the transfer to work.
This is actually documented in BOL:
- For MediaAgents with Optimize for concurrent LAN backups option enabled, opening the incoming port of the CommVault Communications Service improves the backup performance.
So I'd recommend re-enabling the option and adding 8400 as an additional port for the best of both worlds.