If "Optimized For concurrent LAN backups" enabled on MA speed backup Oracle DB slowdown at 3 times

There are two major areas that get impacted when toggling this setting. This is a confusing topic, so bear with me. 

#1 - That option changes the way data is transferred between Commvault processes local to the media agent. This option is also sometimes referred to as SDT (simple data transfer). With “Optimized For concurrent LAN backups” ON (i.e SDT ON), data is transferred from one process to another using the loopback adapter (i.e 127.0.0.1 in most cases). Each process binds to a port on the loopback and uses TCP to send data. If this option is turned OFF (SDT OFF), it uses shared memory to transfer data between processes. I.e data from the client is receive to the Media Agent (lets say on the CVD process), and then the writer process may be CVMA, so CVD puts the data into memory, and CVMA reads it out. I’m not 100% sure if that is the correct scenario but the takeaway is that it does not use the loopback and TCP connections to communicate between processes on the MA if you disable optimize for concurrent lan backups. As you can imagine, this setting could increase memory usage and lower scale, which is why SDT is on by default. It may be worth investigating performance of the loopback adapter to ensure it can transfer data fast (like, gigabytes/sec fast) using a tool like iperf3.

#2 - now, if you disable optimize for concurrent lan backups (SDT off), it also changes the way the client communicates with the Media Agent - especially so if you have a network configuration in place restricting ports. I am a bit rusty on this, but with optimize off (SDT off), I believe data will bypass any firewall tunnels (i.e sending control traffic and data over an established tunneled connection - typically 8403) - it will send control traffic over the firewall tunnel but send DATA traffic directly to the CVD port on the Media agent (i.e 8400). This sometimes offers huge performance especially if you are choosing to encrypt traffic over the network tunnel in the networking options. Of course, disabling SDT bypasses network encryption so be aware of that.

One way you could test this out is to keep optimize for concurrent lan backups ON, but add 8400 as an additional port in the network/firewall configuration. This will bypass the network tunnel for data traffic (with the encryption caveat I mentioned before). Another option if you are using network configurations is to increase the ‘tunnels per route’ option discussed in the second post in this thread. Disabling SDT allows the ‘additional data ports’ networking option to be used, that option is ignored when optimized for concurrent LAN backups is ON.

So the TLDR; You could have a performance issue with the loopback adapter on the media agent, OR you are bypassing some of the firewall protocol and encryption overhead which in-turn is improving the performance - especially on unix clients, disabling SDT allows additional port usage which can benefit some environments.