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The real limiting factor here is going to be your flow arrival rate, rather than the total number of interfaces. We do have customers that are comfortably running several thousand interfaces.
The kind of performance problem you're seeing is usually related to a very high flow rate, or some underlying issue with disk performance for the FSDB. It might be a good idea to take a look at the flow arrival rates for each collector. While the settings page does a good job of showing you the aggregate flow rate over time, it doesn't break out individual collectors. That's a little more involved.
and for the devices you are adding, to estimate the additional flows you will add: https://support.solarwinds.com/Success_Center/Netflow_Traffic_Analyzer_(NTA)/How_do_I_estimate_how_many_flows_per_second…
Adding on to this comment, once you know what your flow rates look like it may be relevant to know that a single poller with proper hardware can support up to 50,000 flows per second, and netflow overall caps out around 300,000 flows if you have 6 or more pollers to spread that load over. To be honest, I would be surprised to see 181 interfaces exceeding the 50k flows/sec unless many of them were high speed interfaces like >10G running high utilization rates.
I've worked with many clients who had 300+ netflow interfaces no problem at all. As mentioned above I would be looking at the perfmon counters on your server to begin to see if you have a disk bottleneck or something similar going on. Does the storage have any other potentially noisy neighbors on the array?
I'm only encountering this issue on my primary server, not any of my APEs. I checked perfmon. I'm getting ~700 v5 flows and ~900 v9 flows per second on that server.
I really want to get this situated, because I'm going to be bringing in a lot of other APEs, at least doubling the current sources.
I checked the datastore the server is on and didn't notice any server that would be hammering the disk IO. Should I just repair NTA on the primary I guess?