Scaling log server and log to storage ratio

Using FoundationDB
1

I’m king of wondering on the impact of the scaling on Log servers, I was doing some load testing last week with 4 logs server per region and the log servers were clearly saturated on CPU (not on I/O it seems), so I quadrupled the number of logs server and I still the process were showing little improvement (maybe 5 to 10% CPU reduction) while everything seemed to stay similar.
This is kind of surprising to me, maybe there is still something I don’t understand with FDB. For the record I was using a ratio of 10% writes to 90 reads and we were reading randomly in a 10M keys space.

I’m wondering if I’m not ending end up reading a lot from the log server, is this something that can happen if yes under which conditions ?

Also I’m wondering what are the current recommendation for the ratio log to server for the SSD-2 storage engine, I found a few posts mentioning 1 to 8 but I’m wondering if it’s still the case.

3

For what it’s worth, I ran into something similar recently; please see Log processes and CPU saturation for that discussion. We didn’t reach any firm conclusions there, but I think there are some plausible hypotheses to test.