Nginx load balancing Maxscale to failover - nginx

Question

I have a simple stream block to stream MySQL TCP traffic to Maxscale instances. 2nd instance acts as a failover only, like:
stream {
upstream maxscale {
zone upstream_maxscale 64k;
server 10.1.0.11:3307;
server 10.1.0.12:3307 backup;
}
server {
listen 3307;
proxy_pass maxscale;
}
}
When connections are low (<30), everything goes fine. But when connection are high (>40, if we can say that 40 connections are high...), nginx error log keeps complaining about something that i don't know how to debug.
recv() failed (104: Connection reset by peer) while proxying and reading from upstream, client: 10.1.0.16, server: 10.1.0.15:3307, upstream: "10.1.0.11:3307", bytes from/to client:15738/64316, bytes from/to upstream:64316/15738
I've tried play with options like reuseport, worker_connections or so_keepalive but no chances.
https://nginx.org/en/docs/stream/ngx_stream_core_module.html
https://docs.nginx.com/nginx/admin-guide/load-balancer/tcp-udp-load-balancer/
Can it be a problem in the Maxscale side?
Here the Maxscale 2.4 listener:
# Listener
[listener-rw]
type=listener
service=readwritesplit
protocol=MariaDBClient
address=10.1.0.11
port=3307
ssl=required
ssl_ca_cert=/var/lib/maxscale/ssl/ca-cert.pem
ssl_cert=/var/lib/maxscale/ssl/server.pem
ssl_key=/var/lib/maxscale/ssl/server.key
ssl_version=MAX
# Service
[readwritesplit]
type=service
router=readwritesplit
servers=sql1,sql2,sql3
user=maxscale
password=324F74A347291B3BE79956AD5F4BB2FAD65E1F9052A976722917701742729400
enable_root_user=1
max_sescmd_history=150
max_slave_connections=100%
lazy_connect=true
slave_selection_criteria=LEAST_CURRENT_OPERATIONS
optimistic_trx=true
connection_keepalive=300
master_failure_mode=fail_on_write
https://nginx.org/en/docs/stream/ngx_stream_core_module.html
https://docs.nginx.com/nginx/admin-guide/load-balancer/tcp-udp-load-balancer/

Answer 1

The MaxScale log (in /var/log/maxscale/maxscale.log) most likely contains either an answer as to why you receive such errors or at least will help you determine what the problem might be.
In case you can't find out the reason for this from the logs alone, I would suggest opening a bug report on the MariaDB Jira under the MaxScale project.