OpenLDAP freeze

[Cyril Grosjean <cgrosjean@janua.fr>]

I've an OpenLDAP 2.4.32 server with millions of entries, which uses to 
serve tens of requests per second.
It works as a stand-alone master server (replication is setup but the slave 
server is down)

I noticed several times, at different times and dates, that no lines are 
logged in the OpenLDAP access log
for a few minutes, as if it was frozen. I try to figure out what could 
explain that ? I checked the usual
system counters (CPU, IO, Memory, free disk space, ...) as well as the 
running processes, but did not notice anything
suspicious.

Since the OpenLDAP server runs in a virtual machine, I'm currently trying 
to monitor the ESX hypervisor.
Any idea of what could go wrong ?