If a ClickHouse node is unresponsive and you don't know what is happening, you can easily check the stacktraces of all the working threads by running:
SELECTarrayStringConcat(arrayMap(x -> demangle(addressToSymbol(x)), trace), '\n') AS trace_functions,count()FROM system.stack_traceGROUP BY trace_functionsORDER BY count()DESCSETTINGS allow_introspection_functions=1FORMAT Vertical;
If you can't run any query but you have access to the node running the process, you can execute the following command to send a
TSTP signal to every thread of the process:
for i in $(ls -1 /proc/$(pidof clickhouse-server)/task/); do kill -TSTP $i; done
Stack traces will be printed in the ClickHouse logs.