edit how to generate core file

Author: Marian Zvada

docs/ops/troubleshoot.md

@@ -0,0 +1,29 @@
+# Troubleshooting tips
+
+## Get core file
+Common settings on the system:
+* set core file size to unlimited: `ulimit -c unlimited`
+
+### Core from unexpected process quit
+
+First you need to activate xrootd to create core file when process exits unexpectedly:
+
+* set env variable upon cmsd/xrootd start `DAEMON_COREFILE_LIMIT="unlimited"`
+    RHEL6: env set in `/etc/sysconfig/xrootd`
+    RHEL7: you can use `/etc/sysconfig/xrootd` but prepare override file `/etc/systemd/system/cmsd@clustered.service.d/override.conf` with content:
+```
+     [Service]
+     EnvironmentFile=-/etc/sysconfig/xrootd
+```
+
+### Core from running process
+* directly if you know PID: 
+```
+$ gdb --pid=PID_NR
+(gdb) gcore
+```
+
+* if you know process name (e.g. xrootd):
+```
+`$ gcore $(pidof xrootd)`
+```