postgres@pgtest05f ~ $ psql
psql (9.4.4)
Type "help" for help.

pgtest05f/postgres M # show max_replication_slots ;
 max_replication_slots
-----------------------
 2
(1 row)

Time: 0.433 ms
pgtest05f/postgres M # \q
postgres@pgtest05f ~ $ psql
psql (9.4.4)
Type "help" for help.

pgtest05f/postgres M # alter system set max_replication_slots to 3;
ALTER SYSTEM
Time: 5.309 ms
pgtest05f/postgres M # \q
postgres@pgtest05f ~ $ /usr/pgsql-9.4/bin/pg_ctl stop -m fast -D /var/lib/pgsql/9.4/data/
waiting for server to shut down.... done
server stopped
postgres@pgtest05f ~ $ /usr/pgsql-9.4/bin/pg_ctl start -D /var/lib/pgsql/9.4/data/
server starting
postgres@pgtest05f ~ $ < 2015-10-26 11:43:25.081 MSK >LOG:  registering background worker "pg_stat_wait collector"
< 2015-10-26 11:43:25.082 MSK >LOG:  registering background worker "repl_mon"
< 2015-10-26 11:43:25.461 MSK >FATAL:  no more LWLocks available

postgres@pgtest05f ~ $ vim /var/lib/pgsql/9.4/data/postgresql.auto.conf
postgres@pgtest05f ~ $ /usr/pgsql-9.4/bin/pg_ctl start -D /var/lib/pgsql/9.4/data/
server starting
postgres@pgtest05f ~ $ < 2015-10-26 11:45:04.244 MSK >LOG:  registering background worker "pg_stat_wait collector"
< 2015-10-26 11:45:04.244 MSK >LOG:  registering background worker "repl_mon"
< 2015-10-26 11:45:04.447 MSK >LOG:  redirecting log output to logging collector process
< 2015-10-26 11:45:04.447 MSK >HINT:  Future log output will appear in directory "pg_log".

postgres@pgtest05f ~ $