title: What is the purpose of `pg_logical` directory inside PostgreSQL data?
body: <p>I've just stumbled upon this error while testing failover of a PostgreSQL 9.4 cluster I've set up. Here I'm trying to promote a slave to be the new master:</p>
  
  <pre><code>$ repmgr -f /etc/repmgr/repmgr.conf --verbose standby promote
  
  2014-09-22 10:46:37 UTC LOG:  database system shutdown was interrupted; last known up at 2014-09-22 10:44:02 UTC
  2014-09-22 10:46:37 UTC LOG:  database system was not properly shut down; automatic recovery in progress
  2014-09-22 10:46:37 UTC LOG:  redo starts at 0/18000028
  2014-09-22 10:46:37 UTC LOG:  consistent recovery state reached at 0/19000600
  2014-09-22 10:46:37 UTC LOG:  record with zero length at 0/1A000090
  2014-09-22 10:46:37 UTC LOG:  redo done at 0/1A000028
  2014-09-22 10:46:37 UTC LOG:  last completed transaction was at log time 2014-09-22 10:36:22.679806+00
  2014-09-22 10:46:37 UTC FATAL:  could not open directory "pg_logical/snapshots": No such file or directory
  2014-09-22 10:46:37 UTC LOG:  startup process (PID 2595) exited with exit code 1
  2014-09-22 10:46:37 UTC LOG:  aborting startup due to startup process failure
  </code></pre>
  
  <p><code>pg_logical/snapshots</code> dir in fact exists on master node and it is empty.</p>
  
  <p><strong>UPD</strong>: I've just manually created empty directories <code>pg_logical/snapshots</code> and <code>pg_logical/mappings</code> and server has started without complaining. <code>repmgr standby clone</code> seems to omit this dirs while syncing. But the question still remains because I'm just curious what this directory is for, maybe I'm missing something in my setup. Simply Googling it did not yield any meaningful results.</p>