[Solr] How To Fix java.io.IOException: directory FOO exists and is a directory, but cannot be listed: list() returned null
Updated: June 11th, 2015
I am throwing up a quick post about a relatively cryptic error that Solr started throwing the other day here at Plaxo. After happily running for a few days, I suddenly started getting pages about failed Solr indexing.
Upon closer examination, I saw the following repeatedly in the log file:
catalina.2009-09-18.log:SEVERE: java.io.IOException: directory 'DATADIR/index' exists and is a directory, but cannot be listed: list() returned null
I tried to see if sending an OPTIMIZE command would help but the server returned the same response.
The reason was these errors was quite simple – Solr was running into the system level limit on allowed number of open files (ulimit). This limit can be seen by running