Apache filehandle issue expected to be mission critical for image uploading
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  The problem is a maximum open files limit per users.
  We had several web servers, and several new Apache2 VirtualHost; Apache2 (www-data) reached the maximum number of open files (1024).
  
  You can check that with ulimit -a  or ulimit -n. (The default is maximum 1024 open files.)
  
  You can modify it with ulimit -n 1500, but it will be lost after reboot.
  
  To make the change permanent, add in /etc/security/limits.conf:

#&lt;domain&gt; &lt;type&gt; &lt;item&gt; &lt;value&gt;
#
www-data soft nofile 1600
www-data hard nofile 65535

  
  Of course, restart your web server.


It explains how to set a per-user limit and increase the number of files that can be opened.  If you don't have enough memory in the server, there may not be many options.