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+# Java VM settings for digilib
+
+The Java virtual machine (Java-VM) only uses a fixed amount of memory for
+its operations. When an operation needs more memory than available it aborts
+with an error ("out of memory error").
+
+digilib can need a lot of memory depending on the size and type of images.
+Since digilib runs as a servlet under Tomcat its in the same VM as the Tomcat
+server.
+
+The amount of memory Tomcat (version 5.0) uses is configured by creating a
+"setenv.sh" script with a line
+
+    CATALINA_OPTS="-Xmx512m"
+
+in Tomcat's `bin` directory (giving 512MB in this case).
+
+You can check the amount of memory your digilib instance has available on the
+bottom of the web page `/server/dlConfig.jsp` in your digilib instance (e.g. 
+[http://digilib.mpiwg-berlin.mpg.de/digitallibrary/server/dlConfig.jsp](http://digilib.mpiwg-berlin.mpg.de/digitallibrary/server/dlConfig.jsp)).
+
+# Installing JAI ImageIO
+
+In principle you should be able to install the 
+[Java Advanced Imaging](http://java.sun.com/javase/technologies/desktop/media/jai/) JAI-ImageIO
+JAR file `jai_imageio.jar` (and native
+library files if available) in the `/WEB-INF/lib/` directory of the
+digilib web application as part of the default installation.
+
+You can see if the Jai-ImageIO plugin is active by checking for the 
+availability of the TIFF image format under "Supported image types" on the 
+[`/server/dlConfig.jsp`](http://digilib.mpiwg-berlin.mpg.de/digitallibrary/server/dlConfig.jsp)
+status page.
+
+But sometimes there are classloader issues. Newer versions of Tomcat refuse to load
+the libraries and I found that in some cases digilib stopped reading TIFF files
+after a period of running.
+
+In these cases it helped to install the JAI files in Tomcats `lib/`
+directory or globally in the local Java JDK
+installation (i.e. in the Java's 'jre/lib/ext/' directory on linux).
+
+# Sample setup
+
+The current digilib setup at the MPIWG (as of December 2010):
+
+*   One frontend server running the lightweight web-multiplexer [pound](http://www.apsis.ch/pound/)
+            on port 80 that distributes requests to three servers runnning digilib
+*   the three servers run digilib under [Jetty](http://www.eclipse.org/jetty/) on port 8080 without Apache
+
+        *   one server is the frontend server (Linux 32bit, Dual
+                2.4GHz Xeon, 2GB RAM)
+    *   the other server is a separate, newer machine (Linux 64bit, Dual 1.8GHz
+                Opteron, 2GB RAM)
+    *   the third server is a separate, newer machine (Linux 32bit, Dual 2.8GHz
+                Xeon, 4GB RAM)
+*   the digilib instances (digilib 2.0b1 as of 12.12.2011) run on Jetty 8.0.4 on Java
+            1.6.0_26 with 1GB of [Java VM memory
+              for digilib](#vm_settings) (-Xmx1024m) with JAI (1.1.3) and JAI-ImageIO (1.1) [installed in the Jetty/lib/ext](#installing_jai)(!)
+*   both digilib servers access all image files over NFS (over GBit
+            Ethernet) from a central file server (Solaris 10, Sun Fire 240, multiple RAIDs
+            on Fibrechannel)
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