Configure disk offload caching

When the number of cache entries reaches the configured limit for WAS, eviction of cache entries occurs. This allows new entries to enter the cache service. The dynamic cache service includes an alternative feature named disk offload, which copies the evicted cache entries to disk for potential future access.

To configure disk offload:

1. Start the WAS administrative console.
2. In the topology tree, expand Servers and click Application Servers.
3. Click the server you want to work with.
4. Click Dynamic Cache Service.
5. Click the Enable disk offload check box in the Disk offload field. You can also set the disk offload location in this field.
6. Specify the location used in the Offload location field.
7. Click Apply or OK.
8. Save the administrative configuration.
9. Restart the application server so that the changes take effect.

The disk offload location must be unique for servers defined on the same node. If you have two or more application servers with servlet caching enabled, and the application servers specify the same disk offload location for their caches, the following exceptions might occur:

• java.lang.NullPointerException
  at com.ibm.ws.cache.CacheOnDisk.readTemplate(CacheOnDisk.java:686)
  at com.ibm.ws.cache.Cache.internalInvalidateByTemplate(Cache.java:828)

• java.lang.NullPointerException
  at com.ibm.ws.cache.CacheOnDisk.readCacheEntry(CacheOnDisk.java:600)
  at com.ibm.ws.cache.Cache.getCacheEntry(Cache.java:341)

If one server is run as root and the other servers are run as nonroot, this problem could occur. For example, if server1 runs as root and server2 runs as wasuser or wasgroup, the cache files in the disk offload location might be created with root permissions. This situation causes the applications running on the nonroot servers to fail when they try to read or write to the cache.

For more information about dynamic cache service settings, see DynamicCache service settings