Enabling disk overflow

When disk overflow is enabled, you can extend the data grid capacity by moving cache entries out of memory and onto disk. Use the diskOverflowEnabled property in the server properties file to enable the disk overflow feature. When enabled, entries that do not fit into the available memory capacity of the container servers are stored on disk. The disk storage is not a persistent store. Entries that are written to disk are deleted when container servers restart, in the same way that cache entries that are stored in memory are lost during a container server restart.

Before you begin

You must enable eXtreme memory for this feature to work. For more information, see Configuring IBM eXtremeMemory.

About this task

When disk overflow is enabled, it attempts to keep the most recently used cache entries in memory. Disk overflow moves cache entries to disk only when the number of entries in memory exceed the maximum memory allocation, as defined by the maxXMSize server property. If more entries exist than can fit in memory, the least recently used entries are moved to disk. As a result, operations that access entries that are on disk are slower than the response time for entries that are in memory. After the initial access, the item stays in memory unless it becomes a least recently used entry again. When an entry is least recently used, it is moved disk in favor of some other entry.

Disk I/O speed has a significant impact on the data grid performance when disk overflow is enabled. The faster the disk I/O speed, the less time that is required to read and write entries to disk. This speed is important when you heavily use disk overflow by placing more entries into the data grid than can be contained by the configured maxXMSize value. The disk I/O speed is also important when certain events within the data grid require several entries to be read off disk, such as replication and placement during failover scenarios. To lessen this impact, use a dedicated disk or solid-state drives for disk overflow.

1. Stop the container server on which you want to enable disk overflow. For more information, see Stopping stand-alone servers that use the IBM eXtremeIO transport.
2. Set the following properties in the server properties file:

   diskOverflowEnabled

   Enables the native overflow disk feature. You must enable eXtreme Memory for this feature to work.

   Default: false

   diskOverflowCapBytes

   Specifies the maximum amount of disk space that is used by this server for disk overflow, in bytes. The default value specifies that there is no limit on how much is stored on disk.

   Default: Long.MAX_VALUE

   diskStoragePath

   Specifies the absolute path to a directory location used for storing overflow content.

   diskOverflowMinDiskSpaceBytes

   Specifies that entries are not moved to disk if there is less than this amount of space free in diskStoragePath, in bytes.

   Default: 0

3. Restart your container servers. For more information, see Starting stand-alone servers (XIO).