| <?xml version="1.0"?> |
| |
| <document> |
| <properties> |
| <title>Indexed Disk Auxiliary Cache</title> |
| <author email="ASmuts@yahoo.com">Aaron Smuts</author> |
| </properties> |
| |
| <body> |
| <section name="Indexed Disk Auxiliary Cache"> |
| <p> |
| The Indexed Disk Auxiliary Cache is an optional plugin for the |
| JCS. It is primarily intended to provide a secondary store to |
| ease the memory burden of the cache. When the memory cache |
| exceeds its maximum size it tells the cache hub that the item |
| to be removed from memory should be spooled to disk. The cache |
| checks to see if any auxiliaries of type "disk" have been |
| configured for the region. If the "Indexed Disk Auxiliary Cache" |
| is used, the item will be spooled to disk. |
| </p> |
| |
| <subsection name="Disk Indexing"> |
| <p> |
| The Indexed Disk Auxiliary Cache follows the fastest |
| pattern of disk caching. Items are stored at the end of a file |
| dedicated to the cache region. The first byte of each disk entry |
| specifies the length of the entry. The start position in the file |
| is saved in memory, referenced by the item's key. Though this still |
| requires memory, it is insignificant given the performance trade |
| off. Depending on the key size, 500,000 disk entries will probably |
| only require about 1 MB of memory. Locating the position of an item is |
| as fast as a map lookup and the retrieval of the item only requires 2 |
| disk accesses. |
| </p> |
| <p> |
| When items are removed from the disk cache, the location of the available |
| block on the storage file is recorded in a sorted preferential array of a |
| size not to exceed the maximum number of keys allowed in memory. This allows |
| the disk cache to reuse empty spots, thereby keeping the file size to a |
| minimum. |
| </p> |
| </subsection> |
| |
| <subsection name="Purgatory"> |
| <p> |
| Writing to the disk cache is asynchronous and made efficient by using a |
| memory staging area called purgatory. Retrievals check purgatory then |
| disk for an item. When items are sent to purgatory they are simultaneously |
| queued to be put to disk. If an item is retrieved form purgatory it will no |
| longer be written to disk, since the cache hub will move it back to memory. |
| Using purgatory insures that there is no wait for disk writes, unecessary |
| disk writes are avoided for boarderline items, and the items are always |
| available. |
| </p> |
| </subsection> |
| |
| <subsection name="Persistence"> |
| <p> |
| When the disk cache is properly shutdown, the memory index is written |
| to disk and the value file is defragmented. When the cache starts |
| up, the disk cache can be configured to read or delete the index file. This |
| provides an unreliable persistence mechanism. |
| </p> |
| </subsection> |
| |
| |
| <subsection name="Configuration"> |
| <p> |
| The simple configuration and is done in the auxiliary |
| cache section of the <code>cache.ccf</code> configuration file. |
| In the example below, I created an Indexed Disk Auxiliary Cache |
| referenced by <code>DC</code>. It uses files located in the |
| "DiskPath" directory. |
| </p> |
| <p> |
| The Disk indexes are equipped with an LRU storage limit. The maximum |
| number of keys is configured by the maxKeySize parameter. |
| </p> |
| <source><![CDATA[ |
| jcs.auxiliary.DC= |
| org.apache.jcs.auxiliary.disk.indexed.IndexedDiskCacheFactory |
| jcs.auxiliary.DC.attributes= |
| org.apache.jcs.auxiliary.disk.indexed.IndexedDiskCacheAttributes |
| jcs.auxiliary.DC.attributes.DiskPath=g:\dev\jakarta-turbine-stratum\raf |
| jcs.auxiliary.DC.attributes.maxKeySize=100000 |
| ]]></source> |
| </subsection> |
| <subsection name="TODO"> |
| <p> |
| The Indexed Disk Auxiliary Cache will eventually be equiped |
| with periodic index storage. |
| </p> |
| </subsection> |
| </section> |
| </body> |
| </document> |
