Cache eviction

Cache eviction is a feature where file data blocks in the cache are released when fileset usage exceeds the fileset soft quota, and space is created for new files.

The process of releasing blocks is called eviction. However, file data is not evicted if the file data is dirty. A file whose outstanding changes are not flushed to home is a dirty file.

You can use automatic cache eviction or define your own policy to decide which file data is evicted. To automatically enable cache eviction, define a fileset soft quota on the cache fileset. Eviction starts when fileset usage reaches the soft quota limit. A time lag might result when an eviction is triggered and the data is evicted. Tuning the soft and hard quota limits minimizes application delays that are caused by the data being cached at a faster rate than it is being evicted.

Cache eviction based on inode quotas is not supported. Cached data from partially fetched files are not evicted from cache. When files are evicted from cache, all data blocks of those files are cleared. The files are then uncached. When a read operation is performed on the files the next time, the files fetch data from home.

Cache eviction is enabled by default on all AFM nodes and is controlled by the afmEnableAutoEviction parameter, and fileset block quota. Cache eviction can also be manually triggered by using the mmafmctl evict command. When a file is evicted, file data is removed from the cache, but the inode stays in the cache. Using eviction, you can build environments, where all objects from home are available but running in limited amount of space.

For example, a cache could be created in flash storage. File eviction opens a powerful method of providing small but high speed and low latency cache filesets to clusters.

mmafmctl Device evict -j FilesetName 
                 [--safe-limit SafeLimit] [--order {LRU | SIZE}]
                 [--log-file LogFile] [--filter Attribute=Value ...] 
                 [--list-file ListFile] [--file FilePath]

This option is applicable for RO/SW/IW/LU filesets. This command can be run manually or run in a script with a custom policy to implement a custom eviction policy. Options can be combined.

--safe-limit SafeLimit - This is a mandatory parameter for the manual evict option, for order and filter attributes. It specifies target quota limit which is used as the low water mark for eviction in bytes – the value must be less than the soft limit. This parameter can be used alone or can be combined with one of the following parameters (order or filter attributes). Specify the parameter in bytes.

--order LRU | SIZE - The order in which files are to be chosen for eviction: LRU - Least recently used files are to be evicted first. SIZE - Larger-sized files are to be evicted first.

--log-file Log File- The file where the eviction log is to be stored. By default no logs are generated.

--filter Attribute=Value - The attributes that you can use to control the way data is evicted from the cache. Valid attributes are: FILENAME=File Name - The name of a file to be evicted from the cache. This option uses an SQL-type search query. If the same file name exists in more than one directory, the cache will evict all the files with that name. The complete path to the file must not be given here. MINFILESIZE=Size - The minimum size of a file to evict from the cache. This value is compared to the number of blocks allocated to a file (KB_ALLOCATED), which might differ slightly from the file size. MAXFILESIZE=Size - The maximum size of a file to evict from the cache. This value is compared to the number of blocks allocated to a file (KB_ALLOCATED), which might differ slightly from the file size.

--list-file ListFile - The specified file is a file containing a list of files to be evicted, one file per line. All files must have fully qualified path names. Filesystem quotas need not be specified. If the list of files to be evicted have filenames with special characters then a policy should be used to generate the listfile. This policy output should be hand-edited to remove all other entries except the filenames and can be passed to the evict command.

--file FilePath - The fully qualified name of the file to be evicted. Filesystem quotas need not be specified.

enforceFilesetQuotaOnRoot - Enable to evict files created by root. By default, files created by root are not evicted.