Eviction in AFM to cloud object storage

AFM to cloud object storage filesets use a storage space management process that is called eviction. There are two types of eviction methods, data eviction and metadata eviction.

Eviction is used to make space for new objects and files on an AFM to cloud object storage fileset. Eviction releases object or file data blocks (space) or its metadata (inode) in the fileset, if the fileset usage exceeds the fileset quota. This feature is useful when the storage that is provided by IBM Storage Scale is less than the cloud object storage. The quotas can be defined when you set the AFM to cloud object storage relationship by using the mmafmcosconfig command with --quota-files NumberOfFiles and --quota-blocks NumberOfBlocks parameters.

The process of releasing blocks or inodes is called eviction. However, file data or metadata is not evicted if the file data is dirty. A file whose outstanding changes are not flushed to the home is called a dirty file. When files or objects are evicted from the cache, all data blocks of those files are cleared. The files or objects are uncached. When a read operation is performed on the files or objects, the files or objects fetch data from a cloud object storage. Similarly, any metadata operation such as lookup or ls populate the inodes that were evicted from the AFM to cloud object storage fileset.

While provisioning to control the number of inodes that are used by a cache or home fileset, you can specify a limit on the number of inodes for that fileset during the fileset creation or change the limits by using the mmchfileset command with the --inode-limit option. For more information, see mmchfileset command.

For more information about the data and metadata eviction, see Evicting files or objects data and Evicting files or objects metadata.