Cache and Home

An AFM fileset is an independent fileset. Each fileset has a distinct set of AFM attributes. IBM Spectrum Scale™ cluster that contains AFM filesets is called a cache cluster. A cache fileset has a relationship with a data source which is called the home.

AFM constantly maintains an active relationship between the cache and the home. Changes are managed per fileset results in modular, scalable architecture capable of supporting billions of files and peta bytes of data. Each AFM-enabled fileset is associated with a single home cluster.

AFM uses a NFS or NSD protocol to communicate between home and cache. A Home is an NFS v3 export or a remote cluster mounted from IBM Spectrum Scale cluster.

Architecturally, AFM works with any NFS home export. However if the home has ACLs, it supports extended attributes and sparse files. You must run mmafmconfig enable command on the home cluster to enable this support.

Note: If you do not run mmafmconfig enable and configure AFM relationship, ACLs and extended attributes are not supported. Sparse file information is not maintained. The following error message AFM: AFM is not enabled for fileset cachefs in filesystem gpfs. Some features will not be supported, see documentation for enabling AFM and unsupported features. appears in mmfs.log.

Any cluster can be a home cluster, a cache cluster, or both. In typical setup, a home is in an IBM Spectrum Scale cluster and a cache is defined in another IBM Spectrum Scale cluster. Multiple AFM-enabled filesets can be defined in one cache cluster, each cache having a relationship with targets with a home, or different cluster.

In IW,RO, and LU modes, multiple caches might point to the same home. But in SW mode, only one-to-one relationship between cache and home is supported. AFM can also be configured as a subscription service, where home is the data feed and all caches can subscribe to this data feed.

Within a single cache cluster, application nodes experience POSIX semantics. File locking across cache and home is not supported.

While performing operations on AFM filesets, ensure that the operations are supported on home over the chosen protocol because the operations done from cache are replayed on the remote as normal filesystem operations. While using NSD protocol with UID remapping, operations such as chown (change ownership ) are not supported.