Overview and concepts
The following topics provide an overview of AFM and describe the concepts.
- 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. - Communication between cache and home
AFM uses the NFSv3 or NSD protocol for communication between clusters. - The backend protocol - NFS versus NSD
The NSD protocol is a stateful protocol. The NFSv3 protocol is a stateless protocol which is very resilient to low bandwidth and lossy networks. - Primary gateway
Each cache fileset in a cluster is served by one of the nodes designated as a gateway in the cluster. The gateway node that is mapped to a fileset is called the primary gateway of the fileset. The primary gateway acts as the owner of the fileset and communicates with the home cluster. - Global namespace
You can combine the home and cache entities to create a global namespace. - Caching modes
There are four caching modes in AFM: Read-Only (RO), Single-Writer (SW), Local-Update (LU) and Independent-Writer (IW). - Revalidation
As users traverse the directory tree of an AFM cache fileset, files and directory metadata information from the home cluster is checked and updated as needed on the cache cluster. This process is called AFM revalidation. - Cached and uncached files
A readdir operation on a directory populates the metadata of the directory in the cache, but it does not populate contents of each file within the directory. A read operation on file generates a request to home to make contents available in cache. The file contents do not need to be in cache to start reading it. - Synchronous or asynchronous operations
AFM operations are serviced either synchronously or asynchronously. Reads and revalidations are synchronous operations and update operations from the cache are asynchronous. - Asynchronous delay
All update operations from the writable cache filesets are on the primary gateway. Queues in the primary gateway are pushed to home asynchronously based on the afmAsyncDelay interval. - Operations with AFM modes
Following are the operations with AFM modes - Read only (RO), Single writer (SW), Local updates (LU), and Independent writer (IW). - Filesets to the same home target
The following are the possibilities of filesets pointing to home as the target: - Conversion of mode
An AFM cache can be converted to another mode in some conditions, to meet all types of requirements of data management by unlinking the fileset and using mmchfileset -p afmmode command. - Internal AFM Directories
AFM uses .afm, .ptrash, and .pconflicts as the internal directories.
Parent topic: Active File Management