Elements of A Cache System

A cache system consists of four major elements:

Cache structure
Permanent storage
Local cache buffers
Local cache vector

Figure 1 shows the four elements and their relationship to each other.

Figure 1. Elements of a Cache System

Each piece of shared data, referred to as a “data item” throughout this topic "Using Cache Services (IXLCACHE)", can be stored in different locations within the cache system. Copies of shared data items are stored in the local cache buffers (fastest access) belonging to each cache user. The shared data also resides either in the cache structure on the coupling facility (next fastest access), on permanent storage (slower access to the data than from either the local cache or cache structure) or both the cache structure and permanent storage. In general, how quickly you can access the data depends on where it is stored.

A description of each element of a cache system follows:

Local cache buffers — Local cache buffers are storage buffers that users allocate in their own storage area. They contain copies of data that is shared among cache users. Users read data from permanent storage or from the cache structure to their local cache buffers, and write data from their local cache buffers to permanent storage, to the cache structure, or to both locations. Each user who accesses the cache structure must have a set of local cache buffers to accommodate the data items to be shared.
The cache structure — The cache structure is a structure in the coupling facility that contains:
- A directory to keep track of named data items that are shared among cache users
- Optionally, data entries that hold data items
Users who are connected to the cache structure can use cache services to access and manage shared data.
Permanent storage — Permanent storage is storage that is the final repository for the data that users share, and might be on a direct access storage device (DASD). Users can read the data from permanent storage to local storage buffers for their use, and then either write the data to the cache structure and maintain the data there, or maintain the data in the local buffers and use the “directory-only” caching method to track the validity of the data. After users make updates to the locally-cached data, they are responsible for ensuring that the changes are made to the permanent storage copy of the data. They make these changes to permanent storage either immediately after the update or at a later time, depending on the cache protocol.
Local cache vector — The local cache vector is a user-defined vector that provides a way for cache users to determine the validity of data in their local cache buffers. There is one local cache vector per user of the cache. Each vector is divided into separate entries with each entry corresponding to a local cache buffer. Each vector entry contains an indicator that the system sets to indicate whether the data in the corresponding local cache buffer is valid. Users must test the indicator to determine the validity of the data in their local cache buffers.
Because the local cache vector is in system storage and not directly addressable by the user, the system provides the IXLVECTR service. IXLVECTR allows the user to test the entries in the vector to determine whether the corresponding local cache buffer is valid, and to dynamically change the number of entries in the vector.