Understanding the Cache Structure

A cache consists of one or more partitions. Conceptually, a partition is a storage area on a disk that you set aside for caching. If you want to have your cache span several disks, configure at least one cache partition for each disk. Each partition can be independently administered. In other words, you can enable, disable, and configure a partition independently of all other partitions.

Storing a large number of cached files in a single location can slow performance; therefore, create several directories, or sections, in each partition. Sections are the next level under partitions in the cache structure. You can have up to 256 sections in your cache across all partitions. The number of cache sections must be a power of 2 (for example, 1, 2, 4, 8, 16, ..., 256).

The final level in the cache structure hierarchy is the subsection. Subsections are directories within sections. Each section has 64 subsections. Cached files are stored in the subsections which is the lowest level in your cache.

The following figure shows an example cache structure with partitions and sections. In this figure, the cache directory structure divides the total cache into three partitions. The first partition contains four cache sections, and the second two partitions each contain two sections.

Each cache section is noted by “s” for section, and then a section number. For the section shown as s3.4, the 3 indicates the power of 2 for the number of cache sections (2³ = 8), and the 4 means the number for the section (for the 8 sections labeled 0 through 7). Therefore, s3.4 means section 5 of 8.

Figure 12–2 Example of a Cache Structure