Understanding the Linux Kernel, Second Edition

Chapter 14. Disk Caches

This chapter deals with disk caches. It shows how Linux uses sophisticated techniques to improve system performances by reducing disk accesses as much as possible.

As mentioned in Section 12.1.1, a disk cache is a software mechanism that allows the system to keep in RAM some data that is normally stored on a disk, so that further accesses to that data can be satisfied quickly without accessing the disk.

Besides the dentry cache, which is used by the VFS to speed up the translation of a file pathname to the corresponding inode, two main disk caches—the buffer cache and the page cache—are used by Linux.

As suggested by its name, the buffer cache is a disk cache consisting of buffers; as we know from Section 13.4.3, each buffer stores a single disk block. The block I/O operations (described in Section 13.4.8.1 in the same chapter) rely on the buffer cache to reduce the number of disk accesses.

Conversely, the page cache is a disk cache consisting of pages; each page in the cache corresponds to several blocks of a regular file or a block device file. Of course, the exact number of blocks contained in a page depends on the size of the block. All such blocks are logically contiguous — that is, they represent an integral portion of a regular file or of a block device file. To reduce the number of disk accesses, before activating a page I/O operation (described in Section 13.4.8.2), the kernel should check whether the wanted data is already stored in the page ...