Understanding the Linux Kernel, 3rd Edition

The Generic Block Layer

The generic block layer is a kernel component that handles the requests for all block devices in the system. Thanks to its functions, the kernel may easily:

• Put data buffers in high memory—the page frame(s) will be mapped in the kernel linear address space only when the CPU must access the data, and will be unmapped right after.

• Implement—with some additional effort—a “zero-copy” schema, where disk data is directly put in the User Mode address space without being copied to kernel memory first; essentially, the buffer used by the kernel for the I/O transfer lies in a page frame mapped in the User Mode linear address space of a process.

• Manage logical volumes—such as those used by LVM (the Logical Volume Manager) and RAID (Redundant Array of Inexpensive Disks): several disk partitions, even on different block devices, can be seen as a single partition.

• Exploit the advanced features of the most recent disk controllers, such as large onboard disk caches , enhanced DMA capabilities, onboard scheduling of the I/O transfer requests, and so on.

The Bio Structure

The core data structure of the generic block layer is a descriptor of an ongoing I/O block device operation called bio. Each bio essentially includes an identifier for a disk storage area—the initial sector number and the number of sectors included in the storage area—and one or more segments describing the memory areas involved in the I/O operation. A bio is implemented by the bio data structure, whose fields ...