Linux System Programming

Chapter 3. Buffered I/O

Recall from Chapter 1 that the block, a filesystem abstraction, is the lingua franca of I/O—all disk operations occur in terms of blocks. Consequently, I/O performance is optimal when requests are issued on block-aligned boundaries in integer multiples of the block size.

Performance degradation is exacerbated by the increased number of system calls required to read, say, a single byte 1,024 times rather than 1,024 bytes all at once. Even a series of operations performed in a size larger than a block can be suboptimal if the size is not an integer multiple of the block size. For example, if the block size is one kilobyte, operations in chunks of 1,130 bytes may still be slower than 1,024 byte operations.

User-Buffered I/O

Programs that have to issue many small I/O requests to regular files often perform user-buffered I/O. This refers to buffering done in user space, either manually by the application, or transparently in a library, not to buffering done by the kernel. As discussed in Chapter 2, for reasons of performance, the kernel buffers data internally by delaying writes, coalescing adjacent I/O requests, and reading ahead. Through different means, user buffering also aims to improve performance.

Consider an example using the user-space program dd:

dd bs=1 count=2097152 if=/dev/zero of=pirate

Because of the bs=1 argument, this command will copy two megabytes from the device /dev/zero (a virtual device providing an endless stream of zeros) to the file pirate ...