Understanding the Linux Kernel, 3rd Edition

General Characteristics of Ext2

Unix-like operating systems use several types of filesystems. Although the files of all such filesystems have a common subset of attributes required by a few POSIX APIs such as stat( ), each filesystem is implemented in a different way.

The first versions of Linux were based on the MINIX filesystem. As Linux matured, the Extended Filesystem (Ext FS) was introduced; it included several significant extensions, but offered unsatisfactory performance. The Second Extended Filesystem (Ext2) was introduced in 1994; besides including several new features , it is quite efficient and robust and is, together with its offspring Ext3, the most widely used Linux filesystem.

The following features contribute to the efficiency of Ext2:

• When creating an Ext2 filesystem, the system administrator may choose the optimal block size (from 1,024 to 4,096 bytes), depending on the expected average file length. For instance, a 1,024-block size is preferable when the average file length is smaller than a few thousand bytes because this leads to less internal fragmentation—that is, less of a mismatch between the file length and the portion of the disk that stores it (see the section "Memory Area Management" in Chapter 8, where internal fragmentation for dynamic memory was discussed). On the other hand, larger block sizes are usually preferable for files greater than a few thousand bytes because this leads to fewer disk transfers, thus reducing system overhead.

• When creating ...