Managing RAID on Linux

Blocks and Inodes

All Unix filesystems use two basic components to organize and store data: blocks and inodes. Just as a physical disk is organized into sectors, data on a filesystem is abstracted into blocks. Blocks have a fixed size, determined at the time the filesystem is created. The block size of a filesystem determines how many bytes are allocated to each block on the filesystem. Generally, block sizes are 1 KB, 2 KB, or 4 KB for 32-bit systems. A block size of 8 KB is also available on 64-bit systems.

Inodes are used to map blocks to physical disk locations on Unix filesystems. Every file created, whether it’s a directory, normal file, or special file, is assigned an inode. Inodes work in much the same way as pointers do in programming languages such as C, Perl, and Java. Inodes also store information about a file, including its type, size, and parent directory. On traditional Unix filesystems, inodes are typically allocated when the filesystem is created. ext2, for example, allocates one inode for every 8 KB worth of data blocks when a new filesystem is initialized, although this value can be manually altered. That means an ext2 filesystem with a block size of 4 KB allocates a single inode for every two data blocks.

When a filesystem runs out of inodes, no new files can be created until existing files are deleted, thereby freeing up inodes that are already in use. For this reason, many new filesystems implement dynamic inode allocation, freeing system administrators from ...