Linux Server Hacks, Volume Two

Hack #70. Reduce Restart Times with Journaling Filesystems

Large disks and filesystem problems can drag down the boot process unless you're using a journaling filesystem. Linux gives you plenty to choose from.

Computer systems can only successfully mount and use filesystems if they can be sure that all of the data structures in each filesystem are consistent. In Linux and Unix terms, consistency means that all of the disk blocks that are actually used in some file or directory are marked as being in use, all deleted blocks aren't linked to anything other than the list of free blocks, all directories in the filesystem actually have parent directories, and so on. This check is done by filesystem consistency check applications, the best known of which is the standard Linux/Unix fsck application. Each filesystem has its own version of fsck (with names like fsck.ext3, fsck.jfs, fsck.reiserfs, and so on) that understands and "does the right thing" for that particular filesystem.

When filesystems are mounted as part of the boot process, they are marked as being in use ("dirty"). When a system is shut down normally, all its on-disk filesystems are marked as being consistent ("clean") when they are unmounted. When the system reboots, filesystems that are marked as being clean do not have to be checked before they are mounted, which saves lots of time in the boot process. However, if they are not marked as clean, the laborious filesystem consistency check process begins. Because today's filesystems ...