Running Linux, 5th Edition - Matthias Kalle Dalheimer [190]
Checking and Repairing Filesystems
It is sometimes necessary to check your Linux filesystems for consistency and to repair them if there are any errors or if you lose data. Such errors commonly result from a system crash or loss of power, making the kernel unable to sync the filesystem buffer cache with the contents of the disk. In most cases, such errors are relatively minor. However, if the system were to crash while writing a large file, that file might be lost and the blocks associated with it marked as "in use," when in fact no file entry is corresponding to them. In other cases, errors can be caused by accidentally writing data directly to the hard drive device (such as /dev/hda), or to one of the partitions.
The program fsck is used to check filesystems and correct any problems. Like mkfs, fsck is a frontend for a filesystem-type-specific fsck.type, such as fsck.ext2 for Second Extended filesystems. (As with mkfs.ext2, fsck.ext2 is a symbolic link to e2fsck, either of which you can execute directly if the fsck frontend is not installed.)
Use of fsck is quite simple; the format of the command is:
fsck -t type device
where type is the type of filesystem to repair, as given in Table 10-1, and device is the device (drive partition or floppy) on which the filesystem resides.
For example, to check an ext3 filesystem on /dev/hda2, you use:
rutabaga# fsck -t ext3 /dev/hda2
fsck 1.34 (25-Jul-2003)
/dev/hda2 is mounted. Do you really want to continue (y/n)? y
/dev/hda2 was not cleanly unmounted, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts.
Pass 5: Checking group summary information.
Free blocks count wrong for group 3 (3331, counted=3396). FIXED
Free blocks count wrong for group 4 (1983, counted=2597). FIXED
Free blocks count wrong (29643, counted=30341). FIXED
Inode bitmap differences: -8280. FIXED
Free inodes count wrong for group #4 (1405, counted=1406). FIXED
Free inodes count wrong (34522, counted=34523). FIXED
/dev/hda2: ***** FILE SYSTEM WAS MODIFIED *****
/dev/hda2: ***** REBOOT LINUX *****
/dev/hda2: 13285/47808 files, 160875/191216 blocks
First of all, note that the system asks for confirmation before checking a mounted filesystem. If any errors are found and corrected while using fsck, you'll have to reboot the system if the filesystem is mounted. This is because the changes made by fsck may not be propagated back to the system's internal knowledge of the filesystem layout. In general, it's not a good idea to check mounted filesystems.
As we can see, several problems were found and corrected, and because this filesystem was mounted, the system informed us that the machine should be rebooted.
How can you check filesystems without mounting them? With the exception of the root filesystem, you can simply umount any filesystems before running fsck on them. The root filesystem, however, can't be unmounted while running the system. One way to check your root filesystem while it's unmounted is to use a boot/root floppy combination, such as the installation floppies used by your Linux distribution. This way, the root filesystem is contained on a floppy, the root filesystem (on your hard drive) remains unmounted, and you can check the hard drive root filesystem from there. See "What to Do in an Emergency" in Chapter 27 for more details about this.
Another way to check the root filesystem is to mount it as read-only . This can be done using the option ro from the LILO boot prompt (see "Specifying boot-time options" in Chapter 17). However, other parts of your system configuration (for example, the programs executed by /etc/init at boot time) may require write access to the root filesystem, so you can't boot the system normally or these programs will fail. To boot the system with the root filesystem mounted as read-only, you might want to boot the system into single-user mode