Although there are potentially many things that will prevent a system from booting, these problems can be generally categorized as either hardware problems or software and configuration problems. The most common hardware-related problem you will probably encounter is a bad hard drive; like all mechanical devices with moving parts, these have a finite lifetime and will eventually fail. Fortunately, the Red Hat exams do not require you to address hardware failures.
Software and configuration problems, however, can be a little more difficult. At first glance, they can look just like regular hardware problems.
In addition to knowing how to mount disk partitions, edit files, and manipulate files, you will need to know how to use several other commands to fix problems from rescue mode or single-user mode. The most useful of these are the df, fdisk, and the fsck commands. To diagnose a problem, you need to know how these commands work at least at a rudimentary level.
df
The Linux df command was covered briefly in Chapter 4. When you use df, you can find mounted directories, the capacity of each partition, and the percentage of each partition that's filled with files. The result shown back in Figure 16-8 illustrates the result in kilobytes. There are a couple of simple variations; the following commands provide output in megabytes and inodes:
# df -m
# df -i
fdisk and parted
The Linux fdisk and parted utilities were covered briefly in Chapter 4. When you use fdisk or parted, you can find the partitions you have available for mounting. For example, the fdisk -l /dev/hda (or parted /dev/hda print) command lists available partitions on the first IDE hard disk:
# fdisk -l /dev/hda
Disk /dev/hda: 15.0GB, 15020457984 bytes
240 heads, 63 sectors/track, 1940 cylinders
Units = cylinders of 15120 * 512 = 7741440 bytes
Device Boot Start End Blocks Id System
/dev/hda1 * 1 949 7174408+ b Win95 FAT32
/dev/hda2 950 963 105840 83 Linux
/dev/hda3 964 1871 6864480 83 Linux
/dev/hda4 1872 1940 521640 f Win95 Ext'd (LBA)
/dev/hda5 1872 1940 521608+ 82 Linux swap
Looking at the output from fdisk, it's easy to identify the partitions configured with a Linux format, /dev/hda2, /dev/hda3, and /dev/hda5. Given the size of each partition, it is reasonable to conclude that /dev/hda2 is associated with /boot, and /dev/hda3 is associated with root (/). Here's a fairly complex output from parted:
# parted /dev/sda print
Model: ATA HDS728080PLA380 (scsi)
Disk /dev/sda: 82.3GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 32.3kB 197MB 197MB primary ext3 boot
2 197MB 15.2GB 15.0GB primary ext3
3 15.2GB 16.2GB 1003MB primary linux-swap
4 16.2GB 82.3GB 66.1GB extended
5 16.2GB 16.3GB 107MB logical ext3
6 16.3GB 26.8GB 10.5GB logical ext3
7 26.8GB 41.8GB 15.0GB logical fat32 lba
8 41.8GB 51.8GB 10.0GB logical ext3
9 51.8GB 82.3GB 30.5GB logical ext3
Information: Don't forget to update /etc/fstab, if necessary.
In this example, it's easy to identify the Linux swap partition. Since /boot partitions are small and normally configured toward the front of a drive (with a boot flag), it's reasonable to associate it with /dev/sda1.
For simple partitioning schemes, this is easy. It gets far more complicated when you have lots of partitions. You should always have some documentation available that clearly identifies your partition layout within your filesystem:
# fdisk -l /dev/hda
Disk /dev/hda: 26.8 GB, 26843545600
255 heads, 63 sectors/track, 3263 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/hda1 * 1 13 104391 83 Linux
/dev/hda2 14 268 2048287+ b Win95 FAT32
/dev/hda3 269 396 1028160 83 Linux
/dev/hda4 397 3263 23029177+ f Win95 Ext'd (LBA)
/dev/hda5 397 1097 5630751 83 Linux
/dev/hda6 1098 1734 5116671 83 Linux
/dev/hda7 1735 1989 2048256 83 Linux
/dev/hda8 1990 2244 2048256 83 Linux
/dev/hda9 2245 2372 1028218+ 83 Linux
/dev/hda10 2373 2499 1020096 82 Linux swap
/dev/hda11 2500 2626 1020096 83 Linux
/dev/hda12 2627 2753 1020096 83 Linux
/dev/hda13 2754 2880 1020096 83 Linux
/dev/hda14 2881 3007 1020096 83 Linux
/dev/hda15 3008 3134 1020096 83 Linux
/dev/hda16 3135 3236 1020096 83 Linux
In this example, it's easy to identify the Linux swap partition. Since /boot partitions are small and normally configured toward the front of a drive, it's reasonable to associate it with /dev/hda1.
However, that is just a guess; some trial and error may be required. For example, after mounting /dev/hda2 on an empty directory, you would want to check the contents of that directory for the typical contents of /boot.
e2label
Based on the previous output from fdisk -l, you could probably use a little help to identify the filesystems associated with the other partitions. The e2label command can help. When you set up a new filesystem, the associated partition is normally marked with a label. For example, the following command tells you that the /usr filesystem is normally mounted on /dev/hda5.
# e2label
Usage: e2label device [newlabel]
# e2label /dev/hda5
/usr
dumpe2fs
You can get a lot more information on each partition with the dumpe2fs command, as shown in Figure 16-9.
Figure 16-9: The dumpe2fs command provides a lot of information.
The dumpe2fs command not only does the job of e2label but also tells you about the format, whether it has a journal, and the block size. Proceed further through the output, and you'll find the locations for backup superblocks, which can help you use the fsck or e2fsck command to select the appropriate superblock for your Linux partition.
On the Job fsck is a "front end" for e2fsck, which is used to check partitions formatted to the ext2 and ext3 filesystems.
Filesystem Check-fsck
You should also know how to use the fsck command. This command is a front end for most of the filesystem formats available in Linux, such as ext2, ext3, reiserfs, vfat, and more. This command is used to check the filesystem on a partition for consistency. In order to use the fsck command effectively, you need to understand something about how filesystems are laid out on disk partitions.
When you format a disk partition under Linux using the mkfs command, it sets aside a certain portion of the disk to use for storing inodes, which are data structures that contain the actual disk block addresses that point to file data on a disk. The mkfs command also stores information about the size of the filesystem, the filesystem label, and the number of inodes in a special location at the start of the partition called the superblock. If the superblock is corrupted or destroyed, the remaining information on the disk is unreadable. Because the superblock is so vital to the integrity of the data on a partition, the mkfs command makes duplicate copies of the superblock at fixed intervals on the partition, which you can find with the dumpe2fs command described earlier.
The fsck command checks for and corrects problems with filesystem consistency by looking for things such as disk blocks that are marked as free but are actually in use (and vice versa), inodes that don't have a corresponding directory entry, inodes with incorrect link counts, and a number of other problems. The fsck command will also fix a corrupted superblock. If fsck fails due to a corrupt superblock, you can use the fsck command with the -b option to specify an alternative superblock. For example, the following command performs a consistency check on the filesystem on disk partition /dev/hda5, using the superblock located at disk block 8193:
# fsck -b 8193 /dev/hda5
