Chapter 6. Managing Filesystems, Swap, and Devices

6.1.1. Filesystem Types

Table 6-1. Linux Filesystem Types

In addition, Linux now supports the Microsoft Joliet extensions to ISO 9660 which can handle long filenames made up of Unicode characters. This is not widely used now but may become valuable in the future, because Unicode has been accepted internationally as the standard for encoding characters of scripts world-wide.

6.1.2. Mounting Filesystems

mount -t iso9660 -r /dev/cdrom /mnt

The mount manual page lists all available mounting options. Not all are of immediate interest, but you might have a need for some of them, someday.

umount /mnt

  It is important to note that removable media, including floppies and CD-ROMs, should not be removed from the drive or swapped for another disk while mounted. This causes the system's information on the device to be out of sync with what's actually there and could lead to no end of trouble. Whenever you want to switch a floppy or CD-ROM, unmount it first, using the umount command, and then remount the device.

If you wish to allow mortal users to mount and unmount certain devices, you have two options. The first option is to include the user option for the device in /etc/fstab (described later in this section). This allows any user to use the mount and umount command for a given device. Another option is to use one of the mount frontends available for Linux. These programs run setuid root and allow ordinary users to mount certain devices. In general, you wouldn't want normal users mounting and unmounting a hard drive partition, but use of CD-ROM and floppy drives might be more lenient on your system.

There are quite a few things that can go wrong when attempting to mount a filesystem. Unfortunately, the mount command will give you the same error message in response to a number of problems:

mount: wrong fs type, /dev/cdrom already mounted, /mnt busy, or other error

rutabaga# mount 
/dev/hda2 on / type ext2 (rw) 
/dev/hda3 on /msdos type msdos (rw) 
/dev/cdrom on /cdrom type iso9660 (ro) 
/proc on /proc type proc (rw,none)

Also, be sure the mount point you're trying to use (such as /mnt) exists. If not, you can simply create it with the mkdir command.

Here, device, mount-point, and type are equivalent to their meanings in the mount command, and options is a comma-separated list of options to use with the -o switch to mount.

# device        directory       type      options
/dev/hda2       /               ext2      defaults
/dev/hda3       /msdos          msdos     defaults
/dev/cdrom      /cdrom          iso9660   ro
/proc           /proc           proc      none

/dev/hda1       none            swap      sw

6.1.3. Automounting Devices

You can automount filesystems wherever you like, but for simplicity's sake, we will assume here that you want to automount all filesystems below one directory which we will call /automount here. If you want your automount points to be scattered over your filesystem, you will need to use multiple automount daemons.

If you have compiled the autofs package yourself, it might be a good idea to start by copying the sample configuration files that you can find in sample directory, and adapt them to your needs. To do this, copy the files sample/auto.master and sample/auto.misc into the /etc directory, and the file sample/rc.autofs under the name autofs wherever your distribution stores its boot scripts. We'll assume here that you use /sbin/init.d.

The first configuration file to edit is /etc/auto.master. This lists all the directories (the so-called mount points) below which the automounter should mount partitions. Since we have decided to use only one partition in this chapter's example, we will need to make only one entry here. The file could look like this:

/automount	/etc/auto.misc

This file consists of lines with two entries each, separated by whitespace. The first entry specifies the mount point, and the second entry names a so-called map file that specifies how and where to mount the devices or partitions to be automounted. You need one such map file for each mount point.

In our case, the file /etc/auto.misc looks like the following:

cd		-fstype=iso9660,ro	:/dev/scd0
floppy		-fstype=auto		:/dev/fd0

Again, this file consists of one-line entries that each specify one particular device or partition to be automounted. The lines have two mandatory and one optional field, separated by whitespaces. The first value is mandatory and specifies the directory, onto which the device or partition of this entry is automounted. This value is appended to the mount point so that the CD-ROM will be automounted onto /automount/cd.

The second value is optional and specifies flags to be used for the mount operation. These are equivalent to those for the mount command itself, with the exception that the type is specified with the option -fstype= instead of -T.

Finally, the third value specifies the partition or device to be mounted. In our case, we specify the first SCSI CD-ROM drive and the first floppy drive, respectively. The colon in front of the entry is mandatory; it separates the host part from the device/directory part, just as with mount. Since those two devices are on a local machine, there is nothing to the left of the colon. If we wanted to automount the directory sources from the NFS server sourcemaster, we would specify something like:

sources    -fstype=nfs,soft    sourcemaster:/sources

After editing the configuration files to reflect your system, you can start the automount daemon by issuing (replace the path with the path that suits your system):

tigger# /sbin/init.d/autofs start

Since this command is very taciturn, you should check whether the automounter has really started. One way to do this is to issue:

tigger# /sbin/init.d/autofs status

but it is difficult to determine from the output whether the automounter is really running. Your best bet, therefore, is to check whether the automount process exists:

tigger# ps -aux | grep automount 

If this command shows the automount process, everything should be all right. If it doesn't, you need to check your configuration files again. It could also be the case that the necessary kernel support is not available: either the automount support is not in your kernel, or you have compiled it as a module but not installed this module. If the latter is the case, you can fix the problem by issuing:

tigger# modprobe autofs

When your automounter works to your satisfaction, you might want to put the modprobe call as well as the autofs call in one of your system's startup configuration files like /etc/rc.local, /sbin/init.d/boot.local or whatever your distribution uses.

If everything is correctly set up, all you need to do is access some directory below the mount point, and the automounter will mount the appropriate device or partition for you. For example, if you type:

tigger$ ls /automount/cd

the automounter will automatically mount the CD-ROM, so that ls can list its contents. The only difference between normal and automounting is that with automounting you will notice is a slight delay, before the output comes.

In order to conserve resources, the automounter unmounts a partition or device if it has not been accessed for a certain amount of time (the default is five minutes).

The automounter supports a number of advanced options; for example, you do not need to read the map table from a file but can also access system databases or even have the automounter run a program and use this program's output as the mapping data. See the man pages for autofs and automount for further details.

6.1.4. Creating Filesystems

When you installed Linux, you may have created filesystems by hand using a command such as mke2fs. (If not, then the installation software created the filesystems for you.) In fact, mke2fs is equivalent to mkfs.ext2. The programs are the same (and on many systems, one is a symbolic link to the other), but the mkfs.fs-type filename makes it easier for mkfs to execute the appropriate filesystem-type specific program. If you don't have the mkfs frontend, you can use mke2fs or mkfs.ext2 directly.

Assuming that you're using the mkfs frontend, a filesystem can be created using this command:

mkfs -t type device blocks

Each filesystem-specific version of mkfs supports several options you might find useful. Most types support the -c option, which causes the physical media to be checked for bad blocks while creating the filesystem. If bad blocks are found, they are marked and avoided when writing data to the filesystem. In order to use these type-specific options, include them after the -t type option to mkfs, as follows:

mkfs -t type -c device blocks

You may not have all available type-specific versions of mkfs installed. If this is the case, mkfs will fail when you try to create a filesystem of a type for which you have no mkfs.type. Many filesystem types supported by Linux have a corresponding mkfs.type available, somewhere.

6.1.5. Checking and Repairing Filesystems

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 could 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

When fsck is executed at boot time, it checks all filesystems other than root before they are mounted. Once fsck completes, the other filesystems are mounted using mount. Check out the files in /etc/rc.d, especially rc.sysinit (if present on your system), to see how this is done. If you want to disable this feature on your system, comment out the lines in the appropriate /etc/rc.d file that execute fsck.

There are several options you can pass to the type-specific fsck. Most types support the options -a, which automatically confirms any prompts that fsck.type may display; -c, which does bad-block checking, as with mkfs; and -v, which prints verbose information during the check operation. These options should be given after the -t type argument to fsck, as in:

fsck -t type -v device

See the manual pages for fsck and e2fsck for more information.

Not all filesystem types supported by Linux have a fsck variant available. To check and repair MS-DOS filesystems, you should use a tool under MS-DOS, such as the Norton Utilities, to accomplish this task. You should be able to find versions of fsck for the Second Extended filesystem, Minix filesystem, and Xia filesystem at least.