Chapter 6. Managing Filesystems, Swap Space, and Devices

6.1.1. Filesystem Types

Table 6-1. Linux filesystem types

[17]Note that the /proc filesystem under Linux is not the same format as the /proc filesystem under SVR4 (say, Solaris 2.x). Under SVR4, each running process has a single "file" entry in /proc, which can be opened and treated with certain ioctl() calls to obtain process information. On the contrary, Linux provides most of its information in /proc through read() and write() requests.

Recently, Linux also received support for UDF, a filesystem that is meant for use with CD-RWs and DVDs.

Finally, there is a slew of filesystems for accessing data on partitions; these are created by operating systems other than the DOS and Unix families. Those filesystems support the Acorn Disk Filing System (ADFS), the AmigaOS filesystems (no floppy disk support except on Amigas), the Apple Mac HFS, and the QNX4 filesystem. Most of the specialized filesystems are useful only on certain hardware architectures; for instance, you won't have hard disks formatted with the Amiga FFS filesystem in an Intel machine. If you need one of those drivers, please read the information that comes with them; some are only in an experimental state.

6.1.2. Mounting Filesystems

Before we tell you how to mount filesystems, we should also mention that some distributions come with automounting setups that require you to simply load a diskette or CD into the respective drive and access it like you would on other platforms. For everybody, there are times, however, when you need to know how to mount and unmount media directly. We'll cover how to set up automounting yourself later.

mount -t type device mount-point

mount: block device /dev/cdrom is write-protected, mounting read-only

Use a command, such as:

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

instead. This is also necessary if you are trying to mount a floppy that has the write-protect tab in place.

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. A useful variant of using mount is mount -a, which mounts all filesystems listed in /etc/fstab except those marked with the noauto option.

The inverse of mounting a filesystem is, naturally, unmounting it. Unmounting a filesystem has two effects: it synchronizes the system's buffers with the actual contents of the filesystem on disk, and it makes the filesystem no longer available from its mount point. You are then free to mount another filesystem on that mount point.

umount /dev/fd0

to unmount the filesystem on /dev/fd0. Similarly, to unmount whatever filesystem is currently mounted on a particular directory, use a command, such as:

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, insert the new disk, and then remount the device. Of course, with a CD-ROM or a write-protected floppy, there is no way the device itself can get out of sync, but you could run into other problems. For example, some CD-ROM drives won't let you eject the disk until it is unmounted.

Reads and writes to filesystems on floppies are buffered in memory as they are for hard drives. This means that when you read or write data to a floppy, there may not be any immediate drive activity. The system handles I/O on the floppy asynchronously and reads or writes data only when absolutely necessary. So if you copy a small file to a floppy, but the drive light doesn't come on, don't panic; the data will be written eventually. You can use the sync command to force the system to write all filesystem buffers to disk, causing a physical write of any buffered data. Unmounting a filesystem makes this happen as well.

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 you could be more lenient about the use of CD-ROM and floppy drives on your system.

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

wrong fs type is simple enough: this means that you may have specified the wrong type to mount. If you don't specify a type, mount tries to guess the filesystem type from the superblock (this works only for minix, ext2, and iso9660). If mount still cannot determine the type of the filesystem, it tries all the types for which drivers are included in the kernel (as listed in /proc/filesystems). If this still does not lead to success, mount fails. device already mounted means just that: the device is already mounted on another directory. You can find out what devices are mounted, and where, using the mount command with no arguments:

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

Here, we see two hard-drive partitions, one of type ext2 and the other of type vfat, a CD-ROM mounted on /cdrom, and the /proc filesystem. The last field of each line (for example, (rw)) lists the options under which the filesystem is mounted. More on these soon. Note that the CD-ROM device is mounted in /cdrom. If you use your CD-ROM often, it's convenient to create a special directory such as /cdrom and mount the device there. /mnt is generally used to temporarily mount filesystems such as floppies.

Of course, other error isn't very helpful. There are several other cases in which mount could fail. If the filesystem in question has data or media errors of some kind, mount may report it is unable to read the filesystem's superblock, which is (under Unix-like filesystems) the portion of the filesystem that stores information on the files and attributes for the filesystem as a whole. If you attempt to mount a CD-ROM or floppy drive, and there's no CD-ROM or floppy in the drive, you will receive an error message, such as:

mount: /dev/cdrom is not a valid block device

Floppies are especially prone to physical defects (more so than you might initially think), and CD-ROMs suffer from dust, scratches, and fingerprints, as well as being inserted upside-down. (If you attempt to mount your Stan Rogers CD as ISO 9660 format, you will likely run into similar problems.)

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.

/dev/cdrom      /cdrom          iso9660   ro,user

The user option allows users other than root to mount the filesystem. If this option is present, a user can execute a command, such as:

mount /cdrom

to mount the device. Note that if you specify only a device or mount point (not both) to mount, it looks up the device or mount point in /etc/fstab and mounts the device with the parameters given there. This allows you to mount devices listed in /etc/fstab with ease.

The option defaults should be used for most filesystems; it enables a number of other options, such as rw (read-write access), async (buffer I/O to the filesystem in memory asynchronously), and so forth. Unless you have a specific need to modify one of these parameters, use defaults for most filesystems and ro for read-only devices such as CD-ROMs. Another potentially useful option is umask, which lets you set the default mask for the permission bits, something that is especially useful with some foreign filesystems.

The command mount -a will mount all filesystems listed in /etc/fstab. This command is executed at boot time by one of the scripts found in /etc/rc.d, such as rc.sysinit (or wherever your distribution stores its configuration files). This way, all filesystems listed in /etc/fstab will be available when the system starts up; your hard-drive partitions, CD-ROM drive, and so on will all be mounted.

6.1.3. Automounting Devices

Note that there are two versions of automount support: Version 3 and Version 4. Version 3 is the one still contained in most distributions, so that's what we describe here.

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 that 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 /etc/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. Because 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. Because 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, such as:

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# /etc/init.d/autofs start

tigger# /etc/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

If that doesn't work, you need to use:

tigger# modprobe autofs4

[18]We'll cover the modprobe command in the next chapter.

If everything is set up correctly, 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 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).

6.1.4. Creating Filesystems

[19]Under Linux the mkfs command historically created a Minix filesystem. On newer Linux systems, mkfs is a frontend for any filesystem type, and Minix filesystems are created using mkfs.minix.

When you installed Linux, you may have created filesystems by hand using a command such as mke2fs. (If not, 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.

mkfs -t type device

[20]Debian users should use superformat instead.

rutabaga# fdformat /dev/fd0
Double-sided, 80 tracks, 18 sec/track. Total capacity 1440 kB. 
Formatting ... done 
Verifying ... done

Using the -n option with fdformat will skip the verification step.

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

To determine what options are available, see the manual page for the type-specific version of mkfs. (For example, for the Second Extended filesystem, see mke2fs.)

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.

[21]Also, the procedure for making an ISO 9660 filesystem for a CD-ROM is more complicated than simply formatting a filesystem and copying files. See the CD-Writing HOWTO for more details.

6.1.5. Checking and Repairing Filesystems

Use of fsck is quite simple; the format of the command is:

fsck -t type device

Many Linux systems automatically check the filesystems at boot time. This is usually done by executing fsck from /etc/rc.d/rc.sysinit. When this is done, the system usually mounts the root filesystem initially as read-only, runs fsck to check it, and then runs the command:

mount -w -o remount /

The -o remount option causes the given filesystem to be remounted with the new parameters; the -w option (equivalent to -o rw) causes the filesystem to be mounted as read-write. The net result is that the root filesystem is remounted with read-write access.

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 executes fsck.

You can pass options to the type-specific fsck. Most types support the option -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

to run fsck with verbose output.

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.