Mounting a Filesystem

Mounting a UNIX filesystem does more than merely make its data available. Mounting eliminates all device boundaries, making the filesystem device-independent (a very important feature in software installation and implementation). Figure 5.3 illustrates the relationship between disk partitions (as basic storage units) with the associated filesystems and with the overall UNIX filesystem.

The root filesystem resides in the first partition of the root disk (the first disk — Disk

#1), which is accessible via a special device file /dev/dsk/c1d0s0 (the naming of special device files can be different among different UNIX flavors). Mounting a root filesystem establishes a logical connection between the special device file /dev/dsk/c1d0s0 and a mounting point for the root filesystem in the overall UNIX hierarchical directory tree. For the root filesystem, the mounting point must be the root directory “/,” and the mounting itself must be performed during the system startup (booting). A mounted root filesystem cannot be dismounted as long as the system lives.

To mount a new filesystem, the corresponding mounting point (or, as we prefer to say, mount-point) is required. A mount-point must be an accessible directory in the already mounted directory hierarchy. It explains why the mounting of the root filesystem must be done during the system startup, as well as why the root filesystem must live as long as the UNIX system itself. The mounting of the root filesystem happens when no hierarchical directory structure exists at all. Obviously it can be performed only by the system itself. In addition, dismounting of the root filesystem would be fatal for the system because the complete UNIX filesystem would be lost without chances for a recovery. A filesystem cannot be accessed if its mount-point is not accessible, and the root filesystem is the beginning of everything. However, once the root filesystem is available, a number of new mount-points can be created and designated to mount other filesystems.

In Figure 5.3, the root filesystem contains several empty directories: /usr, /var, /home, and /project designated to merge other filesystems (any mount-points can easily be added by creating a new directory). While the first three listed filesystems are standard ones (please make clear that they are not mandatory as separate filesystems — they could be part of the root filesystem), the fourth one is very site-specific. This example illustrates a special case where two additional disk partitions (named project and docs) are dedicated to keep specific project-related data, and only the project filesystem is supposed to be mounted onto the root filesystem. In any case all partition sizes and mount-points are arbitrary, and they fully reflect flexibility in creating an overall UNIX filesystem. In this example, partitions’ filesystems are located in disks and partitions that can be accessed via the special device files presented in Table 5.1.

An additional partition of the disk #1 (as it can be seen in the Figure 5.3), identified with /dev/dsk/c1d0s1 is dedicated to the swap partition. While the swap partition is crucial for the operating system, it is not an integral part of the UNIX filesystem and that is why it is not included in this discussion.

Four filesystems, usr, var, home, and project, are merged into the root filesystem, while the fifth one, docs, is merged into the project filesystem. This means that the project filesystem must be mounted before the docs filesystem. Additionally, the project filesystem contains the empty directory ./docs (/project/docs after the project filesystem is mounted) as a mount-point for the docs filesystem.

(root dir) /

Please note that there is no necessary connection (even by convention) between a mount-point for a specific filesystem and a particular disk partition and its associated special device file. The collection of files in a disk partition can be mounted in any directory in an already accessible filesystem. Once the partition’s filesystem is mounted, its top-level directory will take the name of its mount-point. At the same time, the top-level directory of a mounted partition’s filesystem replaces the mount-point directory. As a side effect, the eventual files that could reside in the mount-point directory (if it was not empty) will disappear once the new filesystem is mounted. These data can no longer be seen and accessed, but they are not erased or overwritten. They remain unchanged but hidden for future use; they will reappear once the filesystem is dismounted. Obviously, it is highly recommended to select empty directories for the mount-points. Otherwise, disk space taken by such files will be wasted — the files cannot be accessed, nor used, but they still take up disk space.

A filesystem can only be mounted in one place at one time; that is, a special device file may only designate one mount-point in the directory tree. However, one filesystem can have another filesystem as a subtree within it.

The previous discussion was related to the local filesystems — the filesystems that reside in local disks. This is not necessarily always the case; UNIX also supports remote disks.

Nevertheless, at this time we will only focus on the local filesystems, and the discussion in this chapter will primarily address these issues.

5.3.1.1 The mount Command

The mount command must be used to mount a filesystem. This is a regular UNIX com-mand that can be invoked from the comcom-mand line or a script at any time. However, the command is so crucial for the system that the security precautions require strict superuser privileges for its execution. Even the SUID bit (discussed in Section 2.2.2.2.4) doesn’t work in the case of the mount command; if SUID is set, the system will simply reject execution of the command. Any security risk must be avoided, and SUID always carries a bit of it.

The generic format for the mount command is:

mount[key-options] block-special-file mount-point

The mount command attaches a named filesystem, identified by block-special-file, to the overall filesystem hierarchy at an existing directory, identified by mount-point. A number of options are available.

mount maintains a table of mounted filesystems in the filesystem status file, usually named /etc/mnttab, or /etc/mtab. If invoked without an argument, mount displays the contents of this

TABLE 5.1

Filesystem Locations and Special Device Files

Note: Filesystems are usually named by their mount-points; this convention is implemented here.

Filesystem Special Device File Disk and Partition Mount-Point

usr /dev/dsk/c1d0s3 disk #1 - part. #3 /usr

var /dev/dsk/c1d0s5 disk #1 - part. #5 /var

home /dev/dsk/c1d1s0 disk #2 - part. #1 /home

project /dev/dsk/c1d1s5 disk #2 - part. #5 /project docs /dev/dsk/c1d2s2 disk #3 - whole disk /project/docs

table. If invoked with a single argument, either block-special-file or mount-point only, mount searches the filesystem configuration file (usually named /etc/vfstab, or /etc/fstab) for a matching entry, and mounts the specified filesystem in the specified directory.

The key-options can be generic ones, valid for all filesystem types, or specific for the different filesystem types. The following are the most common options:

-p Print the list of mounted filesystems in a format suitable for use in the filesystem configuration file.

-a Stands for all. Attempt to mount all the filesystems described in the filesystem configuration file. If a type argument is specified with the -t option, mount all file systems of that type. Some UNIX platforms have a special mount command for this purpose.

-f Fake a filesystem status entry (in the filesystem status file /etc/

mtab, or /etc/mnttab), but do not actually mount any filesystem.

-n Mount a filesystem without making an entry in the filesystem status file.

-v Verbose. Display messages indicating each filesystem being mounted.

-ttype Specify a file system “type” (see the later text about filesystem types).

-r Mount the specified file system read-only, even if the config-uration entry specifies that it is to be mounted read-write.

Physically write-protected and read-only filesystems should be mounted read-only. Otherwise errors occur when the system attempts to update access times, even if no write operation is attempted.

-oFS-specific-options Specify the filesystem-specific options — a comma-separated list of options valid for the corresponding filesystem type (see the text about filesystem types).

The following list shows the common options for most local UNIX filesystems.

Options Meaning

defaults Use all default options.

rw / ro Read/write, or read-only; the default is rw.

suid / nosuid SUID execution allowed, or not allowed; the default is suid.

grpid Create files with BSD semantics for the propagation of the group ID. Under this option, files inherit the GID of the directory in which they are created, regardless of the directory’s SGID bit.

noauto Do not mount the filesystem automatically, only explicitly (ignore option -a).

remount A filesystem mounted read-only can be remounted read-write (used in conjunction with rw).

intr / nointr Allow, or do not allow, keyboard interrupts to terminate a process that is waiting for an operation on a locked filesystem; the default is intr.

quota / noquota Filesystem usage limits are enforced, or are not enforced; the default is noquota.

rq Read-write with quota turned on (equivalent to rw,quota).

largefiles / nolargefiles Attempt to enable or disable the creation of files greater than 2GB in size; the filesystem must be created especially to support large files. The default is nolargfiles.

Note: It is highly recommended that you check the manual pages for the mount command before attempting to implement it.

A few examples of how to use the mount command follow; the presented situations are hypothetical.

• To mount the local filesystem /dev/xy0g in the directory /usr:

#mount /dev/xy0g /usr

• To mount the hfs filesystem /dev/dsk/c1d2s0 in the directory /home:

#mount -t hfs /dev/dsk/c1d2s0 /home

• To fake an entry for nd root:

#mount -ft 4.2 /dev/nd0 /

• To list the filesystems that are currently mounted:

#mount

• To mount all ufs file systems:

#mount -at ufs

• To save the current mount state:

#mount -p > /etc/vfstab