Mounting and Unmounting

Linux presents all filesystems as one directory tree. Hence to add a new device with a filesystem on it its filesystem needs to be made part of that one directory tree. The way this is done is by attaching the new filesystem under an existing (preferably empty) directory, which is part of the existing directory tree - the “mount” point.

To attach a new file system to the directory hierarchy you must mount its associated device file. First you will need to create the mount point; a directory where the device will be attached. As directories are part of a filesystem too the mount point exists on a previously mounted device. It should be empty. If is is not the files in the directory will not be visible while the device is mounted to it, but will reappear after the device has been disconnected (or unmounted). This type of security by obscurity is sometimes used to hide information from the casual onlooker.

To mount a device, use the mount command:

mount [options] device_file mount_point

With some devices, mount will detect what type of filesystem exists on the device, however it is more usual to use mount in the form of:

mount [options] -t file_system_type device_file mount_point

Generally, only the root user can use the mount command - mainly due to the fact that the device files are owned by root. For example, to mount the first partition on the second (IDE) hard drive off the /usr directory and assuming it contained the ext2 filesystem, you’d enter the command:

mount -t ext2 /dev/hdb1 /usr

A common device that is mounted is the floppy drive. A floppy disk generally contains the FAT, also known as msdos, filesystem (but not always) and is mounted with the command:

mount -t msdos /dev/fd0 /mnt

Note that the floppy disk was mounted under the /mnt directory. This is because the /mnt directory is the usual place to temporarily mount devices.

To see which devices you currently have mounted, simply type the command mount. Some sample output:

/dev/hda3 on / type ext2 (rw) /dev/hda1 on /dos type msdos (rw) none on /proc type proc (rw)

/dev/cdrom on /cdrom type iso9660 (ro) /dev/fd0 on /mnt type msdos (rw)

Each line shows which device file is mounted, where it is mounted, what filesystem type each partition is and how it is mounted (ro= read only,rw= read/write). Note the strange entry on line three - the proc filesystem. This is a special "virtual" filesystem used by Linux systems to store information about the kernel, processes and current resource usage. It is actually part of the system’s memory - in other words, the kernel sets aside an area of memory in which it stores information about the system. This same area is mounted onto the filesystem so that user programs have access to this information.

The information in the proc filesystem can also be used to see which filesystems are mounted by issuing the command:

$ cat /proc/mounts /dev/root / ext2 rw 0 0 proc /proc proc rw 0 0 /dev/hda1 /dos msdos rw 0 0 /dev/cdrom /cdrom iso9660 ro 0 0 /dev/fd0 /mnt msdos rw 0 0

The difference between /etc/mtab and /proc/mounts is that /etc/mtab is the user space administration kept by mount, and /proc/mounts is the information kept by the kernel. The latter reflects the information in user space. Due to these different implementations the info in /proc/mounts is always up-to-date, while the info in /etc/mtab may become inconsistent.

To release a device and disconnect it from the filesystem, the umount command is used. It is issued in the form:

umount device_file or

umount mount_point

For example, to release the floppy disk, you’d issue the command:

umount /dev/fd0 or

umount /mnt

Again, you must be the root user or a user with privileges to do this. You can’t unmount a device/mount point that is in use by a user (e.g. the user’s current working directory is within the mount point) or is in use by a process. Nor can you unmount devices/mount points which in turn have devices mounted to them.

The system needs to mount devices during boot. In true UNIX fashion, there is a file which governs the behaviour of mounting devices at boot time. In Linux, this file is /etc/fstab. Lines from the file use the following format:

device_file mount_point file_system_type mount_options [n] [n]

The first three fields are self explanatory; the fourth field,mount_optionsdefines how the device will be mounted (this includes information of access mode^ro/^rw, execute permissions and other information) - information on this can be found in the mount man pages (note that this field usually contains the word “defaults” ). The fifth and sixth fields are used by the system utilities dump and fsck respectively - see the next section for details.

There’s also a file called /etc/mtab. It lists the currently mounted partitions in fstab form.

3.1.7 Swap

Swap space in Linux is a partition or file that is used to move the contents of inactive pages of RAM to when RAM becomes full.

Linux can use either a normal file in the filesystem or a separate partition for swap space. A swap partition is faster, but it is easier to change the size of a swap file (there’s no need to repartition the whole hard disk, and possibly install everything from scratch).

When you know how much swap space you need, you should use a swap partition, but if you are in doubt, you could use a swap file first, and use the system for a while so that you can get a feel for how much swap you need, and then make a swap partition when you’re confident about its size. It is recommended to use a separate partition, because this excludes chances of file system fragmentation, which would reduce performance. Also, by using a separate swap partition, it can be guaranteed that the swap region is at the fastest location of the disk. On current HDDs this is at the beginning of the platters (outside rim, first cylinders).

It is possible to use several swap partitions and/or swap files at the same time. This means that if you only occasionally need an unusual amount of swap space, you can set up an extra swap file at such times, instead of keeping the whole amount allocated all the time.

The command mkswap is used to initialize a swap partition or a swap file. The partition or file needs to exist before it can be initialized. A swap partition is created with a disk partitioning tool like fdisk and a swap file can be created with:

dd if=/dev/zero of=swapfile bs=1024 count=65535

When the partition or file is created, it can be initialized with:

mkswap {device|file}

An initialized swap space is taken into use with swapon. This command tells the kernel that the swap space may be used. The path to the swap space is given as the argument, so to start swapping on a temporary swap file one might use the following command:

swapon /swapfile

or, when using a swap partition:

swapon /dev/hda8

Swap spaces may be used automatically by listing them in the file /etc/fstab:

/dev/hda8 none swap sw 0 0 /swapfile none swap sw 0 0

The startup scripts will run the command swapon -a, which will start swapping on all the swap spaces listed in /etc/fstab.

Therefore, the swapon command is usually used only when extra swap is needed. You can monitor the use of swap spaces with free. It will report the total amount of swap space used:

$ free

total used free shared buffers cached Mem: 127148 122588 4560 0 1584 69352 -/+ buffers/cache: 51652 75496 Swap: 130748 57716 73032

The first line of output (Mem:) shows the physical memory. The total column does not show the physical memory used by the kernel, which is loaded into the RAM memory during the boot process. The used column shows the amount of memory used (the second line does not count buffers). The free column shows completely unused memory. The shared column shows the amount of memory shared by several processes; The buffers column shows the current size of the disk buffer cache.

That last line (Swap: ) shows similar information for the swap spaces. If this line is all zeroes, swap space is not activated.

The same information, in a slightly different format, can be shown by using cat on the file /proc/meminfo:

$ cat /proc/meminfo

total used free shared buffers cached

Mem: 130199552 125177856 5021696 0 1622016 89280512 Swap: 133885952 59101184 74784768

MemTotal: 127148 kB MemFree: 4904 kB MemShared: 0 kB Buffers: 1584 kB Cached: 69120 kB SwapCached: 18068 kB Active: 80240 kB Inactive: 31080 kB HighTotal: 0 kB HighFree: 0 kB LowTotal: 127148 kB LowFree: 4904 kB SwapTotal: 130748 kB SwapFree: 73032 kB

To disable a device or swap file, use the swapoff command:

# swapoff /dev/sda3

3.1.8 UUIDs

The term UUID stands for Universal Unique IDentifier. It’s a 128 bit number that can be used to identify basically anything.

Generating such UUIDs can be done using appropriate software. There are 5 various versions of UUIDs, all of them use a (pseudo)random element, current system time and some mostly unique hardware ID, for example a MAC address. Theoretically there is a very, very remote chance of an UUID not being unique, but this is seen as impossible in practice.

On Linux, support for UUIDs was started within the e2fsprogs package. With filesystems, UUIDs are used to represent a specific filesystem. You can for example use the UUID in /etc/fstab to represent the partition which you want to mount.

Usually, a UUID is represented as 32 hexadecimal digits, grouped in sequences of 8,4,4,4 and 12 digits, separated by hyphens.

Here’s what an fstab entry with a UUID specifier looks like:

UUID=652b786e-b87f-49d2-af23-8087ced0c828 / ext4 errors=remount-ro,noatime 0 1

You might be wondering about the use of UUID’s in fstab, since device names work fine. UUIDs come in handy when disks are moved to different connectors or computers, multiple operating systems are installed on the computer, or other cases where device names could change while keeping the filesystem intact. As long as the filesystem does not change, the UUID stays the same.

Note the ’as long as the filesystem does not change’. This means, when you reformat a partition, the UUID will change.

For example, when you use mke2fs to reformat partition /dev/sda3, the UUID will be changed. So, if you use UUIDs in /etc/fstab, you have to adjust those as well.

If you want to know the UUID of a specific partition, use blkid /path/to/partition:

# blkid /dev/sda5

/dev/sda5: UUID="24df5f2a-a23f-4130-ae45-90e1016031bc" TYPE="swap"

Note

It is possible to create a new filesystem and still make it have the same UUID as it had before, at least for ’ext’ type filesystems.

# tune2fs /dev/sda5 -U 24df5f2a-a23f-4130-ae45-90e1016031bc

Note

On most Linux distributions you can generate your own UUIDs using the commanduuidgen.

3.1.9 sync

To improve performance of Linux filesystems, many operations are done in filesystem buffers, stored in RAM. To actually flush the data contained in these buffers to disk, the sync command is used.

sync is called automatically at the right moment when rebooting or halting the system. You will rarely need to use the command yourself. sync might be used to force syncing data to an USB device before removing it from your system, for example.

sync does not have any operation influencing options, so when you need to, just execute "sync" on the command line.