Filesystems and Swapping

Devices

Devices in Linux are represented as special files. These files are either character special files or block special files. Character special files are used for devices that can work with one byte at a time. Examples include

/dev/ttyS* -- the serial ports (waiting for a connection)
/dev/cua* -- the serial ports (dialing out)
/dev/console -- the keyboard/monitor
/dev/tty -- Your current input and output
/dev/audio -- the soundcard
/dev/ps2 -- a ps2 mouse
/dev/atimouse -- an ati mouse
/dev/mouse -- normally a link to whatever mouse is active

There are also lots of block special files. Examples include

/dev/hda -- The first ide hard drive
/dev/sda -- The first scsi hard drive
/dev/fd0H1440 -- the a: floppy disk
/dev/fd0H720 -- the a: floppy disk, formatted to 720K
/dev/rst0 -- the first scsi tape drive
/dev/cdrom -- the cdrom device

You may create a filesystem on almost any block special file. Making a filesystem on a tape drive or a writeable cdrom is a known-bad-idea.

What matters for devices are the major number, which describes the type of device, and minor number, which describes the instance of the device. Devices also have a standard name, which is the name they are most often called. For example, the first ide drive is normally /dev/hda, major =3, minor =0. The second ide drive is normally /dev/hdb, major=3, minor=64.

To make a device file, use the mknod command. For example, mknod /dev/mydisk b 3 64 would make a file called /dev/mydisk which would access the second ide harddrive.

To make many device files, use the MAKEDEV command.

To find out the major and minor numbers for a device, look in the kernel source or (much better) the MAKEDEV command, which can be found in /dev of all places.

Stupidly, there are no device files for network cards.

There are also lots of psuedo-device files. Examples include

/dev/random -- a bunch of random numbers
/dev/zero -- an infinite supply of zeros
/dev/null -- the great bit bucket
/dev/ram[0-9] -- ram used as a ramdisk
/dev/ram -- the physical ram in the system

Hard Drives

Hard drive get a device file for entire hard drive. You also get a device file for every partition on the drive.

/dev/hda is the entire first hard drive
/dev/hda3 is the third partition of the first hard drive

Filesystems

Filesystems are stored on either partitions or disks.

If the filesystem is to be on a hard drive

If the hard drive is bootable

then the hard drive needs a partition table
then the filesystem must be on a partition

If the hard drive is not bootable

You may place the filesystem on a partition (the partition table costs a few K)
You may place the filesystem on the draw device (other operating systems will think the disk unpartitioned)

If the filesystem is to be on a floppy disk

No partition table ... use the raw device

If the filesystem is to be on a ramdisk

No partition table .. use the raw device

Filesystem Types

There are many different kinds of filesystem

EXT2 is the standard Linux filesystem
RiserFS is a new filesystem that requires a kernel patch. It has journalling, which means it is must more restant to suddenly loosing power.
ISO9660 is the filesystem normally found on CD-ROMS
Win95 is a the filesystem found on Windows Machines. It's not as fast as EXT2, and has limitations on the names that files can have. There are no hard links.
To use a filesystem you must have support for that filesystem compiled into the kernel.

Making a Filesystem

The basic command is
mkfs -t type place.

For example,
mkfs -t ext2 /dev/ram0

For ext2 filesystem, you can also type mke2fs /dev/ram0. To see the *many* options, read man mke2fs.

Mounting a Filesystem

You cannot access the contents of a filesystem until it is mounted. The place where it is mounted is called mount point.
To temporaritly mount a filesystem, type
mount /dev/ram0 /mount-point

To set things up so that the filesystem is permenantly mounted, edit the file /etc/fstab.

Unmounting a Filesystem

To remove a filesystem temporarily, type
umount /mount-point
This will fail if any process is using the filesystem, or even has it as