Partitioning and File-systems (cont)

The DD command:

> dd [if=input] [of=output] [bs=blksize] [count=#-blks] [conv=opts]

• Convert/copy/create a file. To be used in several places to create empty files using the /dev/zero device (a virtual block device that returns unlimited null ('\0') bytes of data.)

Unix standard virtual character devices: (man 4 null zero random urandom full)

• Useful for dd as well as many other things:

/dev/null

• Reads return EOF (End Of File), writes are discarded.

/dev/zero

• Reads return null characters ('\0'), writes are discarded.

/dev/random

• Returns random data, sourced from an entropy pool -- can be very slow and block when the pool is empty. Writes update the entropy pool.

/dev/urandom

• A source of pseudo-random data, not sourced from a pool. Writes update the PRNG (seed)

/dev/full

• Writes fail with the error "no space left", reads return null bytes. Used for error-testing programs.

Swap partitions and files:

Swap files or page files are used by the virtual memory system of the OS to augment the amount of available memory by using the hard drive as a secondary level of memory. When additional memory is needed, unused memory is "swapped" to disk, freeing up main memory to be used in a different area.

# mkswap [options] device

• Initialize a swap partition or file (formats the swap space.)

# swapon [-a] [device]

• Enable swapping on all (-a) devices (present in the /etc/fstab file) or a single device.

# swapoff [-a] [device]

• Disable swapping

# swaplabel [-L label] device

• Show/set swap label (a label, discussed later allow us to refer to a device by a name (it's label), rather than some complicated block device enumeration (i.e. /dev/nvme0n1p0 for example.)

> free

• Display amount of free virtual memory available, including swap space

Files:

/proc/swaps

• Lists swap areas in use.

swap files vs partitions:

• Swap space can be allocated to a file as well as a partition. There is very little difference in performance between the two, although partitions can be allocated at the beginning of the disk which is usually faster (for spinning disks anyway,) whereas a swap file could be located anywhere on the disk.
• Swap files can be created and added/removed as needed.
• Make sure swap files are mode 0600 so they are not readable by normal users.

Example:

# dd if=/dev/zero of=/swap bs=1M count=1k

• Creates a blank 1 gigabyte file (/swap) (done as the super-user)

# mkswap /swap

• Formats the blank file so it is a swap file.

# chmod 0600 /swap

• Make sure only root can read/write to the file.

# swapon /swap

• Enables swapping in the new swap file. Should be added to /etc/fstab to make the system start swapping on it on boot-up.

  Example /etc/fstab entry:
  /swap none swap defaults 0 0

Filesystems:

# mount [-o options] [-t type] device dir

• mounts a filesystem located on device at dir in the directory hierarchy.
  -t forces it to use for the filesystem type, in the event that the kernel cannot automatically determine it.
  -o options Specific mount options.

	Option	What it does
*	defaults	Mount with default options (mostly used in /etc/fstab) implies: rw, suid, dev, exec, auto, nouser, and async.
*	noatime	Don't update atime records on files
*	noauto	Don't mount it explicitly (mostly used in /etc/fstab)
	nodev	Don't interpret device files on the file-system
*	noexec	Don't execute binaries on the file-system
	nosuid	Don't allow setuid binaries on this file-system
*	remount	Remounts the file-system with the new options
*	ro	Mount file-system in read-only mode
	rw	Mount FS in read-write mode (default)

* = remember these (may be useful in the future)

Example:
# mount -o ro,remount /dev/sda1 /mnt

• Re-mounts the device /dev/sda1 at /mnt in read-only mode.

# umount dir | device`

• un-mounts a filesystem.

  Examples:
  # umount /dev/sda1

  • Un-mounts the device /dev/sda1

  # umount /mnt

  • Un-mounts the device mounted at /mnt

# fuser [-uv] [-m filesystem] [-n tcp|udp port] [file]

• Display processes (& users -u) using files, filesystems (-m) or sockets (-n).

  Example:
  # fuser -uvm /net

  • Display all users/processes using the /net filesystem.

# lsof

• Like fuser, but more. Lists open files for all processes.