Server Admin Chapter 5

/dev directory

Contains one file per device for almost all devices on a Linux machine

Character devices

Transfer data character-by-character to and from the device

Block devices

Transfer chunks or blocks of data using physical memory to buffer the transfer (faster than character devices)

What does `ls -l` do when looking at a device?

Shows b or c for block/character file.

Shows major and minor number (Ex: 8, 1) for the device

mknod command

Recreates a corrupt device file if major/minor numbers are known

/proc/devices file

Contains list of devices currently used on the system and their major/minor numbers

Common components of all filesystems

Superblock, Inode table, Data blocks

Linux filesystem types

Btrfs, exFAT, ext2, ext3, ext4, zfs

mount command

Used to mount devices to mount point directories (Makes the device's contents visible)

umount command

Used to unmount devices from mount point directories

lsusb command

Displays removable devices connected to your PC

lsblk command

Used to help verify the device file used to represent the partition on your USB drive

mkfs command

Used to make a filesystem on a USB flash drive

df -hT command

(Disk free space) Displays a list of currently mounted filesystems

fuser command (fuser -u )

Displays the users using a particular directory

eject command

Unmounts the filesystem and force the CD or DVD to physically eject a disc

mkisofs

Create a new ISO image from a directory of files

SATA and SCSI drive letter format

First: /dev/sda

Second: /dev/sdb

...

How many partitions does Linux require at minimum?

Two

One mounted to the / directory (root partition)

One mounted to the /boot directory

Tracks

Concentric circles

Sectors

Divide each track into smaller units

Blocks of data

Combination of sectors

Cylinder

Series consisting of the same concentric track on all the metal platters inside a HDD (think vertical)

Partition definitions

Stored in the first readable sector of the hard disk known as the Master Boot Record (MBR) or GUID Partition Table (GPT)

Partition device files lettering format

First primary partition: /dev/sda1

Second primary partition: /dev/sda2

...

Logical drives are named starting with number...

5

How many partitions can be created?

Up to 128

fdisk /dev/sda command

Creates sda partition after installation (partitions a fixed disk)

mkswap /dev/sda4 command

Prepares the swap partition

swapon /dev/sda4 command

Activates the swap partition

cfdisk command

A GUI to create, manipulate, and delete partitions

Note: always reboot after using fdisk and cfdisk.

gdisk command

(GPT fdisk) Creates and works with partitions on a GPT hard disk

parted command

(GNU parted) Creates and modifies partitions on both MBR and GPT HDs

Logical Volume Manager (LVM)

Creates logical volumes that can be mounted to directories within the Linux filesystem hierarchy

Alternative to creating and mounting filesystems that reside on standard partitions

What does the LVM consist of?

Physical volumes, volume groups, and logical volumes

Physical Volumes (PVs)

Unused partitions on hard disk drives that the LVM can use to store information

Volume Groups (VGs)

Contains one or more PVs.

Represents the pools of storage space that are available to the LVm for creating logical volumes.

Logical Volumes (LVs)

Usable volumes that are created by the LVM from the available storage space within a LG

pvcreate /dev/sda4 command

Creates PVs of unused partition you want the LVM to use

pvdisplay command

Displays detailed info about each PV

vgcreate vg00 /dev/sda4 command

Creates a VG named vg00 that uses the dev/sda4 PV

Physical extent (PE)

Sets the block size of the VG

vgdisplay command

Displays detailed info about each VG

lvcreate command

Creates LVs from available space in a VG

lvdisplay command

Displays info about each LV

pvscan, vgscan, and lvscan commands

Display info about PVs, VGs, and LVs

vgextend command

Adds a new PV to an existing VG

lvextend command

Increases the size of an LV

df -h command

(Disk free) Monitors free space used by mounted filesystems (-h indicates human readable option)

du -h /usr | more command

View size of directory and its contents

-s to summarize output

-h for human readable

| more - use with large nums of files

-i to view the total num of inodes and free inodes

Bad Blocks

Unusable areas of a disk

fsck command

Checks the filesystem for errors

NOTE: THE FILESYSTEM MUST FIRST BE UNMOUNTED!

e2fsck command

Checks an ext2, ext3, and ext4 filesystem for errors

-c to check for bad blocks

tune2fs command

Used to adjust tunable filesystem parameters

Disk Quotas

Set user limits to prevent users from using unnecessary space

Soft limits

Disk quotas that the user may briefly exceed for a certain period of time

Hard limits