Linux Filesystem Administration

Linux Filesystem Administration

• Objectives include:
  • Identifying the structure and types of device files in the /dev directory
  • Understanding common filesystem types and their features
  • Mounting and unmounting filesystems
  • Creating and managing filesystems on various storage devices
  • Working with ISO images
  • Using LVM to create and manage logical volumes
  • Monitoring free space
  • Checking for errors
  • Using hard disk quotas to limit user space

The /dev Directory

• A device file represents a system device
  • There is typically one file per device in /dev
  • Specifies data transfer methods
• Character devices transfer data character-by-character
• Block devices transfer data in chunks using physical memory to buffer the transfer
  • Results in faster data transfer
  • Examples: CDs, DVDs, USB flash drives, hard disks, and SSDs
• Major number: a device file number that points to the device's driver in the Linux kernel
  • Different devices can share the same major number if they are the same general type
• Minor number: indicates the particular device
• Characteristics of device files are identified by: the file type (block or character), major number, and minor number
• The mknod command is used to re-create a corrupted device file, use it with the file type, major, and minor numbers
• Lists of devices and their major numbers can be viewed in the /proc/devices files

Filesystems

• Filesystem: an organization and management layer of the physical storage media
  • Used to manage the storage and retrieval of data
• All filesystems share three common components:
  • Superblock
  • Inode table
  • Data blocks
• All storage media need to contain a filesystem before usable

Filesystem Types

• The Linux operating system has access to many filesystems
  • Each with its own strengths and weaknesses
  • Several devices formatted with different filesystems under the same directory tree may be used

Mounting

• Mounting makes a device accessible to users via the logical directory tree
• Mount point: the directory to which a device is attached
  • The contents of the mount point will be temporarily be replaced by the mounted device
  • Any existing directory can be a mount point
• Prevent inaccessible files by creating empty directories specifically for mounting devices
• Root filesystem: a filesystem on the hard drive mounted to the / directory when the Linux system starts. Contains most OS files
• The mount command is used to mount devices to mount point directories
• The umount command unmounts devices from mount point directories

Working with USB Flash Memory Drives

• Most common type of removable media
  • Used to store small files
  • Recognized as SCSI drives by operating systems
  • Often formatted with the DOS FAT or exFAT filesystem
• The mkfs (make filesystem) command creates a new filesystem
  • Specify the filesystem type, device file, and use the -t switch
  • Specify a different filesystem after the -t option
• A USB flash memory drive must be mounted before use
  • To list currently mounted filesystems, use the mount command without options or arguments
  • Alternatively, use the df (disk free space) command
• To check if the /media/USBdrive directory is being used, use the fuser command
• The umount command can take the device name to unmount, or the mount point directory as an argument

Working with CDs, DVDs, and ISO Images

• Most software not downloaded from the internet is packaged on CDs and DVDs
  • Use the mount command to mount, and umount to unmount CD's
  • The device file depends on the technology used by the drive itself
• PATA Drive Configurations:
  • Primary master (/dev/hda)
  • Primary slave (/dev/hdb)
  • Secondary master (/dev/hdc)
  • Secondary slave (/dev/hdd)
• SATA or SCSI drives may use different names
• Fedora Linux creates a file called /dev/cdrom to make CD/DVD identification easier
  • Which is a symbolic link to the correct device file for the first CD or DVD drive
• CDs and DVDs typically use ISO 9660 or UDF filesystem type and are read-only when accessed using Linux
  • Mount with the -r (read-only) option
  • CDs and DVDs cannot be ejected until properly unmounted
• The ISO 9660 filesystem type is not limited to CDs and DVDs
  • Image files, called ISO images, can be created and contain other files
• The mkisofs command creates ISO image from a directory of files

Working with Removable Media Within a GUI Environment

• Processes automatically mount removable media to a directory
  • Provides immediate access
• Inserting a USB flash memory drive, CD, or DVD while in a GUI environment
  • The system automatically mounts the device to a directory
• Many types of removable media devices:
  • External hard drives
  • Digital cameras
  • Media players
  • Smartphones
  • Tablets

Working with Hard Disks and SSDs

• Types of hard disks:
  • PATA
  • SATA
  • SCSI/SAS
• PATA Configurations:
  • Primary master (/dev/hda)
  • Primary slave (/dev/hdb)
  • Secondary master (/dev/hdc)
  • Secondary slave (/dev/hdd)
• SATA and SCSI hard disks are well-suited to Linux servers
  • Faster access speed
  • Multiple hard drives can be attached to a controller
• Different device files are associated with:
  • First SCSI HDD (/dev/sda)
  • Second SCSI HDD (/dev/sdb)
  • Third SCSI HDD (/dev/sdc)
  • Fourth SCSI hard disk drive (/dev/sdd)
  • Fifth SCSI hard disk drive (/dev/sde)
  • Sixth SCSI hard disk drive (/dev/sdf)
  • And so on...
• SSDs use much faster NAND flash storage and provide a hard disk compatible interface

Standard Hard Disk Partitioning

• Partition: physical division of a hard disk
  • Can have its own filesystem
• Linux requires at least two partitions:
  • Root and swap
• It is good practice to use more than two partitions
  • Segregate different types of data
  • Allow for use of multiple filesystem types on one hard disk drive
  • Reduce chance that filesystem corruption will render a system unusable
  • Speed up access to stored data
  • Allow for certain operating system features
• Track: area on a hard disk that forms a concentric circle
• Sector: portion of a track containing information
• Block: combination of sectors
• Cylinder: series consisting of the same concentric track on all of the metal platters inside a hard disk drive
• Partition definitions are stored in the first readable sector of the hard disk
  • Master Boot Record (MBR) or master boot block (MBB)

Working with Standard Hard Disk Partitions

• The fdisk command creates partitions after installation
  • Specify hard disk partition as an argument
  • Variety of options for fdisk prompt to achieve different tasks
• The cfdisk command is an interactive graphical utility for creating, manipulating and deleting partitions
• Reboot computer after using the fdisk and cfdisk commands to reload into memory properly
• The mkswap command prepares the swap partition
• The swapon command activates the swap partition
• The swapoff command deactivates the swap partition
• Edit /etc/fstab file to ensure that new swap partition is activated as virtual memory
• If a hard disk uses a GPT instead of a MBR, then:
  • Use fdisk or cfdisk commands to create and modify partitions before you format them
• The gdisk (GPT fdisk) command: create and work with partitions on GPT hard disk
• The parted (GNU Parted) command: create and modify partitions on both MBR and GPT hard disks
• After creating GPT partitions that should contain a filesystem:
  • Format those partitions with a filesystem using mkfs
  • Mount them to the directory tree with the mount command
  • Update the /etc/fstab file to mount them automatically

Working with the LVM

• Logical Volume Manager (LVM) creates logical volumes
  • Can contain filesystems and can be mounted to directories
  • More flexible than standard partitions; allows use of free space across multiple hard disks
• LVM components:
  • Physical volumes (PVs)
  • Volume groups (VGs)
  • Logical volumes (LVs)
• The pvcreate command creates PVs
• The pvdisplay command displays detailed information about each PV
• The vgcreate command creates a VG that use the space in PVs
• Physical extent (PE) size: block size for saving data
• The vgdisplay command displays detailed information about each VG
• The lvcreate command creates LVs from available space in a VG
• The lvdisplay command displays information about each LV
• Mount points of LVs are worked with like any other disk partition device file
• The pvscan, vgscan, and lvscan commands display information about PVs, VGs, and LVs, respectively
• The vgextend command adds a new PV to an existing VG
• The lvextend command increases the size of an LV

Monitoring Filesystems

• Check mounted filesystems periodically for:
  • Errors
  • Disk space usage
  • Inode usage
• Minimizes problems by reducing the likelihood of insufficient disk space

Disk Usage

• Using more filesystems typically results in less hard disk space per filesystem
  • Can cause errors when filesystems fill up with data
• The df (disk free space) command monitors free space
  • Use the -h option for a more user-friendly format
  • To get information about different filesystems, mount them prior to using the command
• The du (directory usage) command views the size of a directory and its contents in kilobytes
  • Use the -s option to summarise output
  • Use the -h option for a more user-friendly format
• The dumpe2fs command views the total number of inodes and free inodes for ext2, ext3, or ext4 filesystems
  • Use the -h option

Checking Filesystems for Errors

• Filesystem corruption: errors in the filesystem
  • Commonly occurs due to improper system shutdown
• Syncing: saving data to the hard disk
• Bad blocks: unusable areas of a disk, that cannot hold a magnetic charge
• The fsck (filesystem check) command checks for errors
  • Filesystem must be unmounted
  • Use the -f option to perform a full check
• The e2fsck command checks an ext2, ext3, or ext4 filesystem
  • Use the -c option to check for bad blocks
• The tune2fs command is used to change filesystem parameters
  • The -i option sets the interval to force a full system check

Disk Quotas

• Used when several users on a system, must be enough hard disk space for each user's files
• Disk quotas: user limits on filesystem usage
  • Quotas can restrict the number of files/directories a user creates
• Soft limit: a brief period of time that a user can exceed a quota
  • Default is seven days
• Hard limit: the boundary a user is unable to exceed
• The quotaon and quotaoff commands turn quotas on and off
• The edquota command edits user quotas
• The repquota command reports user quotas
• The quota command allows to view their own quotas and current usage

Summary

• Disk devices are represented by device files in the /dev directory
• Filesystems must be created to a disk drive to be used by mounting to the linux directory tree with the mount command
• Hard disks must be partitioned into distinct sections before the creation of filesystems on those partitions
• Many different filesystems are available to Linux
• LVM can be used for creating logical volumes from multiple partitions
• Most removable devices are recognised as SCSI disks
• Automounting performed in GUI enviroments
• Monitoring disk usage with df du, and dumpe2fs commands essential to monitor disk space
• Hard disk quotas can used to limit maximum space usage for different user accounts