Download pdf - LPI 101 Ch12 File Systems

8/9/2019 LPI 101 Ch12 File Systems

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SAIGONLAB 83 Nguyn Th Nh, P9, Q.Tn Bnh, Tp. HCM LPI 102

Chapter 12

File Systems


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Objectives

To understand the Linux file system concepts

Define and identify different file system structures

and types

Describe and mount file systems including MS

Windows partitions

Define and manage disk quotas

Create different file systems

Describe the /proc file system


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File System Structure

Almost everything in Linux is treated as a file, including

commands, most hardware devices, processes, and network

connections

File systems on device must be mount into tree structurebefore accessing :

A diskpartition can be mounted onto any directory

Directory, in this case, is referred to as mount point

File permission used to control user access to the systems


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File system defines how a disk is structured, and different

disks can be formatted with different file systems. However,

all file systems have a special blockthat contains disk

information

The superblock contains information about the file system,

including the locations of its inode tables, free block list, and

root directory.

In general, if the superblock becomes corrupt, the file systemwill become unusable. Therefore, extra copies of the

superblock are scattered in predetermined location across the

file system, Linux do this automatically


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Under UNIX, all information about a file (meta-data) is

stored in an its inode except the file's name, including :

Type

Ownership and associated group Permissions

Time stamps: mtime (modification time), ctime (inode

change time), atime (access time)

Link (hard) count

Pointers to direct and indirect data blocks (where the

contents are stored)


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Specific file systems may include other information such as

the ext2 attributes (see the lsatter and chatter man pages for

details) or access control lists (ACLs) and extended attributes

(EAs).


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Making a File System

The mkfs command is used to create file systems on a disk or

disk partition

When creating a new file system, you will need to specify the

raw disk device, disk size as well as some other parameters,mkfs creates the superblock and inode list. Keep in mind that

the inode list isfixedin size and cannot be extendedon many

file system types

Examples:# mkfs -t ext2 /dev/hda5 102400

# mkfs -t msdos /dev/fd0


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Use the default number of inodes unless you expect to create a

lot of very small files or a few very big ones. A larger block

size (2,048) will give better disk performance but will waste

more space due to partially filled blocks at the end of files.

Smaller block sizes (512) use less space, but disk performance

is worse

You do not have to make the file system the same size as the

disk or partition, but by not doing so, the unused space will bewasted


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Options for mkfs Command:

-c Check the device for bad blocks (mke2fs and mkdosfs)

-L label Set the volume label for the files system (mke2fs)

-n label Set the 11-character volume label for the file system(mke2fs)

-t fstype Specifies the type of file system to be built. If notspecified, the default file system type (currently ext2)

is used

-v Verbose mode


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Mounting a File System

In Linux, file systems must be mounted in order to access

them. Mounting can occur via the command line or by

including an entry in /etc/fstab


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Command Line Mounting

Command format:

mount t o

Examples:

# mount -t ext2 /dev/fd0 /nmt/floppy# mount -t iso9660 /dev/cdrom /mnt/cdrom

# mount -t nfs 10.0.1.1:/usr/local/stuff

/usr/local/stuff

( Refer to mount man pages for more details )


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Mounting with /etc/fstab

The /etc/fstab file is read at boot time. Any file systems listed

within this file are loaded by default. Each line has the form:

LABEL=/ / ext3 defaults 1 1

/dev/hda1 /windows vfat defaults 0 0

/dev/fd0 /mnt/floppy msdos noauto 0 0/dev/cdrom /mnt/cdrom iso9660 noauto,ro 0 0

none /proc proc defaults 0 0

/dev/hda6 /swap swap defaults 0 0


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Mounting a File System

The commands mount, cat /etc/mtab ,and cat /proc/mounts will

display the file systems that arecurrently mounted


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Unmounting a File System

To unmount a file system, simply use the umount command

in format:

umount

Or :umount

Example:

# umount /mnt/cdrom


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Two Way To Fix Device busy Error

To list the PIDs first:

fuser -m

And then: stop processes with kill command (safely)

Stop all processes that are keeping a file system busy, use :fuser k -m


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Unmounting a File System

The eject command will attempt to unmount the file systemand then unmount the CD-ROM tray

Be sure to unmount a floppy drive prior to removal of the

floppy disk. Serious file system errors, and loss of data tothe disk can occur if it is not unmounted

File systems must be unmounted depth first


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Mounting Microsoft (MS-DOS) Partitions

Linux has the ability to mount MS-DOS and MS Windows9x partitions. This allows MS files to be read and written to

while in the Linux environment

The mount command and /etc/fstab allow two MS-DOS filesystem type options: msdos and vfat. It is safest to use the

msdos option when mounting a true MS-DOS partition, and

vfat when mounting an MS Windows partition


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Checking File Systems

Power failures, failing hardware and operator error candamage the file system.

e2fsck Check a Linux ext2 or ext3 file system

tune2fs Adjusts file system parameters such as the maximal mountcount, maximum time between checks, and the number ofblocks reserved for root.

It can also add a journal to an existing ext2 partition tomake it an ext3 partition:

tune2fs -j /dev/hda1

debugfs A fairly dangerous file system debugger

ext2ed An editor for the second extended file system

dumpe2fs Shows information about an ext2 or ext3 file system mostly from the superblock


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Managing File Systems

Kernel File Cache


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Kernel File Cache

The Linux kernel caches its blockdata in memory. Thisimproves performance by reducing data accesses to disk, as

the kernel will frequently find the data it wants in the cache

in memory. The sync command is used to manually flush

the cache to disk.

A very old method of closing down Linux system was to

issue the commands:

$ sync; sync ; halt The newer journaling file systems, such as ext3fs, maintain

file system integrity even if the system crashes. Journaling

file systems recover automatically on mount


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The lost+found Directory

This directory contains links to lost files in a corrupted filesystem

The file system check programs, such as e2fsck, log entries

into the lost and found directory. Lost files stored in

lost+found directory with thefilename stored as an inode

number. To identify the types of recovered files, use the file,

more, or less command


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Corrupt File Systems

If a system crashes or is shutdown improperly, you will needto repair the file system, fsck and e2fsck is best suited for

repairing ext2 partitions. Repaired file systems are modified

to return everything to a consistent state

Some damaged files can be completely lost and the data

unrecoverable. Recoverable files or partial files will be

placed in the lost+found directory


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Utilities

Free disk : to find out how much free disk space is availableon a mounted file system use df command

Options: -k Reports the size in KB (default)

-m Reports the size in MB-t Selects a specific file system type

-a Shows all mount points


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Utilities

DiskUsage :Use du command to list only directory size

Options: -s Summarize for named files

-c Compute a grand total

-h Human readable format-a Detail all files


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Utilities

Quota :

The quota command checks a user's disk usage and limit.

Some Linux administrators choose not to enable quota

checking; it is compiled into the kerneland activatedby the

system administrator


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User Disk Quota

The idea behind quota is to make users stay within their disk

consumption limit. The quota system on Linux allows an

administrator to set the number of inodes a user or a group of

users may possess and the number of disk blocks that may be

allocated to a user or a group of users

Quota are not always appropriate for dedicated servers and

single user workstations. Quota are especially useful for

multi-user machines such as ISP style servers, MAILservers,


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User Disk Quota

Quota have two levels: "soft" and "hard" :

The soft limit can be exceeded, but only for a certain time;

failure to reduce the allocated space before the timer

expires will trigger the quota

Hard limits apply immediately.Users exceeding their quota

on a disk will be advised that they have run out of space (

maybe confused with afull disk)


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Getting a Report on a User's Quota Status

The quota command displays disk usage and limits for a givenuser. By default, quota will display the current user's diskquota information. If logged in as root, use the commandquota -u to view a particular user's disk quota

information


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Turning on Quota at Boot Time

Add or uncomment these lines in /etc/rc.d/rc.sysinit :

if [ -x /usr/sbin/quotacheck ]; then

echo "Checking quota. This may take some time.

/usr/sbin/quotacheck -avug echo Done.fi

if [ -x /usr/sbin/quotaon ]; then

echo Turning on quota.

/usr/sbin/quotaon -auvg

fi


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Turning on Quota at Boot Time

Add "usrquota" to the fourth field containing the worddefaults (man fstab for details):

/dev/hda1 / ext3 defaults 1 1

/dev/hda2 /usr ext3 defaults,usrquota 1 1

If group quota support on a file system is required, replace

"usrquota" with "grpquota"


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Maintaining Quota

The system administrator will need to create two quotarecord files, quota.user and quota.group in the top-level

directory of the file system. They should be owned by root,

and permissions set to read-write for root only


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Maintaining Quota

As root, go to the partition where quota should be enabled.Use these commands:

quotaon, quotaoff : turn the file system quota on and off.

edquota u : is the quota editor. One or moreusers or groups may be specified on the command line.

Only root may edit quota


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The /proc File System

The /proc file system is a virtual file system (no actual diskspace is used) that contains very useful information about the

status of the system

The files and directories under /proc change over time as the

kernel provides system statistics and process information.

Commands such as ps, top, dmesg, use /proc to obtain their

information


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Summary

To understand the Linux file system concepts

Define and identify different file system structures and types

Describe and mount file systems including MS Windows

partitions Define and manage disk quotas

Create different file systems

Describe the /proc file system


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