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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 Structure
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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File System Structure
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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File System Structure
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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Making a File System
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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Making a File System
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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