Cosc 4750
File Systems
• Which would you expect to find in a
“filesystem”?
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Processes
serial ports
inter-process communication channels
Shared memory segments
applications and files
• All of them are in a UNIX filesystem
• UNIX unifies all objects of the O/S into the
filesystem
– consistent programming interface, easy access
from the shell
File system components
• There are 4 main components
– A namespace
• Way of naming things and arranging them in a
hierarchy
– an API
• A set of system calls for navigation and
manipulating nodes
– Security Model
• a scheme for protecting, hiding, and sharing things
File system components (2)
– An implementation
• Code that ties the logical model to an actual disk
• Pathnames
– the directory structure can be as deep as
needed, but no name can be longer and 255
characters
– But a full pathname can not be accessed
properly when it is longer than 4,095 characters
File system components (3)
• Special characters such as the space, *, ?, and
others must either be backslashed or the path must
be in double quotes
– cat My\ File.txt
– cat “My File.txt”
• NEVER put a dash as the first character. The – is
taken as parameter to the program, not as the
filename.
Mount/unmouting file systems
• You can use the mount command and umount
command
• mount a usb drive into the filesystem
– mount /dev/sdc1 /media/usb
• Assuming /dev/hdc4 is the device for the usb drive.
• And the /media/usb directory already exists.
– mount /media/usb
• This assumes usb drive is listed in the /etc/fstab file.
• umount /media/usb
– unmounts the usb drive from the filesystem
Problems unmounting
• Unix will not allow you umount a file system that
is in use.
• umount /home
• umount: /home: device is busy
– A file is open or a user’s current working directory is on
the file system
– To find them, you can use the fuser –mv <file system>
command
• fuser –mv /home
• Show all processes user the /home file system
– You can also use lsof command on RedHat/Fedora and
SuSE installations.
File Structure
• Most UNIX system follow this convention:
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/home (/people) where the user dir. are at
/usr system info and applications
/usr/local local software (that you install)
/usr/bin /bin standard utilities (like ls, grep)
/usr/etc where SA utilities are located.
/etc system configurations
/dev system devices
/var system logs and other system info
/proc kernel and system info, also images of running processes
/boot (linux only), where the kernel and other files are located for
booting.
File Types
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regular files
directories
character device files
block device files
UNIX domain sockets
Named pipes (FIFOs)
Symbolic links
-rwx-----drwx-----crwx-----brwx-----srwx-----prwx-----lrwx------
Setting permissions
• Use chmod
• Can be used in 2 ways
– chmod u+x <file>
– chmod 700 <file>
• Chmod ugo + rx <file> or chmod a + rx <file>
u = user, g=group o= world
• and chmod ugo – w <file> or chmod a – w <file>
• numeric 0 ---, 1 --x , 2 -w-, 3 -wx, 4 r--, 5 r-x, 6 rw-, 7 rwx
• so, chmod 744 <file> is -rwxr--r--
Extra bits
• There is 1 more field for setuid, setgid, and the
“sticky bit”
• chmod 4755 <file>, enable setuid
– The file runs as the owner, instead of who is actually
running it.
• chmod 2755 <file>, enable setgid
– The file runs as the group
• chmod 1755 <file>, enable stick bit
– It used leave the program in memory, but is historical
and linux silently ignores it.
Directory Permissions
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For directories, the permission are
r: view the contents of the directory, ls
x: you can enter the directory, cd
w: you can create a file in the directory
– without it, in linux you can not modify a file either.
• SUN, sgi, etc… implement permissions slightly
differently.
– you don't need write permissions on the directory to
modify a file in the directory.
Links
• Soft links
– the “file” is a redirect to where the “real” file is
• All file and directory permissions apply.
– Can be used across physical hard drives
– deleting the link doesn’t delete the file. The “real” file
can be deleted and the link will not work anymore.
• Hard links
– the “file” is a connected directly to the other file. Only
the file permissions of the linked file apply.
– Can not be used across physical hard drives.
– If you deleted the “real” file, the contents still exist for
the linked file or vise versa
Links (2)
• directories can only be soft linked
• Using hard links, file locking does not work
• ln –s <target file> <linked filename>
– creates a soft link
• ln <target file> <linked filename>
– creates a hard link
Note on size limits
• The size of a file and filesystem are
dependent on block size
FileSystem Type
Max File
Max Filesystem
Ext3
2TB
16TB
Ext4
16TB
50TB
XFS
500TB
500TB
• Block size will effect allocation that can be
made in a file, this are using 4K block size
Extra attributes
• For the ext2, ext3, and ext4 fs
• Use lsattr to view, chattr to set/unset
– Anyone can set
• A, Never update access time
• d, no backup (dump will ignore the file)
• S, write with no buffering (sort of works)
– Causes more problems then is worth.
– Only root can set
• a, only allow write in append mode
• i, make file immutable and undeletable
– listed, but don’t work
• c, keep contents compressed (transparently)
• s, physically erase (write over) when deleted
• u, save contents on deletion to permit recovery
Default permissions
• use the umask command
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the system default is to create all files with -rw-r--r-directories default to drwxr-xr-x
umask lists the default permissions in reverse
so chmod 755 (rwxr-xr-x), umask 022
For better security you may want to use a
• umask 137
(-rw-r----)
• umask 177
(-rw-------)
• command: set umask 137
Ext4 file system
• Ext4 (fourth extended filesystem)
– Journaling file system like ext3
– Adopted into the kernel 2.6.28 released Dec
2008
– Support volumes up 1 exabyte and files up to
16 Terabytes
– Sub directory limit now 64,000 (32,000 limit in
ext3)
Ext4 file system (2)
• Performance improvements
– Delayed allocation, improves performance and
reduces fragmentation
• Block allocation decisions based on actual file size.
• Disadvantage
– Delay allocation may lead to data lost in a
crash, because old file is already removed, but
new data is not written yet.
– Ext3 either the old file or new file exists.
Quotas
• Linux only (others implement differently)
– NOTE: quotas and NFS don't work well together. Locally it will
tell the user they have exceeded quota, via NFS it may silently
discard the file.
• add command usrguota and grpquota to /etc/fstab file for
each partition using quotas
# quotacheck –c /home
– creates /home/aquota.user
# quotacheck –vug /home
– builds a table of disk usage
• See chapter 11 for startup scripts.
Quotas (2)
• Creating quota rules
• command edquota –u <user>
– NOTE: by default invokes the vi editor
– uses blocks instead of kilobytes
• a block equals 1K
– 512000 blocks is 500MB
– 7168000 blocks is 700 MB.
– See chapter 11 for more information
Quotas (3)
• Checking quotas
# repquota –a
– display a listing of all users, with used space, quota
limit, over quota, etc.
• using du
#du – h –c –s /home/ugrad/*
– lists each undergrad's directory with amount of space
used (-h show K, M, or GB -c show total at end)
Logical Volumes
• Technique to deploy large logical volumes,
instead of several smaller physical volumes
– Can span multiple physical hard drives.
– Example:
• (3) 9.1 GB drives, use 100 for /boot on one drive
• 9.1GB x 3 – 100MB gives about 27GB of space to
use, say 20 GB /home and 7Gb for / partation.
– Later another 9.1 GB added to extend the
volume and then (with ext3) used to expand the
/home partition (giving about 29GB).
• Note: LVM supports up to 2TB with LVM 2 and 8 Exabytes with 64bit.
Q&A