Transcript System Administration

System Administration – Part 2
Devices in UNIX
• Devices in UNIX are files:
• A device can be accessed with different file names
• All device files are stored in /dev or its sub-directories
• Device files can be grouped in to 2 main categories:
 b (block devices) include floppy, CD-ROM, disk drives, DVD
All data is read & written in blocks an uses a buffer cache.
Example file listing: brw-rw-rw- fd0

c (character devices) include terminals, printers and tape
drives. Also known as raw devices.
Example file listing: crw-rw-rw- tty1A
• Device file do not contain any data.
How the UNIX File System Is Organized on Disk Drives
• Each drive is organized in the form of a directory structure with
own sub-root
• Each disk must have at least 1 file system on it
• UNIX usually resides on multiple file systems. Mounting
combines them into one logical system
• Root File System: Contains bare-bones UNIX - /root, /bin,
/etc, /dev, /lib
• UNIX often has a swap file system where the kernel temporarily
moves processes out of memory while it is waiting, e.g., I/O
transfers, to swap in once process is ready to run again.
Mounting Disk Drives in UNIX
Secondary Disk drives must be mounted to be known to the kernel:
 When a new file system is created, root does not know of it.
 mount attaches all secondary file systems to the root file system.
 Enables root to be main file system with the root directory the
directory of a unified file system.
Example: mount –F ufs /dev/dsk/ct08d0s1 /oracle
 mount command by it self lists all mounted drives.
 A mount point is the location in the operating system's
directory structure where a mounted file system appears by
the name provided, i.e., a child directory of root.
Example: /oracle is the device’s mount-point in the above
Unmounting Disk Drives in UNIX
• The umount (note spelling!) command disassociates and detaches a
secondary file system . Example (system-dependent, like mount):
umount /oracle
# linux
umount /dev/hda3 /oracle
# linux
umount /dev/dsk/ct08d0s1
# solaris
o If a mounted file system is being used by a user, i.e., the user has
cd’ to /oracle, the umount command will fail.
# umount: /oracle busy
o To use umount, the user must be in a directory closer to root than
the file system being detached.
What’s in a UNIX File System?
Boot block:
Referred to as Master Boot Record (MBR). Contains
small boot program and the partition table.
Superblock: Contains global info about the file sytem, includes
a list of free inodes and data blocks.
inode blocks: Contains the inode value for each file of the file system.
When file is created, its inode entry is allocated here.
Contains array of disk block addresses.
Data blocks: Contains all user created data and programs. Although
disk blocks are numbered consecutively, the file’s data
may be arranged in non-contiguous blocks on the drive.
Note: Blocks are usually 512 bytes (or 1024 bytes in Linux).
When Good File Systems Go Bad
• Every 30 seconds the update daemon writes copies of the
superblock to disk using sync. But what happens if power is lost
before a sync is performed?
• The file system can lose its integrity in ways like:
 two inodes can claim the same disk block
 a used block is marked free
 a free block is not listed in the superblock
• There is a periodic need to check (and sometimes repair) a file
system that may have gone bad.
Checking the File System
fsck – File System Consistency Check
• Used if file system fails to mount. Checks and repairs damaged file
system (dirty, not clean) .
• Damage often occurs from abnormal shutdown due to hardware
failure, power failure or switching off without proper shutdown.
• If cannot be repaired, then reinstallation of system may be required.
Example: # fsck /dev/da0s1a
** phase 1 - Check Blocks and Sizes
** phase 2 - Check Pathnames
** phase 3 - Check Connectivity
** phase 4 - Check Reference Counts
** phase 5 - Check Free List
#
(checks out ok, otherwise answer questions to fix)
Monitoring Free Disk Space
df - reports amount of free space for each file system (separately).
$ df -k
Filesystem 1024-blocks
/dev/hd4
32768
/dev/hd2
4587520
/dev/hd9var 65536
/dev/hd3
819200
/dev/hd1
524288
/proc
/dev/hd10opt 65536
Free
16016
1889420
12032
637832
395848
26004
%Used Iused %Iused Mounted on
52%
2271
14%
/
59% 37791
4%
/usr
82%
518
4%
/var
23%
1829
1%
/tmp
25%
421
1%
/home
/proc
61%
654
4%
/opt
Monitoring Disk Space Used
du - reports used disk space for each subordinate directory (separately).
• Example: directory usage listed in kilobytes:
$ du -k *
152304 ./junk1
1856548 ./junk2
• Example: directory usage in human-readable format:
$ du -h *
149M
./junk1
1.8G
./junk2
1.3K
./junk3
• Example: report usage of all subdirectories and files including
hidden files, sorted by filesize :
$ du -k .[A-z]* * | sort -n
Backing Up Files Using tar
tar – backup and restore files (tape archive, but other devices are OK too)
Example: # cd /home
# tar –cvf /dev/rdsk/f0q18dt ./rick
-c creates a new archive
-v displays the progress of the backup (verbose mode)
-f use the specified backup device
The files being backed up are specified using a relative pathname
so they can be restored in a different directory if needed.
Restoring Files Using tar
Use the –t option to display the table of contents. Use –x to restore the
files.
Example: Create an incremental backup and then restore it.
# tar –cvf /dev/rct0 `find /home/rick –newer .lt –print`
# touch .lt
# tar –tvf /dev/rct0
rw-r--r-- 203/50 470 Jun 4 09:35 2010 ./grades
rwxr-xr-x 203/50 470 Jun 4 10:46 2010 ./test.sh
rwxr-xr-x 203/50 470 Jun 3 02:35 2010 ./a.out
# tar –xvf /dev/rct0
x /home/rick/grades 169 bytes, 1 tape blocks
x /home/rick/test.sh 4855 bytes, 10 tape blocks
x /home/rick/a.out 7505 bytes, 15 tape blocks