System Administration – Part 2

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. Devices in UNIX

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.

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 Mounting Disk Drives in UNIX

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 Free %Used Iused %Iused Mounted on /dev/hd % % / /dev/hd % % /usr /dev/hd9var % 518 4% /var /dev/hd % % /tmp /dev/hd % 421 1% /home /proc /proc /dev/hd10opt % 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 * /junk /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/ Jun 4 09: /grades rwxr-xr-x 203/ Jun 4 10: /test.sh rwxr-xr-x 203/ Jun 3 02: /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