What is the recommended way to execute the fsck command to check and repair the unix filesystem?

Answer:

Its recommendable that you run the fsck from a boot cdrom.

At the maintenance prompt-> halt
At the prompt (ok) -> boot cdrom -s
Then -> fsck -F ufs /dev/rdsk/c0t0d0s0

If you have a BAD SUPERBLOCK message-> 
newfs -Nv /dev/rdsk/c0t0d0s0
(to obtain the block numbers of the Superblock backup's) 

Select a backup block number in the middle of the previous response. 
then run -> 

fsck -F ufs -ob=<backup_block_numer> /dev/rdsk/c0t0d0s0

Explanation about command: fsck

fsck is a Unix utility for checking and repairing file system inconsistencies. File system can become inconsistent due to several reasons and the most common is abnormal shutdown due to hardware failure, power failure or switching off the system without proper shutdown. Due to these reasons the superblock in a file system is not updated and has mismatched information relating to system data blocks, free blocks and inodes .

fsck – Modes of operation :

fsck operates in two modes interactive and non interactive :

Interactive – fsck examines the file system and stops at each error it finds in the file system and gives the problem description and ask for user response whether to correct the problem or continue without making any change to the file system. 

Non interactive :fsck tries to repair all the problems it finds in a file system without stopping for user response useful in case of a large number of inconsistencies in a file system but has the disadvantage of removing some useful files which are detected to be corrupt .

If file system is found to have problem at the booting time non interactive fsck is run and all errors which are considered safe to correct are corrected. But if still file system has problems the system boots in single user mode asking for user to manually run the fsck to correct the problems in file system

Running fsck :

fsck should always be run in a single user mode which ensures proper repair of file system . If it is run in a busy system where the file system is changing constantly fsck may see the changes as inconsistencies and may corrupt the file system .

If the system can not be brought in a single user mode fsck should be run on the partitions ,other than root & usr , after unmounting them . Root & usr partitions can not be unmounted . If the system fails to come up due to root/usr files system corruption the system can be booted with CD and root/usr partitions can be repaired using fsck. 

Command Syntax  :

fsck [ -F fstype] [-V] [-yY] [-o options] special

-F fstype type of file system to be repaired ( ufs , vxfs etc)

-V verify the command line syntax but do not run the command 

-y or -Y Run the command in non interactive mode – repair all errors encountered without waiting for user response.

-o options Three options can be specified with -o flag

b=n where n is the number of next super block if primary super block is corrupted in a file system .

p option used to make safe repair options during the booting process.

f force the file system check regardless of its clean flag.

special – Block or character device name of the file system to be checked/repaired – for example /dev/rdsk/c0t3d0s4 .Character device should be used for consistencies check & repair

fsck phases
fsck checks the file system in a series of 5 pages and checks a specific functionality of file system in each phase.

** phase 1 – Check Blocks and Sizes
** phase 2 – Check Pathnames
** phase 3 – Check Connectivity
** phase 4 – Check Reference Counts
** phase 5 – Check Cylinder Groups

fsck error messages & Corrective action :

1. Corrupted superblock – fsck fails to run 

If the superblock is corrupted the file system still can be repaired using alternate superblock which are formed while making new file system .

the first alternate superblock number is 32 and others superblock numbers can be found using the following command :

newfs -N /dev/rdsk/c0t0d0s6

for example to run fsck using first alternate superblock following command is used 

fsck -F ufs -o b=32 /dev/rdsk/c0t0d0s6

2. Link counter adjustment

fsck finds mismatch between directory inode link counts and actual directory links and prompts for adjustment in case of interactive operation. Link count adjustments are considered to be a safe operation in a file system and should be repaired by giving ‘y’ response to the adjust ? prompt during fsck.

3. Free Block count salvage 

During fsck the number of free blocks listed in a superblock and actual unallocated free blocks count does not match. fsck inform this mismatch and asks to salvage free block count to synchronize the superblock count. This error can be corrected without any potential problem to the file system or files.

4. Unreferenced file reconnection 

While checking connectivity fsck finds some inodes which are allocated but not referenced – not attached to any directory . Answering y to reconnect message by fsck links these files to the lost+found directory with their inode number as their name. 

To get more info about the files in lost+found ‘file’ command can be used to see the type of files and subsequently they can be opened in their applications or text editors to find out about their contents. If the file is found to be correct it can be used after copying to some other directory and renaming it. 

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