Silicon Graphics IRIS Workstation User Manual page 54

50 IRIS WORKSTATION GUIDE
Appendix B:
Super-Block
One of the most common corrupted items is the super-block. The super-block
is prone to corruption because every change to the file system's blocks or
inodes modifies the super-block.
The super-block and its associated parts are most often corrupted when the
computer is halted and the last command involving output to the file system
was not a sync command.
The super-block can be checked for inconsistencies involving file-system size,
inode-list size, free-block list, free-block count, and the free-inode count.
File-System Size and Inode-List Size.
The file-system size must be larger than the number of blocks used by the
super-block and the number of blocks used by the list of inodes. The number
of inodes must be less than 65,535. The file-system size and inode-list size are
critical pieces of information to the fsck program. While there is no way to
actually check these sizes, fsck can check for them being within reasonable
bounds. All other checks of the file system depend on the correctness of these
sizes.
Free-Block List.
The free-block list starts in the super-block and continues through the free-list
blocks of the file system. Each free-list block can be checked for a list count out
of range, for block numbers out of range, and for blocks already allocated
within the file system. A check is made to see that all the blocks in the file
system were found.
The first free-block list is in the super-block. Fsck checks the list count for a
value of less than zero or greater than fifty. It also checks each block number
for a value of less than the first data block in the file system or greater than the
last block in the file system. Then it compares each block number to a list of
already allocated blocks. If the free-list block pointer is non-zero, the next
free-list block is read in and the process is repeated.
When all the blocks have been accounted for, a check is made to see if the
number of blocks used by the free-block list plus the number of blocks claimed
by the inodes equals the total number of blocks in the file system.
If anything is wrong with the free-block list, then fsck may rebuild it, excluding
all blocks in the list of allocated blocks.
Free-Block Count.
The super-block contains a count of the total number of free blocks within
the file system. Fsck compares this count to the number of blocks it found free
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