CHECK(VIII) 8/31/73 CHECK(VIII)
NAME
check - file system consistency check
SYNOPSIS
check [ -lsuib [ numbers ] ] [ filesystem ]
DESCRIPTION
Check examines a file system, builds a bit map of used
blocks, and compares this bit map against the free list
maintained on the file system. It also reads directories
and compares the link-count in each i-node with the number
of directory entries by which it is referenced. If the file
system is not specified, a check of a default file system is
performed. The normal output of check includes a report of
The number of blocks missing; i.e. not in any file nor
in the free list,
The number of special files,
The total number of files,
The number of large files,
The number of directories,
The number of indirect blocks,
The number of blocks used in files,
The highest-numbered block appearing in a file,
The number of free blocks.
The -l flag causes check to produce as part of its output
report a list of the all the path names of files on the file
system. The list is in i-number order; the first name for
each file gives the i-number while subsequent names (i.e.
links) have the i-number suppressed. The entries ``.'' and
``..'' for directories are also suppressed. If the flag as
given as -ll, the listing will include the accessed and
modified times for each file. The -l option supersedes -s.
The -s flag causes check to ignore the actual free list and
reconstruct a new one by rewriting the super-block of the
file system. The file system should be dismounted while
this is done; if this is not possible (for example if the
root file system has to be salvaged) care should be taken
that the system is quiescent and that it is rebooted
immediately afterwards so that the old, bad in-core copy of
the super-block will not continue to be used. Notice also
that the words in the super-block which indicate the size of
the free list and of the i-list are believed. If the
super-block has been curdled these words will have to be
patched. The -s flag causes the normal output reports to be
suppressed.
With the -u flag, check examines the directory structure for
connectivity. A list of all i-node numbers that cannot be
reached from the root is printed. This is exactly the list
of i-nodes that should be cleared (see clri (VIII)) after a
series of incremental restores. (See the bugs section of
restor (VIII).) The -u option supersedes -s.
The occurrence of i n times in a flag argument -ii...i
causes check to store away the next n arguments which are
taken to be i-numbers. When any of these i-numbers is
encountered in a directory a diagnostic is produced, as
described below, which indicates among other things the
entry name.
Likewise, n appearances of b in a flag like -bb...b cause
the next n arguments to be taken as block numbers which are
remembered; whenever any of the named blocks turns up in a
file, a diagnostic is produced.
FILES
Currently, /dev/rp0 is the default file system.
SEE ALSO
fs (V), clri (VIII), restor(VIII)
DIAGNOSTICS
If a read error is encountered, the block number of the bad
block is printed and check exits. ``Bad freeblock'' means
that a block number outside the available space was
encountered in the free list. ``n dups in free'' means that
n blocks were found in the free list which duplicate blocks
either in some file or in the earlier part of the free list.
An important class of diagnostics is produced by a routine
which is called for each block which is encountered in an
i-node corresponding to an ordinary file or directory.
These have the form
b# complaint ; i= i# (class )
Here b# is the block number being considered; complaint is
the diagnostic itself. It may be
blk if the block number was mentioned as an argument
after -b;
bad if the block number has a value not inside the
allocatable space on the device, as indicated by the
devices's super-block;
dup if the block number has already been seen in a
file;
din if the block is a member of a directory, and if an
entry is found therein whose i-number is outside the
range of the i-list on the device, as indicated by
the i-list size specified by the super-block.
Unfortunately this diagnostic does not indicate the
offending entry name, but since the i-number of the
directory itself is given (see below) the problem
can be tracked down.
The i# in the form above is the i-number in which the named
block was found. The class is an indicator of what type of
block was involved in the difficulty:
sdir indicates that the block is a data block in a small
file;
ldir indicates that the block is a data block in a large
file (the indirect block number is not available);
idir indicates that the block is an indirect block
(pointing to data blocks) in a large file;
free indicates that the block was mentioned after -b and
is free;
urk indicates a malfunction in check.
When an i-number specified after -i is encountered while
reading a directory, a report in the form
# ino; i= d# (class ) name
where i# is the requested i-number. d# is the i-number of
the directory, class is the class of the directory block as
discussed above (virtually always ``sdir'') and name is the
entry name. This diagnostic gives enough information to
find a full path name for an i-number without using the -l
option: use -b n to find an entry name and the i-number of
the directory containing the reference to n, then
recursively use -b on the i-number of the directory to find
its name.
Another important class of file system diseases indicated by
check is files for which the number of directory entries
does not agree with the link-count field of the i-node. The
diagnostic is hard to interpret. It has the form
i# delta
Here i# is the i-number affected. Delta is an octal number
accumulated in a byte, and thus can have the value 0 through
377(8). The easiest way (short of rewriting the routine) of
explaining the significance of delta is to describe how it
is computed.
If the associated i-node is allocated (that is, has the
allocated bit on) add 100 to delta. If its link-count is
non-zero, add another 100 plus the link-count. Each time a
directory entry specifying the associated i-number is
encountered, subtract 1 from delta. At the end, the i-
number and delta are printed if delta is neither 0 nor 200.
The first case indicates that the i-node was unallocated and
no entries for it appear; the second that it was allocated
and that the link-count and the number of directory entries
agree.
Therefore (to explain the symptoms of the most common
difficulties) delta = 377 (-1 in 8-bit, 2's complement
octal) means that there is a directory entry for an
unallocated i-node. This is somewhat serious and the entry
should be be found and removed forthwith. Delta = 201
usually means that a normal, allocated i-node has no
directory entry. This difficulty is much less serious.
Whatever blocks there are in the file are unavailable, but
no further damage will occur if nothing is done. A clri
followed by a check -s will restore the lost space at
leisure.
In general, values of delta equal to or somewhat above 0,
100, or 200 are relatively innocuous; just below these
numbers there is danger of spreading infection.
BUGS
Check -l or -u on large file systems takes a great deal of
core.
Since check is inherently two-pass in nature, extraneous
diagnostics may be produced if applied to active file
systems.
It believes even preposterous super-blocks and consequently
can get core images.