V4/man/man8/check.8

.th CHECK VIII 8/31/73
.sh NAME
check \*- file system consistency check
.sh SYNOPSIS
.bd check
[
.bd \*-lsib
[ numbers ]
]
[ filesystem ]
.sh DESCRIPTION
.it 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
.it check
includes a report of
.s3
.lp +4 0
The number of blocks missing; i.e. not in any file
nor in the free list,
.lp +4 0
The number of special files,
.lp +4 0
The total number of files,
.lp +4 0
The number of large files,
.lp +4 0
The number of directories,
.lp +4 0
The number of indirect blocks,
.lp +4 0
The number of blocks used in files,
.lp +4 0
The highest-numbered block appearing in a file,
.lp +4 0
The number of free blocks.
.s3
.i0
The
.bd \*-l
flag causes
.it 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 ``\fB.\fR'' and ``\fB..\fR''
for directories are also suppressed.
.s3
The
.bd \*-s
flag causes
.it 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
.bd \*-s
flag
causes the normal output reports to be suppressed.
.s3
The occurrence
of
.bd i
.it n
times in a flag argument
.bd \*-ii...i
causes
.it check
to store away the next
.it 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.
.s3
Likewise,
.it n
appearances of
.bd b
in a flag like
.bd \*-bb...b
cause the next
.it 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.
.sh FILES
Currently, /dev/rp0 is the default file system.
.sh "SEE ALSO"
fs (V)
.sh DIAGNOSTICS
There are some self-evident diagnostics like
``can't open ...'', ``can't write ....''
If a read error is encountered,
the block number of the bad block is printed and
.it check
exits.
``Bad freeblock'' means that
a block number outside the available space was encountered in the free list.
``\fIn\fR dups in free''
means that \fIn\fR blocks were found in the free list which
duplicate blocks either in some file or in the earlier part of the free list.
.s3
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
.s3
.dt
	\fIb# complaint \fB; i= \fIi# \fB(\fIclass \fB)\fR
.s3
Here
.it b#
is the block number being considered;
.it complaint
is the diagnostic itself.
It may be
.s3
.lp +8 5
\fBblk\fR	if the block number was mentioned as an argument
after
.bd \*-b;
.lp +8 5
\fBbad\fR	if the block number has a value not inside the allocatable space
on the device, as indicated by the devices's super-block;
.lp +8 5
\fBdup\fR	if the block number has already been seen in a file;
.lp +8 5
\fBdin\fR	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.
.s3
.i0
The
.it i#
in the form above is the i-number in which the named block was found.
The
.it class
is an indicator of what type of block was involved in the
difficulty:
.s3
.lp +8 5
\fBsdir\fR	indicates that the block is a data block in a small file;
.lp +8 5
\fBldir\fR	indicates that the block is a data block in a large file
(the indirect block number is not available);
.lp +8 5
\fBidir\fR	indicates that the block is an indirect block
(pointing to data blocks) in a large file;
.lp +8 5
\fBfree\fR	indicates that the block was mentioned after
.bd \*-b
and is free;
.lp +8 5
\fBurk\fR	indicates a malfunction in
.it check.
.s3
.i0
When an i-number specified after
.bd \*-i
is encountered while reading a directory,
a report in the form
.s3
	\fi# \fBino; i= \fId# \fB(\fIclass \fB) \fIname\fR
.s3
where
.it i#
is the requested i-number.
.it d#
is the i-number of the directory,
.it class
is the class of the directory block as discussed above
(virtually always
``sdir'')
and
.it name
is the entry name.
This diagnostic gives enough information
to find a full path name for an i-number
without using the
.bd -l
option:
use
.bd \*-b
.it n
to find an entry name and
the i-number of the directory containing the reference to
.it n,
then recursively use
.bd \*-b
on the i-number of the directory to find its name.
.s3
Another important class of
file system diseases indicated
by
.it 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
.s3
.dt
.ft I
	i#  delta
.ft R
.s3
Here
.it i#
is the i-number affected.
.it 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
.it delta
is to describe how it is computed.
.s3
If the associated i-node is allocated
(that is, has the
.it allocated
bit on)
add 100 to
.it 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
.it delta.
At the end,
the i-number and
.it delta
are printed if
.it 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.
.s3
Therefore (to explain the symptoms of the most common difficulties)
.it 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.
.it 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
.it clri
followed by a
.it "check \*-s"
will restore the lost space at leisure.
.s3
In general,
values of
.it delta
equal to or somewhat above 0, 100, or 200
are relatively innocuous;
just below these numbers there is danger of
spreading infection.
.sh BUGS
Unfortunately,
.it "check \*-l"
on file systems
with more than 3000 or so files
does not work because it runs out of core.
.s3
Since
.it check
is inherently two-pass in nature, extraneous diagnostics
may be produced if applied to active file systems.
.s3
It believes even preposterous super-blocks and
consequently can get core images.