FSCK_LFS(8) System Manager's Manual FSCK_LFS(8)

fsck_lfs — Log-structured File System consistency check and interactive repair

fsck_lfs [-adfpqU] [-b block] [-i idaddr] [-m mode] [-y | -n] filesystem ...

fsck_lfs performs interactive file system consistency checks and repair for each of the file systems specified on the command line. It is normally invoked from fsck(8).

The design of LFS takes care that no file system inconsistencies can happen unless hardware or software failures intervene. fsck_lfs will report and optionally correct any such inconsistencies.

For each corrected inconsistency one or more lines will be printed identifying the file system on which the correction will take place, and the nature of the correction. After successfully correcting a file system, fsck_lfs will print the number of files on that file system, the number of used and free blocks, and the percentage of fragmentation.

If sent a QUIT signal, fsck_lfs will finish the file system checks, then exit with an abnormal return status.

Without the -p option, fsck_lfs audits and interactively repairs inconsistent conditions for file systems. If the file system is inconsistent, the operator is prompted for concurrence before each correction is attempted. It should be noted that some of the corrective actions will result in some loss of data. The amount and severity of data lost may be determined from the diagnostic output. The default action for each consistency correction is to wait for the operator to respond yes or no. If the operator does not have write permission on the file system fsck_lfs will default to a -n action.

The following flags are interpreted by fsck_lfs:

Do not report discrepancies in avail accounting.
block
Use block as the super block for the file system.
Print debugging output.
Force checking of file systems. Normally, if a file system is cleanly unmounted, the kernel will set a “clean flag” in the file system superblock, and fsck_lfs will not check the file system. This option forces fsck_lfs to check the file system, regardless of the state of the clean flag.
idaddr
Use the given disk address to locate the index file inode, rather than looking in the superblock.
mode
Use mode specified in octal as the permission bits to use when creating the lost+found directory rather than the default 1700. In particular, systems that do not wish to have lost files accessible by all users on the system should use a more restrictive set of permissions such as 700.
Assume a no response to all questions asked by fsck_lfs except for ‘CONTINUE?’, which is assumed to be affirmative; do not open the file system for writing.
Specify “preen” mode. Currently, in this mode fsck_lfs does nothing.
Quiet mode, do not output any messages for clean file systems.
Resolve user ids to user names.
Assume a yes response to all questions asked by fsck_lfs; this should be used with great caution as this is a free license to continue after essentially unlimited trouble has been encountered.

Inconsistencies checked are as follows:

  1. Blocks claimed by more than one inode.
  2. Blocks claimed by an inode outside the range of the file system.
  3. Incorrect link counts.
  4. Size checks:
    • Directory size not a multiple of DIRBLKSIZ.
    • Partially truncated file.
  5. Bad inode format.
  6. Directory checks:
    • File pointing to unallocated inode.
    • Inode number out of range.
    • Dot or dot-dot not the first two entries of a directory or having the wrong inode number.
  7. Super Block checks:
    • More blocks for inodes than there are in the file system.
  8. Index File checks:
    • “In use” inodes on free list, or free inodes not on free list.
    • Segment block counts incorrect, or “clean” segments containing live data.

Orphaned files and directories (allocated but unreferenced) are, with the operator's concurrence, reconnected by placing them in the lost+found directory. The name assigned is the inode number. If the lost+found directory does not exist, it is created. If there is insufficient space its size is increased.

Because of inconsistencies between the block device and the buffer cache, the raw device should always be used.

The diagnostics produced by fsck_lfs are fully enumerated and explained in Appendix A of Fsck - The UNIX File System Check Program.

fstab(5), fsck(8), newfs_lfs(8), reboot(8)

The fsck_lfs program was first made available in NetBSD 1.4.

Most of the fsck_lfs program was taken from fsck_ffs(8); what was not was written by Konrad Schroder <perseant@NetBSD.org>.

September 14, 2025 NetBSD 11.99