Chapter 12. File System Check

File systems may be checked for consistency, and optionally repaired, with file system-specific userspace tools. These tools are often referred to as fsck tools, where fsck is a shortened version of file system check.

Note

These file system checkers only guarantee metadata consistency across the file system; they have no awareness of the actual data contained within the file system and are not data recovery tools.
File system inconsistencies can occur for various reasons, including but not limited to hardware errors, storage administration errors, and software bugs.
Before modern metadata-journaling file systems became common, a file system check was required any time a system crashed or lost power. This was because a file system update could have been interrupted, leading to an inconsistent state. As a result, a file system check is traditionally run on each file system listed in /etc/fstab at boot-time. For journaling file systems, this is usually a very short operation, because the file system's metadata journaling ensures consistency even after a crash.
However, there are times when a file system inconsistency or corruption may occur, even for journaling file systems. When this happens, the file system checker must be used to repair the file system. The following provides best practices and other useful information when performing this procedure.

Important

Red Hat does not recommended this unless the machine does not boot, the file system is extremely large, or the file system is on remote storage. It is possible to disable file system check at boot by setting the sixth field in /etc/fstab to 0.

12.1. Best Practices for fsck

Generally, running the file system check and repair tool can be expected to automatically repair at least some of the inconsistencies it finds. In some cases, severely damaged inodes or directories may be discarded if they cannot be repaired. Significant changes to the file system may occur. To ensure that unexpected or undesirable changes are not permanently made, perform the following precautionary steps:
Dry run
Most file system checkers have a mode of operation which checks but does not repair the file system. In this mode, the checker prints any errors that it finds and actions that it would have taken, without actually modifying the file system.

Note

Later phases of consistency checking may print extra errors as it discovers inconsistencies which would have been fixed in early phases if it were running in repair mode.
Operate first on a file system image
Most file systems support the creation of a metadata image, a sparse copy of the file system which contains only metadata. Because file system checkers operate only on metadata, such an image can be used to perform a dry run of an actual file system repair, to evaluate what changes would actually be made. If the changes are acceptable, the repair can then be performed on the file system itself.

Note

Severely damaged file systems may cause problems with metadata image creation.
Save a file system image for support investigations
A pre-repair file system metadata image can often be useful for support investigations if there is a possibility that the corruption was due to a software bug. Patterns of corruption present in the pre-repair image may aid in root-cause analysis.
Operate only on unmounted file systems
A file system repair must be run only on unmounted file systems. The tool must have sole access to the file system or further damage may result. Most file system tools enforce this requirement in repair mode, although some only support check-only mode on a mounted file system. If check-only mode is run on a mounted file system, it may find spurious errors that would not be found when run on an unmounted file system.
Disk errors
File system check tools cannot repair hardware problems. A file system must be fully readable and writable if repair is to operate successfully. If a file system was corrupted due to a hardware error, the file system must first be moved to a good disk, for example with the dd(8) utility.