Chapter 11. File System Check

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

Note

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

11.1. Best Practices for fsck

Generally, running the filesystem 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 filesystem may occur. To ensure that unexpected or undesirable changes are not permanently made, perform the following precautionary steps:
Dry run
Most filesystem checkers have a mode of operation which checks but does not repair the filesystem. In this mode, the checker will print any errors that it finds and actions that it would have taken, without actually modifying the filesystem.

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 filesystem image
Most filesystems support the creation of a metadata image, a sparse copy of the filesystem which contains only metadata. Because filesystem checkers operate only on metadata, such an image can be used to perform a dry run of an actual filesystem repair, to evaluate what changes would actually be made. If the changes are acceptable, the repair can then be performed on the filesystem itself.

Note

Severely damaged filesystems may cause problems with metadata image creation.
Save a filesystem image for support investigations
A pre-repair filesystem 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 filesystems
A filesystem repair must be run only on unmounted filesystems. The tool must have sole access to the filesystem or further damage may result. Most filesystem tools enforce this requirement in repair mode, although some only support check-only mode on a mounted filesystem. If check-only mode is run on a mounted filesystem, it may find spurious errors that would not be found when run on an unmounted filesystem.
Disk errors
Filesystem check tools cannot repair hardware problems. A filesystem must be fully readable and writable if repair is to operate successfully. If a filesystem was corrupted due to a hardware error, the filesystem must first be moved to a good disk, for example with the dd(8) utility.