Chapter 12. File System Check
fsckis a shortened version of file system check.
/etc/fstabat boot-time. For journaling filesystems, this is usually a very short operation, because the filesystem's metadata journaling ensures consistency even after a crash.
12.1. Best Practices for fsck
- 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.
NoteLater 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.
NoteSeverely 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