System information

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.
O p erat e f irst o n a f ilesyst em imag e
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 f ilesyst em imag e f o r su p p o rt in vest ig at io n s
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.
O p erat e o n ly on u n mo u n t ed f ilesyst ems
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 erro rs
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
d d (8) utility.
12.2. Filesyst em-Specific Informat ion for fsck
12.2.1. ext 2, ext 3, and ext 4
All of these filesytems use the e2fsck binary to perform filesystem checks and repairs. The filenames
fsck. ext2, fsck. ext3, and fsck. ext4 are hardlinks to this same binary. These binaries are run
automatically at boot time and their behavior differs based on the filesystem being checked and the
state of the filesystem.
A full filesystem check and repair is invoked for ext2, which is not a metadata journaling filesystem,
and for ext4 filesystems without a journal.
For ext3 and ext4 filesystems with metadata journaling, the journal is replayed in userspace and the
binary exited. This is the default action as journal replay ensures a consistent filesystem after a
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