dos filesystem components checked by fsck

The following two components are checked:

• File Allocation Table (FAT)

• directory entries

dosfs-fsck phase 1: check FATs

Every dos filesystem maintains two or more identical copies of the File Allocation Table (FAT). During Phase 1, fsck performs two types of checks:

• The FATs are checked first for readability. At least one FAT must be readable for fsck to proceed.

• All readable FATs are checked to see if they are in sync. If FATs are in sync, Phase 2 occurs.

dosfs-fsck phase 2: check directory entries (pass 1)

During Phase 2, fsck performs the following checking:

• If the entry is for ``..'', check that the start cluster matches the one for the parent directory.

• If the entry is for ``.'', check that the start cluster matches the one for the current directory.

• If the entry is for a directory, check that file size is zero.

• If the entry is for a file, check that the file size is set to the correct byte count.

• For each directory entry check that:

  • its start cluster number is within range

  • all its FAT entries are within range

  • If the entry is for a long name, then check the sequence number. Also check for the presence of an associated alias name entry.

• number of subdirectories

• number of regular files

• number of hidden files

• byte count per subdirectories

• byte count per regular files

• byte count per hidden files

• for each cluster in the filesystem, number of times it is being claimed. This information is used during cross referencing checking.

dosfs-fsck phase 3: cross reference check

During Phase 3, fsck will perform the following checking:

• allocation units (clusters) claimed by more than one directory entry

• lost clusters (clusters not claimed by any directory entry but still marked FREE in the FAT)

dosfs-fsck phase 4: check directory entries (pass 2)

Phase 4 is the same as Phase 2. It is only needed and performed by fsck if there was a corrective action performed during cross-referencing checking that had invalidated the data collected during Phase 2.

dosfs-fsck phase 4: check boot area (FAT 32 only)

• Check location of backup boot sector.

• Check location and contents of FS info sector.