Fsck is a File System Consistency checKer. It checks Minixfs filesystems for consistency and optionally repairs them too

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Fsck is a File System Consistency checKer. It checks Minixfs filesystems for consistency and optionally repairs them too. It is important that a Minixfs filesystem is checked for errors when, for example, the system crashes or a lock up forces a reboot when programs are using the filesystems. This will occasionally mean that fsck will perform minor repairs which will cause no damage at all. Using a damaged filesystems can result in much more serious damage occuring at a later date which require more destructive repairs to fix. ADVICE ON USE OF FSCK fsck cannot perform miracles; a severely damaged filesystem (such as if a lot of sectors get wiped somehow) may have very little salvageable. However, don't be overwhelmed by the options available. In practice only fsck -n or fsck on its own are used, the -p option is useful in shell scripts where non-interactive repair is needed, but you dont want any attempted destructive repair. If fsck -n does produce what appear like a lot of serious errors then it might be an idea to attempt to backup the data before trying repairs; of course if you have a full backup (you do make regular backups don't you?) available it might be best to forget about repair altogether and just reinitialize the filesystem (with minit) and restore from backup. USING FSCK Fsck can be used in interactive or non-interactive mode. In interactive mode you are prompted before each fix. In non-interactive mode, repairs are carried out automatically or not at all. The option '-y' carries out all repairs that fsck would suggest, the '-p' (preen) option carries out all repairs if the Minixfs filesystems is only slighlty damaged and can be repaired without destroying data, the '-n' option only prints out the damage: it does not repair anything. The '-y' option is not recommended, unless you are sure it is what you want. The options are followed by the drive letter of the partition you want to check for example, fsck -p D: REPAIRS CARRIED OUT In order to know precisely what repairs are carried out you must know a litle about the internal structure of a Minixfs filesystem. Each filesystem is divided into blocks or zones, these are always 1K in size. Each directory/file/symbolic link has an inode associated with it. This is a small part of a disk sector which carries all information about a file except its name. It contains the file access modes (including what kind of entity the inode refers to) as well as it's size and a list of sectors where the file can be found. If this list is not large enough then an 'indirection block' which is a disk block containing a list of further blocks belonging to the file is used (and recorded in the inode). If this is not enough then a 'double indirection block' is used containing a list of further indirection blocks. With this information, all the data in a file can be accessed. A directory is basically identical to a file except it has a different mode number. The 'data' in this file is a list of 'entries' (or links) each entry contains a filename and an inode number where that name can be found. The root directory is always contained in inode 1. A field in the inode called it's 'link count' gives a count of the number directory entries that refer to it. Every directory has two filenames '.' which refers to itself and '..' which refers to the parent directory. The only exception is the root directory where '..' refers to itself as well. Each used inode/block is referenced in a bitmap. If the filesystem crashes before this can be written out to disk then it will be inaccurate. Fortunately fsck can rebuild the bitmaps from the data contained in inodes. This is what the prompts asking about bitmap repair mean. They are harmless and it is strongly advised that any bitmap repairs suggested are made. If you use the filesystem and some used blocks do not have their bits set in the bitmap then other files can use them, overwriting the original data. These are called 'multiply allocated blocks'. fsck will give a list of these blocks and (optionally) allow you to decide which inode is the valid block. Usually the valid data is contained in the file with the latest modify time: the '-y' option removes all of the references. By finding the files the inodes refer to (e.g. with the -i option) you can look to see which file contains the valid data and then interactively remove the other references. The other files where the references are removed then have 'holes' in them, you may want to try to recover them or just delete them after 'fsck' has done its work. This is why it is important that the bitmaps are accurate and should be checked regularly. This is the 'best case' scenario of multiply allocated blocks. If the block overwritten belongs to a directory then much more trouble is caused. After deleting the overwritten reference in a directory lots of inodes may no longer be referenced in a directory, these are called 'orphaned' inodes (they have no parent directory). fsck has no way of knowing their original names, but the data is recoverable. fsck makes entries for these inodes in a directory called lost+found . The name chosen is simply the inode number. You can then analyse the files/directories in lost+found and delete, rename or move them to where they were originally. The worst case is if an indirection block is overwritten. This can cause lots of spurious messages about bad zone numbers and other multiply allocated blocks. fsck cannot currently help much under such circumstances: if a certain inode contains lots of multiple block number or bad zone numbers then it's best to delete all multiply allocated blocks in that inode; either with fsck or finding the inode on the disk and deleting the associated file. Here is a summary of the possible repairs fsck performs: 1. Scan all inodes, allow deletion of multiply allocated blocks, inodes with bad modes, truncation of inodes which reference too many zones. 2. Scan all directories, check for valid names and the existance of '.' and '..' entries and allow fixing. 3. Check filesystem conectivity (that each inode has a directory entry) and allow repair of any problems and reconnection of orphaned inodes. 4. Check bitmaps and repair; check inode link counts and allow repair. MISCELLANEOUS OPTIONS If the root directory gets destroyed it must be handled separately because the lost+found directory will be killed also. If this is the case then the '-R' option will reform the root directory. Unfortunately the directory increment is determined from the root directory, so you must manually tell fsck the directory increment with the '-d' option followed by the increment. If you get this wrong then a large number of errors about directory entries will be erroneously reported. If for some reason the root inode is no longer a directory Minixfs may get confused and not recognise the filesystem anymore. By asking fsck to continue it will still check the root inode as though it were a directory: if it looks OK fsck allows you to make it a directory again. However you will have to reboot before the filesystem will be recognised by Minixfs again. The '-i' option followed by a comma separated list of numbers will print out the pathnames of the corresponding inodes. E.g. fsck -i 1,2,3 D: This will print out *all* the possible names so e.g. fsck -i 1 d: may give something like: \. \.. \dir\.. \dir2\.. etc. The '-z' option allows an inode to be zeroed if for example it is severely trashed and fixing would do more harm than good (e.g. if an indirection block has been overwritten). -z may be followed by a comma separated list of inodes to zero. NB use this option with extreme caution, the data the inode refers to will be effectively deleted (it will still be on the disk but no longer accessible directly). Using this option will also cause related "errors" to appear (though all can be fixed) such as inode and bitmap errors and links a free inode. The '-e' option tries to find out the filesystem size by reading as far as it can through a partition: this is an experimental option and isn't too useful at present.

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