The badsect utility makes a file to contain a bad sector. Normally, bad sectors are made inaccessible by the standard formatter, which provides a forwarding table for bad sectors to the driver. If a driver supports the bad blocking standard it is much preferable to use that method to isolate bad blocks, since the bad block forwarding makes the pack appear perfect, and such packs can then be copied with dd(1). The technique used by this program is also less general than bad block forwarding, as badsect cannot make amends for bad blocks in the i-list of file systems or in swap areas.
On some disks, adding a sector which is suddenly bad to the bad sector table currently requires the running of the standard DEC formatter. Thus to deal with a newly bad block or on disks where the drivers do not support the bad-blocking standard badsect may be used to good effect.
The badsect utility is used on a quiet file system in the following way: First mount the file system, and change to its root directory. Make a directory BAD there. Run badsect giving as argument the BAD directory followed by all the bad sectors you wish to add. (The sector numbers must be relative to the beginning of the file system, but this is not hard as the system reports relative sector numbers in its console error messages.) Then change back to the root directory, unmount the file system and run fsck(8) on the file system. The bad sectors should show up in two files or in the bad sector files and the free list. Have fsck(8) remove files containing the offending bad sectors, but do not have it remove the BAD/ nnnnn files. This will leave the bad sectors in only the BAD files.
The badsect utility works by giving the specified sector numbers in a mknod(2) system call, creating an illegal file whose first block address is the block containing bad sector and whose name is the bad sector number. When it is discovered by fsck(8) it will ask ""HOLD BAD BLOCK ?"". A positive response will cause fsck(8) to convert the inode to a regular file containing the bad block.
The badsect utility refuses to attach a block that resides in a critical area or is out of range of the file system. A warning is issued if the block is already in use.