ugh.book

UFS: The Root of All Evil 273

Only the Most Perfect Disk Pack Need Apply

One common problem with Unix is perfection: while offering none of its

own, the operating system demands perfection from the hardware upon

which it runs. That’s because Unix programs usually don’t check for hard-

ware errors—they just blindly stumble along when things begin to fail,

until they trip and panic. (Few people see this behavior nowadays, though,

becuase most SCSI hard disks do know how to detect and map out blocks

as the blocks begin to fail.)

The dictionary defines panic as “a sudden overpowering fright; especially a

sudden unreasoning terror often accompanied by mass flight.” That’s a

pretty good description of a Unix panic: the computer prints the word

“panic” on the system console and halts, trashing your file system in the

process. We’ve put a list of some of the more informative(?) ones in

Figure 4.

The requirement for a perfect disk pack is most plainly evident in the last

two of these panic messages. In both of these cases, UFS reads a block of

data from the disk, performs an operation on it (such as decreasing a num-

Message Meaning

panic: fsfull The file system is full (a write failed), but
Unix doesn’t know why.

panic: fssleep fssleep() was called for no apparent rea-
son.

panic: alloccgblk: cyl groups
corrupted

Unix couldn’t determine the requested

disk cylinder from the block number.

panic: DIRBLKSIZ > fsize A directory file is smaller than the mini-
mum directory size, or something like
that.

dev = 0xXX, block = NN, fs = ufs
panic: free_block: freeing free
block

Unix tried to free a block that was already

on the free list. (You would be surprised

how often this happens. Then again,

maybe you wouldn’t.)

panic: direnter: target
directory link count

Unix accidentally lowered the link count

on a directory to zero or a negative num-

ber.

FIGURE 4. Unix File System Error Messages.