1CRASH(8) System Manager's Manual CRASH(8)
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6 crash - what to do when the system crashes
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9 This section gives at least a few clues about how to proceed if the
10 system crashes. It can't pretend to be complete.
11 Bringing it back up. If the reason for the crash is not evident (see
13 below for guidance on `evident') you may want to try to dump the system
14 if you feel up to debugging. At the moment a dump can be taken only on
15 magtape. With a tape mounted and ready, stop the machine, load address
16 44, and start. This should write a copy of all of core on the tape
17 with an EOF mark. Caution: Any error is taken to mean the end of core
18 has been reached. This means that you must be sure the ring is in, the
19 tape is ready, and the tape is clean and new. If the dump fails, you
20 can try again, but some of the registers will be lost. See below for
21 what to do with the tape.
22 In restarting after a crash, always bring up the system single-user.
24 This is accomplished by following the directions in boot(8) as modified
25 for your particular installation; a single-user system is indicated by
26 having a particular value in the switches (173030 unless you've changed
27 init) as the system starts executing. When it is running, perform a
28 dcheck and icheck(1) on all file systems which could have been in use
29 at the time of the crash. If any serious file system problems are
30 found, they should be repaired. When you are satisfied with the health
31 of your disks, check and set the date if necessary, then come up multi-
32 user. This is most easily accomplished by changing the single-user
33 value in the switches to something else, then logging out by typing an
34 EOT.
35 To even boot UNIX at all, three files (and the directories leading to
37 them) must be intact. First, the initialization program /etc/init must
38 be present and executable. If it is not, the CPU will loop in user
39 mode at location 6. For init to work correctly, /dev/tty8 and /bin/sh
40 must be present. If either does not exist, the symptom is best
41 described as thrashing. Init will go into a fork/exec loop trying to
42 create a Shell with proper standard input and output.
43 If you cannot get the system to boot, a runnable system must be
45 obtained from a backup medium. The root file system may then be doc‐
46 tored as a mounted file system as described below. If there are any
47 problems with the root file system, it is probably prudent to go to a
48 backup system to avoid working on a mounted file system.
49 Repairing disks. The first rule to keep in mind is that an addled disk
51 should be treated gently; it shouldn't be mounted unless necessary, and
52 if it is very valuable yet in quite bad shape, perhaps it should be
53 dumped before trying surgery on it. This is