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code segment = base 0×0, limit 0xfffff, type 0×1b = DPL 0, pres 1, def32 1, gran 1 processor eflags = interrupt enabled, resume, IOPL=0 current process = 5 (syncer) interrupt mask = bio trap number = 12 panic: page fault ………………………………………………………………………………………. If you’re an inexperienced sysadmin, messages like this can turn your blood cold, but don’t fret yet. FreeBSD sometimes gives somewhat friendly messages that describe what’s wrong, which give you a specific place to start looking, or at least a term to Google. I’ve seen panics that give very specific instructions on kernel options that should be set to prevent their recurrence. Other panic messages, like this one, are much more puzzling. The only word that looks even vaguely familiar in this panic message is the fourth line from the bottom, where we see that the current process is something called “syncer”. Most people don’t know what the syncer is, and most of those who recognize it know better than to try to fix it. The mysterious panic is among the worst situations you can have in FreeBSD. Responding to a Panic If you get a system panic, the first thing to do is get a copy of the panic message. Since FreeBSD is no longer running at this point, the standard UNIX commands will not work the system won’t let you SSH in or out, and even simple commands like script(1) will not work. The console might be utterly locked up, or it could be in a debugger. In either event, you need the error message. The first time I received an error message like the preceding one, I scrambled for paper and pen. Eventually I found an old envelope and a broken stub of pencil, and crawled between the server rack and the rough brick wall. I balanced the six-inch black-and-white monitor that I’d dragged back there in one hand, while with my other hand I held the old envelope against the wall. Apparently I had a third hand to copy the panic message to the envelope, because it somehow got there. Finally, scraped and cramped, I slithered back out of the rack and victoriously typed the whole mess into an email. Surely the crack FreeBSD developers would be able to look at this garbage and tell me exactly what had happened. After all of this struggle, the initial response was quite frustrating: “Can you send a backtrace?” I’ve seen many, many messages to a FreeBSD mailing list reporting problems like this, and they always get this same response. Most of the people who send these messages are never heard from again, and I understand exactly how they feel. When you’ve been dealing with a server that crashes, or (worse) keeps crashing, the last thing you want to do is reconfigure it. The problem with the panic message on my envelope was that it only gave a tiny scrap of the story. It was so vague, in fact, that it was like describing a stolen car as “red, with a scratch on the fender.”If you don’t give the car’s make, model, and VIN number or license plate, you cannot expect the police to make much headway. Similarly, without more information from your crashing kernel, the FreeBSD developers can’t catch the criminal code. There’s a simple way around this problem, however: Set up your server to handle a panic before the 453

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