Date: Thu, 06 Jul 2000 22:56:31 +1000
From: Andrew Morton <andrewm@uow.edu.au>
>
> 2) Even with journaling filesystems, there will be cases you will need
> to run some kind of filesystem consistency checker.
Can this be done while the fs is online?
Multics operating system was able to run its filesystem recovery tool
while the filesystem was online. Then again, Multics was also designed
so that if a circuit breaker snapped off and one of its three memory
cabinets got uncleanly shutdown, only processes that had memory pages on
the downed memory subsystem would get killed. The thinking was: just
because you lost 1/3 of your memory and you have to kill off 22 user's
processes, why should you have to ruin the the other 45 users's day?
:-)
In practice, though, I'm not aware of any filesystem consistency checker
since the days of Multics that could do this. It's possible, but you
have to put all sorts of very careful interlocking between the checking
code and the filesystem code, and this adds a *lot* of complexity. In
the case of multics, the filesystem consistency checker was actually
part of the kernel (it ran in Ring 0), and this tends to go against the
general Unix and Linux design principles of keeping as much as possible
in userspace.
- Ted
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