Re: Lost file-system story

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> [...], you do indeed seem to think that async-mounted Unix-based
> filesystems should be able to be repaired, at least some of the time,

There's a huge difference between "this isn't promsied" and "this never
happens".

They _can_ be repaired...some of the time.  When they can, it is
because, by coincidence, it just so happens that the stuff that got
written produces a filesystem fsck can repair.

> The probablility of any Unix-based filesystem being repariable after
> a crash is zero (0) if it has been mounted with MNT_ASYNC, and if
> there was _any_ activity that affected its structure since mount time
> up to the time of the crash.

This is simply false.  I just tried it.  On a 5.1 i386 system, I used
fdisk and disklabel to make a half-gig partition, newfsed it, mounted
it normally, copied a file into it, unmounted it, mounted it async,
removed the file, and hit the power switch.  After the machine came
back up, I tried fsck on the filesystem.  It said it was clean.  I used
fsck -f.  It was happy.  I mounted it and, as far as I can tell, fsck
was correct in thinking the filesystem was OK.  So, there is an
existence-proof-by-example that there are circumstances under which a
filesystem mounted async can be changed and still be left in a state
fsck can repair.

> It still might survive after some types of changes, but it _probably_
> won't.

Right.  But that's not "probability ... is zero (0)".

> Linux ext2 is not a Unix-based filesystem and Linux itself is not a
> Unix-based kernel.

It's about as Unix-based as NetBSD is.  Unless you mean something
strange by "Unix-based" - what _do_ you mean by it?

> For Unix-based filesystems and their repair tools, any probablility
> of recovery less than one is as good as if it were zero.

That's not how I feel about it when I've lost a filesystem.  I'll take
a filesystem with a nonzero probability of recovering something useful
from over one that guarantees to trash everything any day (other things
being equal, of course).

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