[Bug 208691] "panic: ffs_valloc: dup alloc" as soon as UFS root partition is mounted

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=208691

--- Comment #3 from Kirk McKusick <mckusick at FreeBSD.org> ---
To your question of recovering when you have a corrupted root filesystem, that
is not a problem because the kernel always comes up with the root filesystem
read-only. The panics that you can get will only happen when attempting to
write the filesystem. So if you are single-user, you will have the opportunity
to fix the filesystem while it is mounted read-only. It only switches to
read-write as it exits single-user and goes to multi-user.

The long-term fix is to note in the superblock when the filesystem has panic'ed
and to force a full fsck when finding this flag set. This is part of a bigger
project that reduces the number of panics in the filesystem that should be
coming in over the next year.

As a general rule, you should run with write cache disabled if you want full
recoverability after a crash.

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