> Surely ".." is of some value (perhaps not now, but maybe in the
> future) to e2fsck is the disk gets corrupted?
Not really. You can use it as an extra sanity-check (and as far as I know
fsck does), but that's true of just about any redundant information - that
doesn't necessarily make it actually useful. It's less so if you journal
your metadata anyway.
So yes, you can use ".." to check that both the parent and the child agree
about each other, but even if they don't there's not all that much you can
do with the information.
Actually, there is something that can be done with the information; if
part of the parent's directory blocks have been corrupted, the child
directory will be disconnected from the directory tree. The '..' link
will tell you where in the directory tree the child should be
reconnected, although not what the name of that child should be.
This information shows up in the fsck transcript, and can be used as
a hint by the system administrator about how to restore the disconnected
directory from the lost+found directory to the correct place in the
directory tree.
I've considered automating this to make it a little easier for system
administrators to actually fix things. (For example: reconnecting the
directory to the parent directory if possible, and putting a symbolic
link in lost+found so the administrator knows where to find the child
directory so he/she can rename it to the correct name.)
Even better would be to store the name of the child directory in the
child's directory block; this additional amount of redundant information
would allow for a completely automated recovery procedure in many
cases, which would be a big win.
- Ted
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