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Re: boot problem, disk mess.. thinking of suicide.

From: Oliver Fromme <check+j6auf200rs4zrznn@xxxxxxxxxx>
Date: 28 Sep 2006 11:41:59 GMT

Vladimir Mitiouchev wrote:
 > Oliver Fromme wrote:
 > > And if you did, you should newfs(8) the file system when
 > > you replaced the cables with good ones.
 > Q: Why fsck is not enough?

fsck(8) is only able to detect certain (not even all, I
believe) inconsistencies in the meta data of a file system.
And if you're lucky, it can repair it without losing data.

However, fsck can neither detect nor repair any damage in
the contents of files.

Obviously, the bad IDE cable that you had caused some damage
to the meta data of your file systems.  It seems like some
write accesses succeeded, while others did not, and maybe
some write operations even wrote wrong data to the disk.
Given the average ratio of meta-data to file-data, chances
are that file contents also suffered from the accident.

In other words, it is possible that you have garbage in
some files, without you knowing.  It could be in obvious
places, such as kernel sources ...  In that case you will
get compilation errors next time you try to compile a
kernel.  Unless, of course, the garbage happens to be
valid C code, which is entirely possible, for example if
only a few bits toggled in a numeric constant.  Then you
won't notice, but your new kernel might exhibit subtle
misbehaviour, and it'll be _very_ tough to find out what's
going on.  I suspect that quite a number of FreeBSD PRs
are just caused by toggled bits due to bad hardware,
wasting significant amounts of developer resources for
hunting the "bug".

I could go on with many more examples, but you probably
got the idea.  That's the reason why you should use newfs
and restore from backup when an unknown amount of damage
happened to a file system due to hardware failure.

(I also mentioned that a RAID1 setup could prevent such
damage.  To detect damage in files, you can also use an
intrusion-detection tool like tripwire, mtree or similar.)

Best regards

Oliver Fromme,  secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
Dienstleistungen mit Schwerpunkt FreeBSD: http://www.secnetix.de/bsd

Any opinions expressed in this message may be personal to the author
and may not necessarily reflect the opinions of secnetix in any way.

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