How are alternate superblocks repaired?

Thomas Watt tango at
Wed Oct 3 17:16:18 UTC 2007

Hi Ted,

That was pretty funny being "protected from preemption"!

It turns out I did discover a bug in my script that I previously sent, and have
fixed it.  Only filesystem blocksize of 2048 needs testing/verification.

Sorry for the resend - it appears my mailer decided I needed to loosen the
priviledges to send the script.

Here is the reworked script attached:


-- Tom

-----Original Message-----
>From: Theodore Tso <tytso at>
>Sent: Oct 2, 2007 5:59 PM
>To: Thomas Watt <tango at>
>Cc: Andreas Dilger <adilger at>, ext3-users at
>Subject: Re: How are alternate superblocks repaired?
>On Tue, Oct 02, 2007 at 03:38:47PM -0400, Thomas Watt wrote:
>> In case you are interested, here is link to a web page on Structure Marking:
>I actually have used a Multics system way back when (I was actually
>logged into MIT Multics when it was finally shutdown[1]).  The com_err
>library and the ss library in e2fsprogs was largely inspired from
>Multics, and I do use structure magic numbers in memory to protect
>against programming errors, which is basically a very simple structure
>marking technique.
>I'm a bit dubious about how useful simply structure matching would be
>for modern Linux systems, since a large number of errors really are
>silent bit flips in the data, that wouldn't be detected simply by
>checking the expected structure ID at the beginning of the on-disk
>object.  We are planning on adding checksum to metadata for ext4,
>which will help a lot in terms of detected bad metadata.
>Regards,   ("You are protected from preemption"  :-)
>					- Ted
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