[linux-lvm] Re: How to handle Bad Block relocation with LVM?

[Eric Hopper <hopper@omnifarious.org>]

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I manually relocated a few bad blocks on a bad IBM drive I had when I
replaced the drive.  It took a lot of time and effort.  I had to run the
dd command many times very carefully to make it work.

One big problem for me was that read-ahead obscured which actual sectors
were in error.  I needed a 'raw' LVM device, but I don't think such a
thing exists for LVM1 on Linux 2.4.x.

What I did was used pvmove to move the PE containing the bad block to a
different spot on the hard drive, then allocated a new LV that was one
LE long, and forced it to allocate the PE containing the bad block. 
Then I used dd to carefully copy over the LE in sections, narrowing down
the location of the bad sectors until I had copied everything that could
possibly be read.

After that, I ran fsck on the filesystem that had originally contained
the bad block, and I was fine.  I checked carefully, and it didn't even
seem that I had lost any data.

Long, time consuming process though.

Actually, it may have been even ickier than I first thought.

It could be that pvmove wouldn't work, and I had to shorten the LV
containing the bad block (the BLV) to contain all PEs prior to the bad
one, allocate a new LV (the NLV) containing all the bad PE, lengthen the
BLV by 1 PE, using a brand new PE, then lengthen it to its original
length so it would contain all the PEs after that bad PE, the do the
procedure I outlined above.

Now that I think of it, I'm nearly positive that pvmove didn't work.  I
had dearly wished for some kind of option to pvmove that would force it
to try as hard as it could to get good reads of all the sectors in a PE,
then move the LE to a new PE, even if there were errors.

Have fun (if at all possible),
-- 
Eric Hopper <hopper@omnifarious.org>
Omnifarious Software

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