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Re: [linux-lvm] Drive gone bad, now what?



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On Fri, 24 Oct 2003, Gert van der Knokke wrote:
> Rickard Olsson wrote:
> > John> If you had just a simple concatenation of all the disks, then
> > you are
> > John> toast.  How do you expect LVM to restore the missing 60gb if
> > there's
> > John> no parity information or mirrored blocks?  It's impossible!
>
> I didn't expect lvm to restore the missing data, I guessed it would just
> let me access the rest of the data.

To elaborate a bit, there are three cases to consider:

o  An LV with no extents stored on the failed drive.  This LV is intact
   and LVM should be able to provide it as if nothing had happened,
   because nothing *has* happened to this LV.

o  An LV all of whose extents were stored on the failed drive.  This LV's
   content is entirely lost and may only be recovered from other media.

o  An LV, some but not all of whose extents were stored on the failed
   drive.  Once the lost storage has been replaced, LVM *could* present
   this LV, which would contain a damaged filesystem.  'fsck' might be
   able to repair the filesystem enough to recover files which were stored
   in the undamaged extents, or a filesystem debugger might be available
   to facilitate manual recovery.  Some of the content is lost and may
   only be recovered from other media, but other content is undamaged.

   I don't yet know LVM well enough to say whether it *does* handle this
   case, but it can in theory and I would expect it to be written to do
   so.  Others' comments suggest that this is so.

BTW I've used various logical-volume schemes for years on top of hardware
RAID.  It's a combination that works well.  I don't understand why some
people want to put (soft) RAID on top of LVM rather than underneath it.

- -- 
Mark H. Wood, Lead System Programmer   mwood IUPUI Edu
MS Windows *is* user-friendly, but only for certain values of "user".
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