Re: [linux-lvm] Restoring data after losing a drive

["Saad Shakhshir" <saads@alum.mit.edu>]

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Right now after running 'vgreduce --removemissing' it resized the volume
group to include only the available drives.  I got back my one small logical
volume (/dev/fileserver/home) and the data is all intact.  The other large
logical volume which spanned the rest of the drives (including the one that
went bad) isn't there anymore.  So it thinks that the majority of the volume
group is unused space, however there is data on there.  If I now create a
logical volume on the remaining space in the volume group, will it
automatically see the data that is there?

Where does the filesystem information get stored in terms of inode
locations?  Will that get overwritten now if I create a new logical volume?

On 5/12/07, Stuart D. Gathman <stuart@bmsi.com> wrote:
>
> On Sat, 12 May 2007, Saad Shakhshir wrote:
>
> > The data on the damaged disk is not recoverable at this point.  However
> > there is data on the other remaining good disks that was part of that
> one
> > large logical volume.  At this point I want to get back the remaining
> data
> > that was in that logical volume and on the remaining good physical
> volumes
> > without the data that was on the physical volume I lost.  I know that
> the
> > data is still intact on those drives... I just need to know how to get
> my
> > system to recognise that it's still there.
>
> The solution involves restoring the metadata to memory or a replacement
> hard
> drive from /etc/lvm (if it is intact) or a backup.  I'll let the experts
> talk about details.
>
> However, the LVM should be able to handle losing a PV and still bring LVs
> for that PV online.  Any attempted IO would result in errors, of course.
> But the metadata for a PV should be automatically loadable even with
> the PV missing.  When the missing PV blows a hole in your large LV,
> it would simplify recovering the pieces if the missing data got I/O
> errors.
> If you replace the drive and restore the metadata, the missing data will
> have whatever is on the drive.  It might help recovery to write a pattern
> to the replacement drive to help recognize the missing data.
>
> You will also need to be a filesystem wizard to navigate with a huge hole
> like that.  If you have one large file with a regular record format, it
> might be simplest to scan all blocks for records - then paste any missing
>
> I used to run into such problems a lot, and developed a filesystem where
> each block is tagged with the inode of the file it belonged to.  The
> recovery program can recover each and every readable block into the
> proper file in the proper location regardless of how much of the
> filesystem
> is missing or garbled due to horrendous errors.  There is no inode table
> either - any block can be an inode (so blowing away the first part of the
> filesystem doesn't lose all your files).  There are drawbacks to
> this approach, of course.  E.g. block size is reduced by the header
> present on every block (and is therefore not a power of 2).
>
> --
>               Stuart D. Gathman <stuart@bmsi.com>
>     Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703
> 591-6154
> "Confutatis maledictis, flammis acribus addictis" - background song for
> a Microsoft sponsored "Where do you want to go from here?" commercial.
>
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