Re: recovery problems, might be driver-related

[Neil Brown <neilb@suse.de>]

On Sun, 07 Feb 2010 14:38:18 +0100
Stefan Hübner <stefan.huebner@stud.tu-ilmenau.de> wrote:

> Hi Everybody,
> 
> I've recently come across some RAID-Recovery problem that were kind of
> not-so-easy-to-understand.  I was trying to recover damaged RAID5s,
> where one
> disk died and another dropped out (most likely due to read error recovery
> timeout / not reporting back while error recovery was active) during
> resync with
> a spare/new disk.  After taking double backups I tried to recreate the
> raid with
> the needed working disk images (to make the superblocks consistent).  During
> that action mdadm told me that the last-dropped disk contained a valid ext3
> filesystem and was obviously part of an md-array.

mdadm only checks the superblock at the start of the device to see if it
looks like an ext3 filesystem.  So if an md array has a valid filesystem,
then it is very likely that at least one of the devices in the array will
appear to have a valid filesystem to mdadm.

> 
> This happened with NAS-Devices from 2 different Vendors (namely Thecus and
> Synology), which made me think it must be a md-raid thing.  Does md-raid
> create
> a filesystem after a disk dropped out?  Or may something in the system
> happen to
> cause this strange behaviour?

No, nothing would try to create a filesystem on a device just because it has
dropped out of a RAID.


> 
> All in all: after recreating the raids the filesystem contained on it was
> totally damaged (could not even be mounted).  fsck ran multiple days with
> excessive data loss.

Maybe there was meant to be another layer between the md array and the
filesystem - maybe LVM ??  If there should have been an LVM and wasn't the
filesystem would definitely look very corrupt even though the superblock
might appear to be in the right place.


NeilBrown


> 
> P.S.: the mdadm-lines to recreate the RAIDs were derived from mdadm -E -
> outputs
> of the original partitions, so I believe that it should have worked (on
> other
> recoveries I did before it also worked well).
> 
> Does anyone have an idea what is going on there?  Or may it have happened -
> well, I don't want to say something that could get me sued.
> 
> All the best,
> Stefan Hübner
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