Re: xfs_repair force_geometry

["Michael L. Semon" <mlsemon35@gmail.com>]

On 05/14/2013 01:11 AM, Benedikt Schmidt wrote:
> First of all: Thanks for your very fast and helpful response.
>
> I copied actually only the partition, not the whole disk: /dd_rescue
> --force -r1 /dev/sdd1 /dev/sdc1/
> The cause for this is that I don't have enough space left on another
> device to store a whole copy of the faulty disk. I thought it would be
> possible, like in some examples I found with google, that you can rescue
> a partition directly.

Understood.  This seems like a valid option.  Had fdisk, cfdisk, and 
gdisk been more cooperative over the past year, this would have been my 
first option.

> /file -s /dev/sdc1/ says:
> //dev/sdc1: data/

This is different from what I got, but maybe Eric sees something in your 
answer.

> The disks look like this (/fdisk -l/):
> /Disk /dev/sdc: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors//
> //Units = Sektoren of 1 * 512 = 512 bytes//
> //Sector size (logical/physical): 512 bytes / 4096 bytes//
> //I/O size (minimum/optimal): 4096 bytes / 4096 bytes//
> //Disk identifier: 0xcba506ee//
> //
> //   Gerät  boot.     Anfang        Ende     Blöcke   Id System//
> ///dev/sdc1             256   732566645   366283195   83 Linux//
> //
> //Disk /dev/sdd: 2000.4 GB, 2000397852160 bytes, 3907027055 sectors//
> //Units = Sektoren of 1 * 512 = 512 bytes//
> //Sector size (logical/physical): 512 bytes / 512 bytes//
> //I/O size (minimum/optimal): 512 bytes / 512 bytes//
> //Disk identifier: 0x3c34826b//
> //
> //   Gerät  boot.     Anfang        Ende     Blöcke   Id System//
> ///dev/sdd1              63  3907024064  1953512001   83 Linux/
>
> If it is not possible to rescue the partition this way I will have to
> extend my to RAID5 so that I can put the copy of the faulty disk on this
> one, like Michael explained in his answer. I just hoped that I can avoid
> this, because it would save me more than 100€.

I had not much information to use, so I set up the safest possible 
scenario and hoped that you were getting results that were close to that.

If the few extra files that you're rescuing aren't worth 100 euros, then 
it's not worth 100 euros to make a duplicate of a dump of an 
already-damaged filesystem.

The crazy, reckless guide is this:

1) use `xfs_repair -n /dev/sdc1`.  If that looks nice,...

2) use `xfs_repair /dev/sdc1`...

   a) A repaired partition is a good sign.  Mount that partition!

   b) If the "attempting to find secondary superblock" search ends in 
"Sorry, could not find valid secondary superblock," then maybe something 
went wrong in the original data transfer.  You might have to give this 
step some time to complete, and it will print dots for a while.  Either 
that or the failures in your hard drive really did hit all of the 
superblocks.

   c) If the "attempting to find secondary superblock" finds something, 
it might make everything well but spit some files into lost+found.  If 
the repair goes badly, there's a chance you'll be using dd to look for 
your data.

   d) If it's something else--xfs_repair segfaults, needs to be run 
again, whatever, mention it--and at least you'll be closer to the real 
answer.

3) If all else fails, and especially when a backup is handy, you could 
try `xfs_repair -L` to zero the log.  This helps when xfs_repair asks 
you to mount the filesystem to allow metadata updates to happen, but 
Linux has an oops as the filesystem is mounted.  In many other 
scenarios, it can work against you.  This is the second-to-last resort.

> As last information: The content of this copy is not totally lost,
> actually only the last few files I have added. All the other stuff is
> already stored on the RAID5, only the latest stuff is not contained in
> this backup. So I don't loose everything if something goes wrong (at
> least one thing :-) ).

Really, it becomes a question of whether it would be faster to search 
for the data using dd and grep, use xfs_repair and hope it works, or 
recreate the data from scratch.

The hope is that dd_rescue does a credible job for you, and that XFS can 
be made to mount something, somewhere, so that you can grab those last 
few files.  The very last resort would be to do all of this repair stuff 
on the original damaged partition, but the safety net goes away after that.

Michael

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