Re: Data Offset

On Mon, 04 Jun 2012 20:26:05 +0200 Pierre Beck [off-list ref] wrote:

I'll try and clear up some confusion (I was in IRC with freeone3000).

/dev/sdf is an empty drive, a replacement for a failed drive. The Array 
attempted to assemble, but failed and reported one drive as spare. This 
is the moment we saved the --examines.

In expectation of a lost write due to drive write-cache, we executed 
--assemble --force, which kicked another drive.

@James: remove /dev/sdf for now and replace /dev/sde3, which indeed has 
a very outdated update time, with the non-present drive. Post an 
--examine of that drive. It should report update time Jun 1st.

We tried to re-create the array with --assume-clean. But mdadm chose a 
different data offset for the drives. A re-create with proper data 
offset will be necessary.

OK, try:

   git clone -b data_offset git://neil.brown.name/mdadm
   cd mdadm
   make

   ./mdadm -C /dev/md1 -e 1.2 -l 5 -n 5 --assume-clean -c 512 \ 
      /dev/sdc3:2048s /dev/sdb3:2048s ???  /dev/sdd3:1024s ???

The number after ':' after a device name is a data offset.  's' means sectors.
With out 's' it means Kilobytes.
I don't know what should be at slot 2 or 4 so I put '???'.  You should fill it
in.  You should also double check the command and double check the names of
your devices.
Don't install this mdadm, and don't use it for anything other than
re-creating this array.

Good luck.

NeilBrown

Greetings,

Pierre Beck


Am 04.06.2012 05:35, schrieb NeilBrown:

quoted

On Fri, 1 Jun 2012 19:48:41 -0500 freeone3000[off-list ref]  wrote:

quoted

Sorry.

/dev/sde fell out of the array, so I replaced the physical drive with
what is now /dev/sdf. udev may have relabelled the drive - smartctl
states that the drive that is now /dev/sde works fine.
/dev/sdf is a new drive. /dev/sdf has a single, whole-disk partition
with type marked as raid. It is physically larger than the others.

/dev/sdf1 doesn't have a mdadm superblock. /dev/sdf seems to, so I
gave output of that device instead of /dev/sdf1, despite the
partition. Whole-drive RAID is fine, if it gets it working.

What I'm attempting to do is rebuild the RAID from the data from the
other four drives, and bring the RAID back up without losing any of
the data. /dev/sdb3, /dev/sdc3, /dev/sdd3, and what is now /dev/sde3
should be used to rebuild the array, with /dev/sdf as a new drive. If
I can get the array back up with all my data and all five drives in
use, I'll be very happy.

You appear to have 3 devices that are happy:
   sdc3 is device 0   data-offset 2048
   sdb3 is device 1   data-offset 2048
   sdd3 is device 3   data-offset 1024

nothing claims to be device 2 or 4.

sde3 looks like it was last in the array on 23rd May, a little over
a week before your report.  Could that have been when "sde fell out of the
array" ??
Is it possible that you replaced the wrong device?
Or is it possible the the array was degraded when sde "fell out" resulting
in data loss?

I need more precise history to understand what happened, as I cannot suggest
a fixed until I have that understanding.

When did the array fail?
How certain are you that you replaced the correct device?
Can you examine the drive that you removed and see what it says?
Are you certain that the array wasn't already degraded?

NeilBrown