Here's the adjusted command.

mdadm --create --assume-clean --level=5 --metadata=1.2 --chunk=512
--size=1952795136 --raid-devices=4 /dev/md0 missing \
92589cc2:9d5ed86c:1467efc2:2e6b7f09 \
390bd4a2:07a28c01:528ed41e:a9d0fcf0 \
4156ab46:bd42c10d:8565d5af:74856641

For the --size option, I'm not quite sure I understood what you tried
to explain to me. I re-read the manpage and I came up with this 2
equations:

(My understanding of your explanation) Used Dev size (3905590272)
divided by 2 = size (1952795136)
(My understanding from the manpages) Used Dev size (3905590272)
divided by chunk size (512) = size (7628106)

As for the device, I should order them with the device UUID (as shown
above) and I replace those UUID with the /dev/sdX3 that returns the
same device uuid from a mdadm -E command I will currently get? i.e.
mdadm -E /dev/sdd3 returns a device uuid of
92589cc2:9d5ed86c:1467efc2:2e6b7f09 , my first device with be
/dev/sdd3...?

One last question, after running mdadm --create command, can I run
mdadm -E and verify the values I get (chunk size, used dev size...)
match the ones I got from my first mdadm -E command, and if it
doesn't, to rerun the mdadm --create command to eventually get
matching values?

On Mon, Dec 8, 2014 at 10:14 AM, Robin Hill [off-list ref] wrote:

On Mon Dec 08, 2014 at 09:13:13AM -0500, Emery Guevremont wrote:

quoted

On Mon, Dec 8, 2014 at 4:48 AM, Robin Hill [off-list ref] wrote:

quoted

On Sat Dec 06, 2014 at 03:49:10PM -0500, Emery Guevremont wrote:

quoted

On Sat, Dec 6, 2014 at 1:56 PM, Robin Hill [off-list ref] wrote:

quoted

On Sat Dec 06, 2014 at 01:35:50pm -0500, Emery Guevremont wrote:

quoted

The long story and what I've done.

/dev/md0 is assembled with 4 drives
/dev/sda3
/dev/sdb3
/dev/sdc3
/dev/sdd3

2 weeks ago, mdadm marked /dev/sda3 as failed. cat /proc/mdstat showed
_UUU. smarctl also confirmed that the drive was dying. So I shutdown
the server and until I received a replacement drive.

This week, I replaced the dying drive with my new drive. Booted into
single user mode and did this:

mdadm --manage /dev/md0 --add /dev/sda3  a cat of /proc/mdstat
confirmed the resyncing process. The last time I checked it was up to
11%. After a few minutes later, I noticed that the syncing stopped. A
read error message on /dev/sdd3 (have a pic of it if interested)
appear on the console. It appears that /dev/sdd3 might be going bad. A
cat /proc/mdstat showed _U_U. Now I panic, and decide to leave
everything as is and to go to bed.

The next day, I shutdown the server and reboot with a live usb distro
(Ubuntu rescue remix). After booting into the live distro, a cat
/proc/mdstat showed that my /dev/md0 was detected but all drives had
an (S) next to it. i.e. /dev/sda3 (S)... Naturally I don't like the
looks of this.

I ran ddrescue to copy /dev/sdd onto my new replacement disk
(/dev/sda). Everything, worked, ddrescue got only one read error, but
was eventually able to read the bad sector on a retry. I followed up
by also cloning with ddrescue, sdb and sdc.

So now I have cloned copies of sdb, sdc and sdd to work with.
Currently running mdadm --assemble --scan, will activate my array, but
all drives are added as spares. Running mdadm --examine on each
drives, shows the same Array UUID number, but the Raid Devices is 0
and raid level is -unknown- for some reason. The rest seems fine and
makes sense. I believe I could re-assemble my array if I could define
the raid level and raid devices.

I wanted to know if there are a way to restore my superblocks from the
examine command I ran at the beginning? If not, what mdadm create
command should I run? Also please let me know if drive ordering is
important, and how I can determine this with the examine output I'll
got?

Thank you.

Have you tried --assemble --force? You'll need to make sure the array's
stopped first, but that's the usual way to get the array back up and
running in that sort of situation.

If that doesn't work, stop the array again and post:
 - the output from mdadm --assemble --force --verbose /dev/md0 /dev/sd[bcd]3
 - any dmesg output corresponding with the above
 - --examine output for all disks
 - kernel and mdadm versions

Good luck,
    Robin

quoted

You'll see from the examine output, raid level and devices aren't
defined and notice the role of each drives. The examine output (I
attached 4 files) that I took right after the read error during the
synching process seems to show a more accurate superblock. Here's also
the output of mdadm --detail /dev/md0 that I took when I got the first
error:

ARRAY /dev/md/0 metadata=1.2 UUID=cf9db8fa:0c2bb553:46865912:704cceae
name=runts:0
   spares=1


Here's the output of how things currently are:

mdadm --assemble --force /dev/md127 /dev/sdb3 /dev/sdc3 /dev/sdd3
mdadm: /dev/md127 assembled from 0 drives and 3 spares - not enough to
start the array.

dmesg
[27903.423895] md: md127 stopped.
[27903.434327] md: bind<sdc3>
[27903.434767] md: bind<sdd3>
[27903.434963] md: bind<sdb3>

cat /proc/mdstat
root@ubuntu:~# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0]
[raid1] [raid10]
md127 : inactive sdb3[4](S) sdd3[0](S) sdc3[5](S)
      5858387208 blocks super 1.2

mdadm --examine /dev/sd[bcd]3
/dev/sdb3:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : cf9db8fa:0c2bb553:46865912:704cceae
           Name : runts:0
  Creation Time : Tue Jul 26 03:27:39 2011
     Raid Level : -unknown-
   Raid Devices : 0

 Avail Dev Size : 3905591472 (1862.33 GiB 1999.66 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : active
    Device UUID : b2bf0462:e0722254:0e233a72:aa5df4da

    Update Time : Sat Dec  6 12:46:40 2014
       Checksum : 5e8cfc9a - correct
         Events : 1


   Device Role : spare
   Array State :  ('A' == active, '.' == missing)
/dev/sdc3:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : cf9db8fa:0c2bb553:46865912:704cceae
           Name : runts:0
  Creation Time : Tue Jul 26 03:27:39 2011
     Raid Level : -unknown-
   Raid Devices : 0

 Avail Dev Size : 3905591472 (1862.33 GiB 1999.66 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : active
    Device UUID : 390bd4a2:07a28c01:528ed41e:a9d0fcf0

    Update Time : Sat Dec  6 12:46:40 2014
       Checksum : f69518c - correct
         Events : 1


   Device Role : spare
   Array State :  ('A' == active, '.' == missing)
/dev/sdd3:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : cf9db8fa:0c2bb553:46865912:704cceae
           Name : runts:0
  Creation Time : Tue Jul 26 03:27:39 2011
     Raid Level : -unknown-
   Raid Devices : 0

 Avail Dev Size : 3905591472 (1862.33 GiB 1999.66 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : active
    Device UUID : 92589cc2:9d5ed86c:1467efc2:2e6b7f09

    Update Time : Sat Dec  6 12:46:40 2014
       Checksum : 571ad2bd - correct
         Events : 1


   Device Role : spare
   Array State :  ('A' == active, '.' == missing)

and finally kernel and mdadm versions:

uname -a
Linux ubuntu 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:41:14 UTC
2012 i686 i686 i386 GNU/Linux

mdadm -V
mdadm - v3.2.3 - 23rd December 2011

The missing data looks similar to a bug fixed a couple of years ago
(http://neil.brown.name/blog/20120615073245), though the kernel versions
don't match and the missing data is somewhat different - it may be that
the relevant patches were backported to the vendor kernel you're using.

With that data missing there's no way to assemble though, so a re-create
is required in this case (it's a last resort, but I don't see any other
option).

quoted

/dev/sda3:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : cf9db8fa:0c2bb553:46865912:704cceae
           Name : runts:0  (local to host runts)
  Creation Time : Mon Jul 25 23:27:39 2011
     Raid Level : raid5
   Raid Devices : 4

 Avail Dev Size : 3905591472 (1862.33 GiB 1999.66 GB)
     Array Size : 5858385408 (5586.99 GiB 5998.99 GB)
  Used Dev Size : 3905590272 (1862.33 GiB 1999.66 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : b2bf0462:e0722254:0e233a72:aa5df4da

    Update Time : Tue Dec  2 23:15:37 2014
       Checksum : 5ed5b898 - correct
         Events : 3925676

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : spare
   Array State : A.A. ('A' == active, '.' == missing)

quoted

/dev/sdb3:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : cf9db8fa:0c2bb553:46865912:704cceae
           Name : runts:0  (local to host runts)
  Creation Time : Mon Jul 25 23:27:39 2011
     Raid Level : raid5
   Raid Devices : 4

 Avail Dev Size : 3905591472 (1862.33 GiB 1999.66 GB)
     Array Size : 5858385408 (5586.99 GiB 5998.99 GB)
  Used Dev Size : 3905590272 (1862.33 GiB 1999.66 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : 92589cc2:9d5ed86c:1467efc2:2e6b7f09

    Update Time : Tue Dec  2 23:15:37 2014
       Checksum : 57638ebb - correct
         Events : 3925676

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 0
   Array State : A.A. ('A' == active, '.' == missing)

quoted

/dev/sdc3:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : cf9db8fa:0c2bb553:46865912:704cceae
           Name : runts:0  (local to host runts)
  Creation Time : Mon Jul 25 23:27:39 2011
     Raid Level : raid5
   Raid Devices : 4

 Avail Dev Size : 3905591472 (1862.33 GiB 1999.66 GB)
     Array Size : 5858385408 (5586.99 GiB 5998.99 GB)
  Used Dev Size : 3905590272 (1862.33 GiB 1999.66 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : 390bd4a2:07a28c01:528ed41e:a9d0fcf0

    Update Time : Tue Dec  2 23:15:37 2014
       Checksum : fb20d8a - correct
         Events : 3925676

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 2
   Array State : A.A. ('A' == active, '.' == missing)

quoted

/dev/sdd3:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : cf9db8fa:0c2bb553:46865912:704cceae
           Name : runts:0  (local to host runts)
  Creation Time : Mon Jul 25 23:27:39 2011
     Raid Level : raid5
   Raid Devices : 4

 Avail Dev Size : 3905591472 (1862.33 GiB 1999.66 GB)
     Array Size : 5858385408 (5586.99 GiB 5998.99 GB)
  Used Dev Size : 3905590272 (1862.33 GiB 1999.66 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : 4156ab46:bd42c10d:8565d5af:74856641

    Update Time : Tue Dec  2 23:14:03 2014
       Checksum : a126853f - correct
         Events : 3925672

         Layout : left-symmetric
     Chunk Size : 512K

   Device Role : Active device 1
   Array State : AAAA ('A' == active, '.' == missing)

At least you have the previous data anyway, which should allow
reconstruction of the array. The device names have changed between your
two reports though, so I'd advise double-checking which is which before
proceeding.

The reports indicate that the original array order (based on the device
role field) for the four devices was (using device UUIDs as they're
consistent):
    92589cc2:9d5ed86c:1467efc2:2e6b7f09
    4156ab46:bd42c10d:8565d5af:74856641
    390bd4a2:07a28c01:528ed41e:a9d0fcf0
    b2bf0462:e0722254:0e233a72:aa5df4da

That would give a current device order of sdd3,sda3,sdc3,sdb3 (I don't
have the current data for sda3, but that's the only missing UUID).

The create command would therefore be:
    mdadm -C -l 5 -n 4 -c 512 -e 1.2 -z 1952795136 \
        /dev/md0 /dev/sdd3 /dev/sda3 /dev/sdc3 missing

mdadm 3.2.3 should use a data offset of 2048, the same as your old
array, but you may want to double-check that with a test array on a
couple of loopback devices first. If not, you'll need to grab the
latest release and add the --data-offset=2048 parameter to the above
create command.

You should also follow the instructions for using overlay files at
https://raid.wiki.kernel.org/index.php/Recovering_a_failed_software_RAID
in order to safely test out the above without risking damage to the
array data.

Once you've run the create, run a "fsck -n" on the filesystem to check
that the data looks okay. If not, the order or parameters may be
incorrect - check the --examine output for any differences from the
original results.

Just to double check, would this be the right command to run?

mdadm --create --assume-clean --level=5 --size=5858385408
--raid-devices=4 /dev/md0 missing /dev/sdb3 /dev/sdc3 /dev/sdd3

Are there any other options I would need to add? Should I specify
--chunk and --size (and if I entered the right size)?

You don't need --assume-clean as there's a missing device, so no scope
for rebuilding one of the disks (which is all the flag prevents). It
won't do any harm leaving it in though.

The size should be the per-device size in kiB (which is half the Used
Dev Size value listed in the --examine output, as that's given in
512-byte blocks) and I gave you the correct value above. I'd recommend
including this as it will ensure that mdadm isn't calculating the size
any different from the version originally used to create the array.

The device order you've given is incorrect for either the original
device numbering or the numbering you posted as being the most recent.
The order I gave above is based on the order as in the latest --examine
results you gave. If you've rebooted since then, you'll need to verify
the order based on the UUIDs of the devices though (again, the original
order should be the one I gave above, based on the device role order in
your original --examine output). If you're using different disks, you'll
need to be sure which one was mirrored from which original. If you use
the incorrect order, you'll get a lot of errors in the "fsck -n" output
but, as long as you don't actually write to the array, it shouldn't
cause any data corruption as only the metadata will be overwritten.

There shouldn't be any need to specify the chunk size, as 512k should be
the default value, but I'd probably still stick it in anyway, just to be
on the safe side.

Similarly with the metadata version - 1.2 is the default (currently
anyway, I'm not certain with 3.2.3), so shouldn't be necessary. Again,
I'd add it in to be on the safe side.

quoted

By the way thanks for the help.

No problem.

Cheers,
    Robin
--
     ___
    ( ' }     |       Robin Hill        [off-list ref] |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |