Re: convert raid10 to raid0

[Neil Brown <neilb@suse.de>]

On Wed, 07 Jul 2010 21:29:49 +0200
Tóth Csaba <csaba.toth@i3rendszerhaz.hu> wrote:

> Hey List,
> 
> i have a serious problem: have a file server with 4 disk (4x750GB),
> using them in a software RAID10 array. One of the HDDs have a bad sector
> (because some administration problems, i cannot replace them), and the
> motherboard sometimes drops the connection with two HDDs. Not with the
> same as the bad sector has, but with a resync i always managed the
> problem. But now 3 drive form the RAID10 array dropped.
> 
> In the past the sdc6 and the sdd6 partitions were the partitions i
> resynced the others, so i know exactly where to look for the proper
> datas. Can i use them as a RAID0 array somehow?

It should be sufficient to:
  mdadm -S /dev/md5
  mdadm -C /dev/md5 --level raid0 --raid-devices 2 --chunk 512 \
      --metadata=1.1 /dev/sdc6 /dev/sdd6

to get your data available.
There is a possible small complication though.  If you use a different
version of mdadm to the one you used to create the array at first, it might
choose a different 'data offset'.
To be sure this doesn't happen, use

 mdadm -E /dev/sdc6

and take note of the "Data Offset".  It would probably be a good idea to keep
a copy of "mdadm -E" of all of the devices, just to be on the same side.

Then after you create the raid0, use the same command
   mdadm -E /dev/sdc6
to check the Data Offset again and make sure it is the same.
I suspect it will be, so everything will be fine.
However if it isn't don't try to access the array.   Post the details and I'll
figure out what to do next.

NeilBrown


> In past mdadm warning letters i saw the config:
> 
> Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5]
> [raid4] [multipath] [faulty]
> md5 : active raid10 sdc6[0] sdd6[3] sdb6[1](F) sda6[2](F)
>       1434619904 blocks super 1.1 512K chunks 2 near-copies [4/2] [U__U]
> 
> md1 : active raid1 sdb5[0] sdc5[1]
>       14763635 blocks super 1.1 [2/2] [UU]
> 
> md0 : active raid1 sdd5[0] sda5[2](F)
>       14659200 blocks [2/1] [U_]
> 
> unused devices: <none>
> 
> 
> Now i have:
> 
> minerva ~ # cat /proc/mdstat
> Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5]
> [raid4] [multipath] [faulty]
> md5 : inactive sdd6[3]
>       717310119 blocks super 1.1
> 
> md1 : active raid1 sdc5[0] sdb5[1]
>       14763635 blocks super 1.1 [2/2] [UU]
> 
> md0 : active raid1 sda5[1] sdd5[0]
>       14659200 blocks [2/2] [UU]
> 
> unused devices: <none>
> minerva ~ # mdadm --detail /dev/md5
> /dev/md5:
>         Version : 1.1
>   Creation Time : Thu Nov 26 14:50:07 2009
>      Raid Level : raid10
>   Used Dev Size : 717309952 (684.08 GiB 734.53 GB)
>    Raid Devices : 4
>   Total Devices : 1
>     Persistence : Superblock is persistent
> 
>     Update Time : Wed Jul  7 10:34:32 2010
>           State : active, FAILED, Not Started
>  Active Devices : 1
> Working Devices : 1
>  Failed Devices : 0
>   Spare Devices : 0
> 
>          Layout : near=2
>      Chunk Size : 512K
> 
>            Name : minerva:5  (local to host minerva)
>            UUID : a73f5902:accc2488:aecfdc14:05a6e2f4
>          Events : 3981420
> 
>     Number   Major   Minor   RaidDevice State
>        0       0        0        0      removed
>        1       0        0        1      removed
>        2       0        0        2      removed
>        3       8       54        3      active sync   /dev/sdd6
> 
> 
> 
> 
> If i try to add sdc6 to the md5 array it fails, because it tries to add
> as a fifth device:
> 
> minerva ~ # mdadm /dev/md5 -a /dev/sdc6
> mdadm: add new device failed for /dev/sdc6 as 4: Invalid argument
> 
> 
> 
> Please help, i don't want to destroy the datas, and i have never job
> with RAID10 under linux.
> 
> thanks in advance,
> regards,
> Csaba
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