remove mdadm raid1 without loosing data

Question

I have 2 hdd's in raid1 using mdadm.

Now i want to completely disable raid but i want to keep data on disk 1, and use disk 2 separately

Is there a way to do this.

Ill be honest, i know what raid is, but i have no idea how it is handled by mdadm.

I'm using Ubuntu 14.04

Configuration:

mdadm: /dev/md does not appear to be an md device
/dev/md0:
        Version : 1.2
  Creation Time : Tue Jul  7 10:13:17 2015
     Raid Level : raid1
     Array Size : 16768896 (15.99 GiB 17.17 GB)
  Used Dev Size : 16768896 (15.99 GiB 17.17 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Sun Oct 11 15:50:16 2015
          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : rescue:0
           UUID : ec52104c:85983ebd:8a13b660:706bae89
         Events : 32

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
/dev/md1:
        Version : 1.2
  Creation Time : Tue Jul  7 10:13:20 2015
     Raid Level : raid1
     Array Size : 523968 (511.77 MiB 536.54 MB)
  Used Dev Size : 523968 (511.77 MiB 536.54 MB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Sat Sep 19 12:41:03 2015
          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : rescue:1
           UUID : decb590d:aac47b81:568a2afa:3dee7600
         Events : 30

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       1       8       18        1      active sync   /dev/sdb2
/dev/md2:
        Version : 1.2
  Creation Time : Tue Jul  7 10:13:20 2015
     Raid Level : raid1
     Array Size : 1073610560 (1023.87 GiB 1099.38 GB)
  Used Dev Size : 1073610560 (1023.87 GiB 1099.38 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon Oct 12 09:25:14 2015
          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : rescue:2
           UUID : 1daa4480:ddf3f359:575a1f97:e232bbdd
         Events : 195

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync   /dev/sda3
       1       8       19        1      active sync   /dev/sdb3
/dev/md3:
        Version : 1.2
  Creation Time : Tue Jul  7 10:13:20 2015
     Raid Level : raid1
     Array Size : 1839089920 (1753.89 GiB 1883.23 GB)
  Used Dev Size : 1839089920 (1753.89 GiB 1883.23 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon Oct 12 09:04:16 2015
          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : rescue:3
           UUID : 051aac4e:ecac8794:538fe663:c9212747
         Events : 92

    Number   Major   Minor   RaidDevice State
       0       8        4        0      active sync   /dev/sda4
       1       8       20        1      active sync   /dev/sdb4  

.

WARNING: GPT (GUID Partition Table) detected on '/dev/sda'! The util fdisk doesn't support GPT. Use GNU Parted.


Disk /dev/sda: 3000.6 GB, 3000592982016 bytes
256 heads, 63 sectors/track, 363376 cylinders, total 5860533168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1  4294967295  2147483647+  ee  GPT

WARNING: GPT (GUID Partition Table) detected on '/dev/sdb'! The util fdisk doesn't support GPT. Use GNU Parted.


Disk /dev/sdb: 3000.6 GB, 3000592982016 bytes
256 heads, 63 sectors/track, 363376 cylinders, total 5860533168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1  4294967295  2147483647+  ee  GPT
Partition 1 does not start on physical sector boundary.

Disk /dev/md0: 17.2 GB, 17171349504 bytes
2 heads, 4 sectors/track, 4192224 cylinders, total 33537792 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000

Disk /dev/md0 doesn't contain a valid partition table

Disk /dev/md1: 536 MB, 536543232 bytes
2 heads, 4 sectors/track, 130992 cylinders, total 1047936 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000

Disk /dev/md1 doesn't contain a valid partition table

Disk /dev/md3: 1883.2 GB, 1883228078080 bytes
2 heads, 4 sectors/track, 459772480 cylinders, total 3678179840 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000

Disk /dev/md3 doesn't contain a valid partition table

Disk /dev/md2: 1099.4 GB, 1099377213440 bytes
2 heads, 4 sectors/track, 268402640 cylinders, total 2147221120 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000

Disk /dev/md2 doesn't contain a valid partition table

Solution

You cannot remove a disk directly from the array, unless it is failed, so we first have to fail it (if the drive it is failed this is normally already in failed state and this step is not needed):

mdadm --fail /dev/md0 /dev/sda1

and now we can remove it:

mdadm --remove /dev/md0 /dev/sda1

This can be done in a single step using:

mdadm /dev/md0 --fail /dev/sda1 --remove /dev/sda1

Then mount the disk as a normal disk:

sudo mount -t auto /dev/sda1 /media/disk

Comments

[pedja]

ok, but raid will then become degraded with only on hdd, could there be any kind of issues if i leave it like that?

[qugu]

If that drive is not used to boot up the system, you should experience no issues using it normally. I have done the same procedure in a similar setup when needed to fix unreadable sectors and then return disk to array.