My Current MDADM Setup

Just some notes on my current MDADM setup, so I do not forget.

So my current hard drives are:

# lsscsi
[0:0:0:0]    disk    ATA      Samsung SSD 860  1B6Q  /dev/sda 
[4:0:0:0]    disk    ATA      WDC WD4003FZEX-0 1A01  /dev/sdb 
[5:0:0:0]    disk    ATA      WDC WD5000AAKS-0 3B01  /dev/sdc 
[5:0:1:0]    disk    ATA      ST2000DM001-1CH1 CC29  /dev/sdd 
[10:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sde 
[11:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sdf 

The first one is my primary and Operating Systems and all that. Seems obvious.

The second one contains a large partition for my /home on sdb1.

Partition sdc1 of the sdc drive is a simple 500Gb partition containing some odds and ends. Not big enough for the RAID, not fast enough for the operating system.

Starting my raid setup I can do with:

# mdadm --assemble /dev/md127 /dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdb2 mdadm: /dev/md127 has been started with 4 drives.

Just so you know, it's a simple mirror setup, so I am using 4 drives to mirror a 2 TB partition.

And then I simple mount the damn thing with:

# mount /dev/md127 /mnt/raid/

Then I can watch the status with detail:

mdadm --detail /dev/md127

Adding a device to a raid with mdadm

Reordering partitions to make room

So my current hard drives are:

# lsscsi
[0:0:0:0]    disk    ATA      Samsung SSD 860  1B6Q  /dev/sda 
[4:0:0:0]    disk    ATA      WDC WD4003FZEX-0 1A01  /dev/sdb 
[5:0:0:0]    disk    ATA      WDC WD5000AAKS-0 3B01  /dev/sdc 
[5:0:1:0]    disk    ATA      ST2000DM001-1CH1 CC29  /dev/sdd 
[10:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sde 
[11:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sdf 

I had a lot of unused space on /dev/sdb, and thought I could rearrange the partitions and use it as an additional device in my (software) RAID.

I used Parted¹, but GParted² is also very nice. GParted can also scan my /dev/md127, which is a raid device.

Of course there's the general problem nowadays of looking what kind of MBR/GPT or whatever boot schema you use.

So:

Disk /dev/sdb: 3.64 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model: WDC WD4003FZEX-0
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: D344E388-FA66-4B30-BA00-4D3B12A07B79

Device          Start        End    Sectors  Size Type
/dev/sdb1        2048       4095       2048    1M BIOS boot
/dev/sdb2        4096    1028095    1024000  500M Linux filesystem
/dev/sdb3     1028096 5273437500 5272409405  2.5T Linux filesystem
/dev/sdb4  5273438208 7420921855 2147483648    1T Linux filesystem

So it's a 4 TB hard drive, and it turns out that I only use /dev/sdb3. (The other partitions are from an old Linux install before I got a nice SDD and reinstalled Linux/Windows on that.)

So I could first rearrange it: resize /dev/sdb3 to 2T and move it to /dev/sdb1.

That worked fine using Parted. It just took some time to actually move all the data.

And then create a new /dev/sdb2 of 2T for my raid with gdisk³ (the GPT version of fdisk).

I had to look up the appropriate partition type in GPT⁴ for mdadm.

# gdisk /dev/sdb
GPT fdisk (gdisk) version 1.0.9

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Command (? for help): n
Partition number (2-128, default 2): 
First sector (34-7814037134, default = 3907006464) or {+-}size{KMGTP}: 
Last sector (3907006464-7814037134, default = 7814035455) or {+-}size{KMGTP}: 
Current type is 8300 (Linux filesystem)


Hex code or GUID (L to show codes, Enter = 8300): fd00
Changed type of partition to 'Linux RAID'

Command (? for help): p
Disk /dev/sdb: 7814037168 sectors, 3.6 TiB
Model: WDC WD4003FZEX-0
Sector size (logical/physical): 512/4096 bytes
Disk identifier (GUID): D344E388-FA66-4B30-BA00-4D3B12A07B79
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 7814037134
Partitions will be aligned on 2048-sector boundaries
Total free space is 3693 sectors (1.8 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048      3907006463   1.8 TiB     8300  
   2      3907006464      7814035455   1.8 TiB     FD00  Linux RAID

Adding the device

Current raid setup:

# mdadm --assemble /dev/md127 /dev/sdd1 /dev/sde1 /dev/sdf1
mdadm: /dev/md127 has been started with 3 drives.

# mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Wed Mar  6 22:16:05 2013
        Raid Level : raid1
        Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
      Raid Devices : 3
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Wed Aug 16 09:08:07 2023
             State : clean 
    Active Devices : 3
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : resync

              Name : micemouse:0
              UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
            Events : 7285

    Number   Major   Minor   RaidDevice State
       4       8       65        0      active sync   /dev/sde1
       5       8       81        1      active sync   /dev/sdf1
       3       8       49        2      active sync   /dev/sdd1

# cat /proc/mdstat
Personalities : [raid1] 
md127 : active raid1 sde1[4] sdd1[3] sdf1[5]
      1953380160 blocks super 1.2 [3/3] [UUU]
      
unused devices: <none>

Adding a device:

mdadm --add /dev/md127 /dev/sdb2
mdadm: added /dev/sdb2
  
md127 : active raid1 sdb2[6](S) sde1[4] sdd1[3] sdf1[5]
      1953380160 blocks super 1.2 [3/3] [UUU]
      
unused devices: <none>
 mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Wed Mar  6 22:16:05 2013
        Raid Level : raid1
        Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
      Raid Devices : 3
     Total Devices : 4
       Persistence : Superblock is persistent

       Update Time : Sat Aug 19 16:47:58 2023
             State : clean 
    Active Devices : 3
   Working Devices : 4
    Failed Devices : 0
     Spare Devices : 1

Consistency Policy : resync

              Name : micemouse:0
              UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
            Events : 7286

    Number   Major   Minor   RaidDevice State
       4       8       65        0      active sync   /dev/sde1
       5       8       81        1      active sync   /dev/sdf1
       3       8       49        2      active sync   /dev/sdd1

       6       8       18        -      spare   /dev/sdb2

[

root@localhost ~]# mdadm --grow /dev/md127 --raid-devices=4
raid_disks for /dev/md127 set to 4

  # mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Wed Mar  6 22:16:05 2013
        Raid Level : raid1
        Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
      Raid Devices : 4
     Total Devices : 4
       Persistence : Superblock is persistent

       Update Time : Sat Aug 19 16:49:32 2023
             State : clean, degraded, recovering 
    Active Devices : 3
   Working Devices : 4
    Failed Devices : 0
     Spare Devices : 1

Consistency Policy : resync

    Rebuild Status : 0% complete

              Name : micemouse:0
              UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
            Events : 7290

    Number   Major   Minor   RaidDevice State
       4       8       65        0      active sync   /dev/sde1
       5       8       81        1      active sync   /dev/sdf1
       3       8       49        2      active sync   /dev/sdd1
       6       8       18        3      spare rebuilding   /dev/sdb2

And now we wait.

# cat /proc/mdstat
Personalities : [raid1] 
md127 : active raid1 sdb2[6] sde1[4] sdd1[3] sdf1[5]
      1953380160 blocks super 1.2 [4/3] [UUU_]
      [>....................]  recovery =  0.1% (2956032/1953380160) finish=1156.0min speed=28118K/sec
      
unused devices: <none>

Ready in about 12 hours.

References

[1] Parted User’s Manual

https://www.gnu.org/software/parted/manual/parted.html

[2] GParted is a free partition editor for graphically managing your disk partitions.

https://gparted.org/

[3] gdisk(8) - Linux man page

https://linux.die.net/man/8/gdisk/

[4] Raid Wiki Kernel Org - Partition Types

https://raid.wiki.kernel.org/index.php/Partition_Types

WP Guru - How to add a drive to software RAID with mdadm

https://wpguru.co.uk/2021/01/expand-software-raid-mdadm/

Mounting and umounting my MDADM disk array

In my dmesg, when connecting my USB harddrives, I get the following messages:

[ 3455.138687] usb 1-1.5: new high-speed USB device number 3 using ehci-pci
[ 3455.218450] usb 1-1.5: New USB device found, idVendor=1058, idProduct=1021, bcdDevice=20.21
[ 3455.218462] usb 1-1.5: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 3455.218468] usb 1-1.5: Product: Ext HDD 1021
[ 3455.218472] usb 1-1.5: Manufacturer: Western Digital
[ 3455.218476] usb 1-1.5: SerialNumber: 574D43315431373033343231
[ 3455.260880] usb-storage 1-1.5:1.0: USB Mass Storage device detected
[ 3455.261279] scsi host10: usb-storage 1-1.5:1.0
[ 3455.261525] usbcore: registered new interface driver usb-storage
[ 3455.267542] usbcore: registered new interface driver uas
[ 3455.287643] usb 1-1.6: new high-speed USB device number 4 using ehci-pci
[ 3455.367335] usb 1-1.6: New USB device found, idVendor=1058, idProduct=1021, bcdDevice=20.21
[ 3455.367347] usb 1-1.6: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 3455.367353] usb 1-1.6: Product: Ext HDD 1021
[ 3455.367357] usb 1-1.6: Manufacturer: Western Digital
[ 3455.367361] usb 1-1.6: SerialNumber: 574D43315432313138383334
[ 3455.367897] usb-storage 1-1.6:1.0: USB Mass Storage device detected
[ 3455.368175] scsi host11: usb-storage 1-1.6:1.0
[ 3456.301327] scsi 10:0:0:0: Direct-Access WD Ext HDD 1021 2021 PQ: 0 ANSI: 4
[ 3456.302077] sd 10:0:0:0: Attached scsi generic sg4 type 0
[ 3456.302737] sd 10:0:0:0: [sde] 3907024896 512-byte logical blocks: (2.00 TB/1.82 TiB)
[ 3456.304758] sd 10:0:0:0: [sde] Write Protect is off
[ 3456.304771] sd 10:0:0:0: [sde] Mode Sense: 17 00 10 08
[ 3456.306754] sd 10:0:0:0: [sde] No Caching mode page found
[ 3456.306763] sd 10:0:0:0: [sde] Assuming drive cache: write through
[ 3456.335251] sde: sde1
[ 3456.335455] sd 10:0:0:0: [sde] Attached SCSI disk
[ 3456.428905] scsi 11:0:0:0: Direct-Access WD Ext HDD 1021 2021 PQ: 0 ANSI: 4
[ 3456.430291] sd 11:0:0:0: Attached scsi generic sg5 type 0
[ 3456.431366] sd 11:0:0:0: [sdf] 3907024896 512-byte logical blocks: (2.00 TB/1.82 TiB)
[ 3456.433375] sd 11:0:0:0: [sdf] Write Protect is off
[ 3456.433387] sd 11:0:0:0: [sdf] Mode Sense: 17 00 10 08
[ 3456.435502] sd 11:0:0:0: [sdf] No Caching mode page found
[ 3456.435515] sd 11:0:0:0: [sdf] Assuming drive cache: write through
[ 3456.457754] sdf: sdf1
[ 3456.457974] sd 11:0:0:0: [sdf] Attached SCSI disk

The partitions on those drives look like follows:

/dev/sdd1 2048 3907024895 3907022848 1.8T fd Linux raid autodetect
/dev/sde1 2048 3907024895 3907022848 1.8T fd Linux raid autodetect
/dev/sdf1 2048 3907024895 3907022848 1.8T fd Linux raid autodetect

Mounting said drives as a mirrored drive om /dev/md127:

mdadm --assemble /dev/md127 /dev/sdd1 /dev/sde1 /dev/sdf1
mdadm: /dev/md127 has been started with 3 drives.

To check the status:

mdadm --detail /dev/md127

/dev/md127:
           Version : 1.2
     Creation Time : Wed Mar  6 22:16:05 2013
        Raid Level : raid1
        Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
      Raid Devices : 3
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Sat Mar 19 07:07:49 2022
             State : clean 
    Active Devices : 3
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : resync

              Name : micemouse:0
              UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
            Events : 6587

    Number   Major   Minor   RaidDevice State
       4       8       65        0      active sync   /dev/sde1
       5       8       81        1      active sync   /dev/sdf1
       3       8       49        2      active sync   /dev/sdd1

It's also possible to use:

mdadm --assemble --scan

Adding a drive

Previously I had only two drives, but I added one as follows:

# mdadm --grow /dev/md127 --add /dev/sdd1 --raid-devices=3
mdadm: added /dev/sdd1
raid_disks for /dev/md127 set to 3
Every 2.0s: cat /proc/mdstat                  sherlock: Sat Nov 24 15:28:29 2018

Personalities : [raid1]
md127 : active raid1 sdd1[3] sdf1[2] sde1[1]
      1953380160 blocks super 1.2 [3/2] [UU_]
      [>....................]  recovery =  0.0% (1446912/1953380160) finish=957.
9min speed=33959K/sec

unused devices: <none>

That took some time.

RAID-1 Degraded - missing device

I have got two hard disks, both the same type/brand. They are two USB2.0 connected WesternDigital drives that work pretty well. I've setup a software-based RAID1 system using them both, by means of the MDADM Linux program. I find it works very well.

Thankfully I noticed something odd about the hard disks I used. It is probably due to Partial RAID1 mounting that I tried recently that I screwed something up.

The raid1 I setup contains two disks, as visible in the lsscsi command:

root@micemouse:~# lsscsi
[0:0:0:0]    disk    ATA      WDC WD5000AAKS-0 01.0  /dev/sda 
[4:0:1:0]    cd/dvd  SAMSUNG  DVD-ROM SDR-430  1.06  /dev/sr0 
[10:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sdb 
[11:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sdc

As you can see in the printout below, it is set to "Degraded", and we're missing one of the devices.

root@micemouse:~# mdadm --detail /dev/md127
/dev/md127:
        Version : 1.2
  Creation Time : Wed Mar  6 22:16:05 2013
     Raid Level : raid1
     Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
  Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
   Raid Devices : 2
  Total Devices : 1
    Persistence : Superblock is persistent

    Update Time : Sat May 16 15:50:47 2015
          State : clean, degraded 
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           Name : micemouse:0  (local to host micemouse)
           UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
         Events : 3748

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1       8       33        1      active sync   /dev/sdc1

We are missing /dev/sdb1.

Rebuilding the array

Let's re-add the missing device.

root@micemouse:~# mdadm --remove /dev/md127 /dev/sdb1
mdadm: hot remove failed for /dev/sdb1: No such device or address
root@micemouse:~# mdadm --add /dev/md127 /dev/sdb1
mdadm: added /dev/sdb1

The array is rebuilding itself.

root@micemouse:~# mdadm -D /dev/md127
/dev/md127:
        Version : 1.2
  Creation Time : Wed Mar  6 22:16:05 2013
     Raid Level : raid1
     Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
  Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Sat May 16 15:54:20 2015
          State : clean, degraded, recovering 
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1

 Rebuild Status : 0% complete

           Name : micemouse:0  (local to host micemouse)
           UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
         Events : 3750

    Number   Major   Minor   RaidDevice State
       2       8       17        0      spare rebuilding   /dev/sdb1
       1       8       33        1      active sync   /dev/sdc1

Trying out "watch -n 2 cat /proc/mdstat" to check on its progress (every two seconds in this case).

Every 2.0s: cat /proc/mdstat                            Sat May 16 16:00:38 2015

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [ra
id10]
md127 : active raid1 sdb1[2] sdc1[1]
      1953380160 blocks super 1.2 [2/1] [_U]
      [>....................]  recovery =  0.2% (5763072/1953380160) finish=6519
.5min speed=4978K/sec

unused devices: <none>

Recovery in progress, but it looks a long long way away yet.

Much later...

After a very long time, /proc/mdstat mentioned that things were finished:

Every 60.0s: cat /proc/mdstat                           Sun May 17 22:23:29 2015

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [ra
id10]
md127 : active raid1 sdb1[2] sdc1[1]
      1953380160 blocks super 1.2 [2/2] [UU]

unused devices: <none>

And a closer look at the raid device, shows:

root@micemouse:~# mdadm -D /dev/md127
/dev/md127:
        Version : 1.2
  Creation Time : Wed Mar  6 22:16:05 2013
     Raid Level : raid1
     Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
  Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Sun May 17 20:46:50 2015
          State : clean 
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : micemouse:0  (local to host micemouse)
           UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
         Events : 5216

    Number   Major   Minor   RaidDevice State
       2       8       17        0      active sync   /dev/sdb1
       1       8       33        1      active sync   /dev/sdc1

The important part being "State: clean".

References

RAID1 - State : clean, degraded

https://www.howtoforge.com/community/threads/raid1-state-clean-degraded.66744/

Mdadm - recover degraded Array

https://www.thomas-krenn.com/en/wiki/Mdadm_recover_degraded_Array

Partial RAID-1 mounting

I recently (read: ages ago) needed to bring my RAID1 disks to a friend, and it seemed silly for a mirrored disk array to bring more than 1 harddisk. Surely a single harddisk would cover it? Mount it read-only and away we go.

Let's take /dev/sdb1 drive of an array {/dev/sdb1, /dev/sdc1}.

The simplest way is to just start up the raid array using mdadm with a single disk². But for some reason I failed to get this to work.

One way to do it (according to [1]), is to assume that the raidarray uses ext4 as an underlying filesystem and then just fiddle with the offsets.

The command "mdadm --examine /dev/sdb1" shows "Data Offset : 262144 sectors".

Then create a loop device, starting from that offset multiplied by the mdadm sector size (512).

losetup --find --show --read-only --offset $((262144*512)) /dev/sdb1

Than, simply mount the loop device.

mount /dev/loop0 /mnt/raid1/

References

[1] StackExchange - How to mount/recover data on a disk that was part of a mdadm raid 1 on another machine?

http://unix.stackexchange.com/questions/64889/how-to-mount-recover-data-on-a-disk-that-was-part-of-a-mdadm-raid-1-on-another-m

[2] sleeplessbeastie's notes - How to mount software RAID1 member using mdadm

https://blog.sleeplessbeastie.eu/2012/05/08/how-to-mount-software-raid1-member-using-mdadm/

Mirroring a.k.a. RAID 1 - Followup

When connecting my two mirrored drives created with mdadm to another machine (my labtop in this case), this happens:

[ 3006.059308] usb 2-1.1: new high-speed USB device number 3 using ehci_hcd
[ 3006.145807] usb 2-1.1: New USB device found, idVendor=1058, idProduct=1021
[ 3006.145818] usb 2-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 3006.145823] usb 2-1.1: Product: Ext HDD 1021
[ 3006.145828] usb 2-1.1: Manufacturer: Western Digital
[ 3006.145832] usb 2-1.1: SerialNumber: 574D43315431373033343231
[ 3006.149589] scsi8 : usb-storage 2-1.1:1.0
[ 3007.154933] scsi 8:0:0:0: Direct-Access WD Ext HDD 1021 2021 PQ: 0 ANSI: 4
[ 3007.157133] sd 8:0:0:0: [sdb] 3907024896 512-byte logical blocks: (2.00 TB/1.81 TiB)
[ 3007.157508] sd 8:0:0:0: Attached scsi generic sg2 type 0
[ 3007.158940] sd 8:0:0:0: [sdb] Write Protect is off
[ 3007.158956] sd 8:0:0:0: [sdb] Mode Sense: 17 00 10 08
[ 3007.160824] sd 8:0:0:0: [sdb] No Caching mode page present
[ 3007.160835] sd 8:0:0:0: [sdb] Assuming drive cache: write through
[ 3007.166300] sd 8:0:0:0: [sdb] No Caching mode page present
[ 3007.166312] sd 8:0:0:0: [sdb] Assuming drive cache: write through
[ 3007.182900] sdb: sdb1
[ 3007.188055] sd 8:0:0:0: [sdb] No Caching mode page present
[ 3007.188067] sd 8:0:0:0: [sdb] Assuming drive cache: write through
[ 3007.188073] sd 8:0:0:0: [sdb] Attached SCSI disk
[ 3007.764265] md: bind
[ 3013.631277] usb 2-1.2: new high-speed USB device number 4 using ehci_hcd
[ 3013.717235] usb 2-1.2: New USB device found, idVendor=1058, idProduct=1021
[ 3013.717246] usb 2-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 3013.717251] usb 2-1.2: Product: Ext HDD 1021
[ 3013.717256] usb 2-1.2: Manufacturer: Western Digital
[ 3013.717261] usb 2-1.2: SerialNumber: 574D43315432313138383334
[ 3013.718543] scsi9 : usb-storage 2-1.2:1.0
[ 3014.722760] scsi 9:0:0:0: Direct-Access WD Ext HDD 1021 2021 PQ: 0 ANSI: 4
[ 3014.724734] sd 9:0:0:0: Attached scsi generic sg3 type 0
[ 3014.725314] sd 9:0:0:0: [sdc] 3907024896 512-byte logical blocks: (2.00 TB/1.81 TiB)
[ 3014.727731] sd 9:0:0:0: [sdc] Write Protect is off
[ 3014.727744] sd 9:0:0:0: [sdc] Mode Sense: 17 00 10 08
[ 3014.729563] sd 9:0:0:0: [sdc] No Caching mode page present
[ 3014.729575] sd 9:0:0:0: [sdc] Assuming drive cache: write through
[ 3014.735276] sd 9:0:0:0: [sdc] No Caching mode page present
[ 3014.735287] sd 9:0:0:0: [sdc] Assuming drive cache: write through
[ 3014.753371] sdc: sdc1
[ 3014.758786] sd 9:0:0:0: [sdc] No Caching mode page present
[ 3014.758796] sd 9:0:0:0: [sdc] Assuming drive cache: write through
[ 3014.758802] sd 9:0:0:0: [sdc] Attached SCSI disk
[ 3015.063908] md: bind
[ 3015.153298] md: raid1 personality registered for level 1
[ 3015.153537] bio: create slab at 1
[ 3015.153667] md/raid1:md127: not clean -- starting background reconstruction
[ 3015.153670] md/raid1:md127: active with 2 out of 2 mirrors
[ 3015.153729] md127: detected capacity change from 0 to 2000261283840
[ 3015.161179] md127: unknown partition table

Turns out, a device is automatically made, called /dev/md127.

Mirroring a.k.a. RAID 1

Introduction

I have recently bought 2 WD (Western Digital) 2 TB Elements external (USB 2.0) harddrives. I am using them for making Backups, therefore I considered a NAS to be overkill.

This article is pure for me to remember the steps I took to get RAID 1 working, and which devices I have, and what my logs said.

The steps followed are basically the steps as mentioned in [1].

Steps

# yum install mdadm
# apt-get update && apt-get install mdadm
# fdisk /dev/sdb
# fdisk /dev/sdc
# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1
# mkfs.ext3 -j -Lraid1 -v -m1 /dev/md0
-- -v = verbose
-- -m1 = percentage of disk used for Root, 1% is more than enough (=20gb)
-- -j = journaling
-- -L = label, in this case "raid1".
# mkdir /mnt/raid1
# mount /dev/md0 /mnt/raid1
# df -H

What my system said in "dmesg", once the drives were under power and plugged into USB ports.

[ 1128.092533] usb 2-1.3: new high-speed USB device number 8 using ehci_hcd
[ 1128.189507] Initializing USB Mass Storage driver...
[ 1128.189698] scsi10 : usb-storage 2-1.3:1.0
[ 1128.189820] usbcore: registered new interface driver usb-storage
[ 1128.189824] USB Mass Storage support registered.
[ 1129.190936] scsi 10:0:0:0: Direct-Access     WD       Ext HDD 1021     2021 PQ: 0 ANSI: 4
[ 1129.192184] sd 10:0:0:0: Attached scsi generic sg2 type 0
[ 1129.195482] sd 10:0:0:0: [sdb] 3907024896 512-byte logical blocks: (2.00 TB/1.81 TiB)
[ 1129.197278] sd 10:0:0:0: [sdb] Write Protect is off
[ 1129.197284] sd 10:0:0:0: [sdb] Mode Sense: 17 00 10 08
[ 1129.199146] sd 10:0:0:0: [sdb] No Caching mode page present
[ 1129.199153] sd 10:0:0:0: [sdb] Assuming drive cache: write through
[ 1129.204101] sd 10:0:0:0: [sdb] No Caching mode page present
[ 1129.204118] sd 10:0:0:0: [sdb] Assuming drive cache: write through   
[ 1129.220472]  sdb: sdb1
[ 1129.225647] sd 10:0:0:0: [sdb] No Caching mode page present
[ 1129.225653] sd 10:0:0:0: [sdb] Assuming drive cache: write through
[ 1129.225658] sd 10:0:0:0: [sdb] Attached SCSI disk
[ 1131.676246] usb 2-1.8: new high-speed USB device number 9 using ehci_hcd
[ 1131.770350] scsi11 : usb-storage 2-1.8:1.0
[ 1132.770974] scsi 11:0:0:0: Direct-Access     WD       Ext HDD 1021     2021 PQ: 0 ANSI: 4
[ 1132.772018] sd 11:0:0:0: Attached scsi generic sg3 type 0
[ 1132.773413] sd 11:0:0:0: [sdc] 3907024896 512-byte logical blocks: (2.00 TB/1.81 TiB)
[ 1132.775265] sd 11:0:0:0: [sdc] Write Protect is off
[ 1132.775274] sd 11:0:0:0: [sdc] Mode Sense: 17 00 10 08
[ 1132.777192] sd 11:0:0:0: [sdc] No Caching mode page present
[ 1132.777198] sd 11:0:0:0: [sdc] Assuming drive cache: write through
[ 1132.781952] sd 11:0:0:0: [sdc] No Caching mode page present
[ 1132.781958] sd 11:0:0:0: [sdc] Assuming drive cache: write through
[ 1132.800861]  sdc: sdc1
[ 1132.805944] sd 11:0:0:0: [sdc] No Caching mode page present
[ 1132.805950] sd 11:0:0:0: [sdc] Assuming drive cache: write through
[ 1132.805956] sd 11:0:0:0: [sdc] Attached SCSI disk

Originally, the drives came preformatted with NTFS, but that was easily fixed.

root@micemouse:~# fdisk /dev/sdb

Command (m for help): p

Disk /dev/sdb: 2000.4 GB, 2000396746752 bytes
255 heads, 63 sectors/track, 243201 cylinders, total 3907024896 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: 0x000575ba

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048  3907024895  1953511424    7  HPFS/NTFS/exFAT

Command (m for help):

root@micemouse:~# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1
mdadm: /dev/sdb1 appears to be part of a raid array:
    level=raid0 devices=0 ctime=Thu Jan  1 01:00:00 1970
mdadm: partition table exists on /dev/sdb1 but will be lost or
       meaningless after creating array
mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
mdadm: /dev/sdc1 appears to be part of a raid array:
    level=raid0 devices=0 ctime=Thu Jan  1 01:00:00 1970
mdadm: partition table exists on /dev/sdc1 but will be lost or
       meaningless after creating array
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

Time to grab a cup of something for the next part. Formatting 2 Terabytes takes a while.

root@micemouse:~# mkfs.ext3 -j -Lraid1 -v -m1 /dev/md0
mke2fs 1.42 (29-Nov-2011)
fs_types for mke2fs.conf resolution: 'ext3'
Filesystem label=raid1
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
122093568 inodes, 488345040 blocks
4883450 blocks (1.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=0
14904 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
    4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
    102400000, 214990848

Allocating group tables: done                           
Writing inode tables: done                           
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

Status

# watch -n 2 cat /proc/mdstat   
# tail -f /proc/mdstat
# mdadm --query /dev/md0

maartenl@micemouse:~$ tail -f /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdc1[1] sdb1[0]
      1953380160 blocks super 1.2 [2/2] [UU]
      [>....................]  resync =  0.1% (2340864/1953380160) finish=1801.8min speed=18046K/sec
     
unused devices: <none>
tail: /proc/mdstat: file truncated
Personalities : [raid1]
md0 : active raid1 sdc1[1] sdb1[0]
      1953380160 blocks super 1.2 [2/2] [UU]
      [>....................]  resync =  0.1% (2340864/1953380160) finish=1801.8min speed=18046K/sec
     
unused devices: <none>

References

[1] How to configure usb mybooks as RAID 1

http://martystech.lucidway.org/2008/01/how-to-configure-usb-mybooks-as.html

Linux - creating software reaid one arrays

http://www.cyberciti.biz/faq/linux-creating-software-raid-one-arrays/

Software RAID HOWTO

http://tldp.org/HOWTO/Software-RAID-HOWTO.html

RAID - Wikipedia

http://en.wikipedia.org/wiki/RAID

Linux RAID HOWTO

https://raid.wiki.kernel.org/index.php/Linux_Raid