Migrate from RAID 6 to RAID 5 with mdadm


lian-li-qo08bI have been using a quite secure setup for the last couples of years with a 4 drive RAID 6 setup. This setup can tolerate two disk failures without any data loss. Recently though, I have been getting close to the edge of the filesystem and could use some extra space and since I have both monthly backup to an external hard drive and nightly offsite backup I am actually not very afraid of a data loss on a RAID 5 setup. So I have planned to change my 4 disk RAID 6 to a 4 disk RAID 5 without any spares.

A word of caution: Please do not do any of the actions below before a backup has been made.

Changing the raid level

Using mdadm it is very easy to change the raid level. The command below changes the raid level from my previous RAID 6 setup with 4 disks to a RAID 5 with 3 active disks and a spare. This reason for using 3 disks and a spare is that mdadm recommends using having a spare when downgrading. Since I am no hurry this is fine with me. Additionally, the command below saves some critical data to a backup file during the process to ease recovery if anything should go wrong or power should be lost. This should be done to a hard disk that is not a part of the raid. The backup of file is not big. In my case it was about 30 MB and was saved to my root that resides on the system SSD.

mdadm --grow /dev/md0 --level=raid5 --raid-devices=3 --backup-file=/root/mdadm-backupfile

This process can take a long while (around 24 hours in my case), but during the raid is fully functional and it is all done in the background. To monitor the progress use either of the commands below (assuming that your raid device is md0). Please note that this is not output from the actual process, but output from my functional raid 5 when it is all done and I am writing this blog entry:

[root@kelvin ~][22:00]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sde1[0] sdb1[3] sdd1[4] sdc1[1]
      2930280960 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4] [UUUU]
      bitmap: 2/8 pages [8KB], 65536KB chunk
unused devices: <none>
[root@kelvin ~][22:00]# mdadm --detail /dev/md0
        Version : 1.2
  Creation Time : Tue Feb  1 23:24:46 2011
     Raid Level : raid5
     Array Size : 2930280960 (2794.53 GiB 3000.61 GB)
  Used Dev Size : 976760320 (931.51 GiB 1000.20 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent
  Intent Bitmap : Internal
    Update Time : Thu Aug 29 22:00:58 2013
          State : active
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0
         Layout : left-symmetric
     Chunk Size : 512K
           Name : kelvin:0  (local to host kelvin)
           UUID : 3f335d48:4b43c1d5:6ee5c975:1428eb56
         Events : 1186983
    Number   Major   Minor   RaidDevice State
       0       8       65        0      active sync   /dev/sde1
       1       8       33        1      active sync   /dev/sdc1
       4       8       49        2      active sync   /dev/sdd1
       3       8       17        3      active sync   /dev/sdb1

Growing the raid from 3 to 4 active disks

After this operation my raid had 3 active disk and a spare. I would like to expand my raid, so I will convert the spare into an active disk using the following command:

mdadm --grow -n 4 /dev/md0

This process is also done while the raid is active and took about 12 hours. My server actually crashed during the process, probably from a XBMC bug, but the process resumed without a hitch when I came home and rebooted it. 🙂

Resizing the filesystem to the disk

Finally I needed to expand the filesystem (ext4 in my case) to use all the new space. So I changed to init level 1 (single user mode) to make sure that all the users are off the system.

init 1

Next I unmounted the raid system

umount /home/

Forced a check to ensure that the filesystem is healthy

e2fsck -f /dev/md0

This took about 5 minutes. Finally I resized my ext4 partition to fill all the available space on md0:

resize2fs /dev/md0

This took about 10 minutes. I could have mounted and changed back to init level 2 (debian multi-usermode), but I decided to restart and had everything up and running shortly after. I now have plenty of space left on my /home partition:

[root@kelvin ~][22:08]# df -hTl
Filesystem     Type      Size  Used Avail Use% Mounted on
/dev/sda2      ext4       50G   18G   30G  37% /
/dev/md0       ext4      2.7T  1.5T  1.3T  56% /home
/dev/sda1      ext2      291M   92M  184M  34% /boot

As a finally note, I noticed strange activity in my munin logging on the disks after the reboot. All the drives in the raid were utilized at around 5% even though nothing was happening. Using iotop it turned out to be a command called “ext4lazyinit” running in the background:

When lazy_itable_init extended option is passed to mke2fs, it
considerably speed up filesystem creation because inode tables are
not zeroed out, thus contains some old data. When this fs is mounted
filesystem code should initialize (zero out) inode tables…

For purpose of zeroing inode tables it introduces new kernel thread
called ext4lazyinit, which is created on demand and destroyed, when it
is no longer needed…

From: http://lists.openwall.net/linux-ext4/2010/09/08/11



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3 Responses to Migrate from RAID 6 to RAID 5 with mdadm

  1. John Vickers says:

    Hi, nice article and I was thinking of doing exactly this on my QNAP TS-420 which has 4x3TB drives – it took a week to go from raid 5 to raid 6 by using the QNAP desktop utilities. Now I want to go the other way. Does the method above preserve data? I do of course have backups.

  2. I have no idea on how the QNAP TS-420 works or if even uses linux mdadm, but the method I described above on my Linux box preserves all data, but as always in these cases make sure you have a complete backup just in case.

  3. John Vickers says:

    I tried this on my QNAP TS-420 but I got an error message from MDADM that I could not decipher so I took the easy route by resetting the QNAP and restoring my data to the new RAID 5 volume.

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