Software RAID 1 Configuration

Question

I have created a software RAID 1 quite some while ago and it always seemed to work for me. However I am not completely sure that I have configured everything right and do not have the experience to check so I would be very grateful for some advice or just verification that all seems right so far.

I am using Linux Fedora 20 (32 bit with plans to upgrade to 64bit) The RAID 1 should consist of two 1TB SATA hard drives.

This is the output of mdadm --detail /dev/md0

/dev/md0:
        Version : 1.2
  Creation Time : Sun Jan 29 11:25:18 2012
     Raid Level : raid1
     Array Size : 976761424 (931.51 GiB 1000.20 GB)
  Used Dev Size : 976761424 (931.51 GiB 1000.20 GB)
   Raid Devices : 2
  Total Devices : 1
    Persistence : Superblock is persistent

    Update Time : Sat Jun  7 10:38:09 2014
          State : clean, degraded 
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           Name : argo:0  (local to host argo)
           UUID : 1596d0a1:5806e590:c56d0b27:765e3220
         Events : 996387

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

The RAID is mounted successfully:

friedrich@argo:~ ➤ sudo mount -l | grep md0
/dev/md0 on /mnt/raid type ext4 (rw,relatime,data=ordered)

Basically my question are:

• Why do I only have 1 active device?
• What does the State removed at bottom mean?

Also I noticed some strange error messages that I see on the console on system start and shutdown and always repeating in the background when I switch with Ctrl + Alt + F2:

...
ata2: irq_stat 0x00000040 connection status changed
ata2: SError: { CommWake DevExch }
ata2: COMRESET failed (errno=-32)
ata2: exception Emask 0x10 SAct 0x0 SErr 0x4040000 action 0xe frozen
ata2: irq_stat 0x00000040 connection status changed
ata2: SError: { CommWake DevExch }
ata2: exception Emask 0x10 SAct 0x0 SErr 0x4040000 action 0xe frozen
...

Are these errors related to the RAID? Something seems wrong with the SATA devices.. All together the system works (I can read and write to the mounted raid) but I always had these strange errors on startup shutdown (probably always in the background).

Thx for your help

Answer 1

It seems your that one of the disks was dropped out of the array. This is indicated first by the removed state which you have already noticed, and then by the following line:

State : clean, degraded

Here is what I would do if I had the same problem:

• Check carefully dmesg for any reason why the disk was dropped out of the array. You can try with dmesg | grep md: and dmesg | grep sdb (I assume the disk which is missing was sdb) and see if there are errors logged. I guess that what happened is that your drive was disconnected at some time and it was dropped from the array. It could have been dropped due to any reason like faulty cables, or broken disk controller or other.

• Identify the letter of the drive which was dropped out of the array and check it with smartctl for any issues: smartctl -a /dev/disk0. Look for possible errors under WHEN_FAILED and START OF READ SMART DATA SECTION.

• If the disk turns out to be healthy (no errors indicated by ), you can re-add it to your array: mdadm -a /dev/md0 /dev/sdb. You can monitor the rebuild progress by running cat /proc/mdstat.

Of course, I would advise you to backup your data before doing any of this stuff, as executing wrong commands may leave you without your data!