Raid5 setup uses only 2 out of 3 drives

divot_powell · 09-29-2009, 06:59 AM

Hi,

I'm setting up a raid 5 array of 3x1.5TB drives and am a little confused with the output that I am getting. I want the 3 drives in the array with the parity spread across them so I have 3TB total space and one backup drive.

I'm following this howto and I'm a little confused at the output I am getting.

Using the command:

Code:

sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sda1 /dev/sdb1 /dev/sdc1

Gives the output:

Code:

raid level 5 set md0 active with 2 out of 3 devices, algorithm 2

However I wanted a raid array using all three drives.

Running:

Code:

sudo mdadm --detail /dev/md0

Gives:

Code:

/dev/md0:
        Version : 00.90.03
  Creation Time : Tue Sep 29 12:45:07 2009
     Raid Level : raid5
     Array Size : 2930271872 (2794.53 GiB 3000.60 GB)
  Used Dev Size : 1465135936 (1397.26 GiB 1500.30 GB)
   Raid Devices : 3
  Total Devices : 3
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Tue Sep 29 12:47:37 2009
          State : clean, degraded, recovering
 Active Devices : 2
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 1

         Layout : left-symmetric
     Chunk Size : 64K

 Rebuild Status : 0% complete

           UUID : c3a9f41d:150589c9:02f70688:dbf4c2a5 (local to host servey-lite)
         Events : 0.4

    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       1       8        1        1      active sync   /dev/sda1
       3       8       33        2      spare rebuilding   /dev/sdc1

Clearly stating that one of the drives is spare. It's almost as if it's using striped raid instead of raid5.

Am I doing something wrong? Or am I just being impatient and I need to allow the spare to rebuild first?

I'm using Ubuntu 8.04 Server LTS.

Thanks in advance for your help,

Dan

divot_powell · 09-29-2009, 07:07 AM

Having literally just made this post I finally found this page:

Quote:

When creating a RAID5 array, mdadm will automatically create a degraded array with an extra spare drive. This is because building the spare into a degraded array is in general faster than resyncing the parity on a non-degraded, but not clean, array. This feature can be overridden with the --force option.

I've used the --force option when creating the array and everything appears to be working as the way I wanted it to, and the sync appears to be running much faster.

If you have noticed anything that I have missed or done incorrectly, please let me know.