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I am trying to create a software raid5 array using mdadm:

$ linux # mdadm --create --verbose /dev/md0 --level=5 --raid-devices=4 --spare-devices=0 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
mdadm: layout defaults to left-symmetric
mdadm: chunk size defaults to 64K
mdadm: array /dev/md0 started.

However when inspecting /proc/mdstat

Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdd1[4] sdc1[2] sdb1[1] sda1[0]
      2930279808 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
      [>....................]  recovery =  0.3% (2970496/976759936) finish=186.1min speed=87172K/sec

unused devices: <none>

It seems one drive isn't active, so I check the details of the array:

/dev/md0:
        Version : 00.90.03
  Creation Time : Tue Jul 21 16:29:53 2009
     Raid Level : raid5
     Array Size : 2930279808 (2794.53 GiB 3000.61 GB)
  Used Dev Size : 976759936 (931.51 GiB 1000.20 GB)
   Raid Devices : 4
  Total Devices : 4
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Tue Jul 21 16:29:53 2009
          State : clean, degraded, recovering
 Active Devices : 3
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 1

         Layout : left-symmetric
     Chunk Size : 64K

 Rebuild Status : 0% complete

           UUID : ce8b2f40:821d003c:0027688e:a70977ec
         Events : 0.1

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
       2       8       33        2      active sync   /dev/sdc1
       4       8       49        3      spare rebuilding   /dev/sdd1

And it seems there are only 3 active devices, with one spare.

Is it just me, or something wrong here?

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seems like you're missing some info – Matt Rogish Jul 21 '09 at 15:08
Yeah, pressed enter too soon :-) Here is the entire question. – Yannick M. Jul 21 '09 at 15:13
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3 Answers

up vote 8 down vote accepted

This tutorial covers building a raid5 array with mdadm. You can't use raid5 with less than three devices.

What do you mean by a spare? Raid5 is designed to provide redundancy to survive the failure of one device. If it doesn't do that then it isn't raid5 anymore.

Edit after question edit: 

   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.

That seems to be it, mdadm is setting up the new array as degraded with a spare, and then putting the spare in to build the array.

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Yes, but apart from the parity info distributed under the 4 drives, it seems it is talking about a spare drive. (Active: 3, Working: 4, Spare Devices: 1) And I am wondering if something is wrong... – Yannick M. Jul 21 '09 at 15:17
Also, as you can see in the tutorial you linked, /proc/mdadm suggest the 3 drives are all fine [UUU] whereas my array says one isn't active [UUU_]. I was fearing a faulty drive, but it seems it is spare. – Yannick M. Jul 21 '09 at 15:20
Nothing to worry about then. Thanks :-) – Yannick M. Jul 21 '09 at 15:24
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up vote 1 down vote

It also shows as currently recovering and your spare drive is showing rebuilding. I'd be interested to see what your output looks like after it finishes.

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I will paste the output after it finishes. – Yannick M. Jul 21 '09 at 15:25
I just confirmed on a test box what I suspect you will see and that is that once the recovery is done, the fourth device will not be listed as a spare any more. Precisely as the answer above states. – TCampbell Jul 21 '09 at 15:34
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up vote 0 down vote

if you got clean disks, and you are making a brand new array, the faster option around the initial rebuild is

(in my case)

mdadm --create --verbose --force --assume-clean /dev/md0 --level=5 --raid-devices=4 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1
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In my case creating a brand new, clean array, was what I was actually looking for, and, actually, what I expected from mdadm to do in the first place. – wireman Jan 24 '11 at 2:54
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