LinuxQuestions.org - New to mdadm. Can someone interpret my /proc/mdstat?

Hi,

I have Ubuntu Server 11.04 running in Virtualbox on my Mac (this is so I can test setting up the server a thousand times before actually building a physical one).

In virtualbox, I have given the server:

8GB drive for the OS
3x4GB drives for a RAID 5 array.

after having run mdadm, I check it by looking at /proc/mdstat.

Code:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid 5] [raid4] [raid10]

md0 : active raid5 sdd1[3] sdc1[1] sdb1[0]

      8383488 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]

(Hopefully there are no typos above, I had to type it in by hand because copying is not currently working)
My concern is that sdd1 is marked as [3]. I have almost no understanding of mdadm (but I am actively trying to learn). This website http://unthought.net/Software-RAID.H...D.HOWTO-6.html would suggest that - if I am not using a spare - the highest number any drive should be exhibiting would be n-1 (where n is the number of drives in the array). In my case, that should be [2], no? But here is is showing [3].

typing:

Code:

mdadm --detail /dev/md0

gets me the following output:

Code:

/dev/md0:

        Version : 1.2

  Creation Time : Sun Sep 25 13:33:42 2011

    Raid Level : raid5

    Array Size : 8383488 (8.00 GiB 8.58 GB)

  Used Dev Size : 4191744 (4.00 GiB 4.29 GB)

  Raid Devices : 3

  Total Devices : 3

    Persistence : Superblock is persistent



    Update Time : Sun Sep 25 13:47:16 2011

          State : clean

 Active Devices : 3

Working Devices : 3

 Failed Devices : 0

  Spare Devices : 0



        Layout : left-symmetric

    Chunk Size : 512K



          Name : ubuntu:0  (local to host ubuntu)

          UUID : 8b7364ba:f9534103:4fd09587:d5c2659e

        Events : 18

Looking at this output I am mildly reassured because it shows 3 active devices and 0 spare devices. Can someone explain what I am supposed to glean from /proc/mdstat?

Thanks
Ben