On Tue Feb 05, 2013 at 12:20:48PM +0000, Brian Candler wrote:

(Ubuntu 12.04.2, kernel 3.2.0-37-generic)

I created a RAID5 array with 22 data disks and 2 hot spares, like this:

    # mdadm --create /dev/md/dbs -l raid5 -n 22 -x 2 -c 512 -b internal /dev/sd{b..y}

However I'm having difficultly understanding the mdstat output.

    # cat /proc/mdstat
    Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
    md127 : active raid5 sdw[24] sdy[23](S) sdx[22](S) sdv[20] sdu[19] sdt[18] sds[17] sdr[16] sdq[15] sdp[14] sdo[13] sdn[12] sdm[11] sdl[10] sdk[9] sdj[8] sdi[7] sdh[6] sdg[5] sdf[4] sde[3] sdd[2] sdc[1] sdb[0]
          61532835840 blocks super 1.2 level 5, 512k chunk, algorithm 2 [22/21] [UUUUUUUUUUUUUUUUUUUUU_]
          [=>...................]  recovery =  6.0% (176470508/2930135040) finish=706.9min speed=64915K/sec
          bitmap: 0/22 pages [0KB], 65536KB chunk

    unused devices: <none>
    # 

Problems:

1. The UUUU_ and [22/21] suggests that one disk is bad, but is that true?
And if so which one?

No, that's normal. A RAID5 (or RAID6) array is created in a degraded
form, then the last disk(s) are recovered (it's the quickest way of
getting the array ready for use).

Output from "dmesg | grep -3 sd" is at end of this mail, and it doesn't show
any errors.

All the disks have the same event counter in the metadata:

# for i in /dev/sd{b..y}; do mdadm --examine $i | grep Events; done | sort | uniq -c
     24          Events : 594

2. /proc/mdstat shows the member disks numbered 0..20 and 22..24, what
happened to 21 ?

21 would (I think) be the "missing" one from the original array creation
(with 22..24 as the spares). The numbers themselves don't really signify
anything.

HTH,
    Robin
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