Raid 5 questions

[David Greaves <david@xxxxxxxxxxxx>]

Hi

I've perused the archives and read the docs but I'm not sure on this 
point :)

If I create an array with 3 250Gb disks and use the 'missing' option I 
should get a degraded raid5 array with 750Gb
Now, when another 250Gb disk arrives I want to add that as the missing 
disk - will that work?

Now, since I actually have the 3 disks and I'm waiting for the 4th, lets 
try it with 2 and add a third...

Clear out the superblocks so we're starting from scratch
cu:~# mdadm --stop /dev/md0
cu:~# mdadm --zero-superblock /dev/sda
cu:~# mdadm --zero-superblock /dev/sdb
cu:~# mdadm --zero-superblock /dev/sdc

Make the array.
cu:~# mdadm -v -C /dev/md0 --chunk=128 --level=5 --raid-devices=3 
/dev/sda /dev/sdb missing
mdadm: layout defaults to left-symmetric
mdadm: size set to 245117312K
mdadm: array /dev/md0 started.

What have we got?
cu:~# mdadm --detail /dev/md0
/dev/md0:
       Version : 00.90.01
 Creation Time : Thu Jun  3 16:19:26 2004
    Raid Level : raid5
    Array Size : 490234624 (467.52 GiB 502.00 GB)
   Device Size : 245117312 (233.76 GiB 251.00 GB)
  Raid Devices : 3
 Total Devices : 2
Preferred Minor : 0
   Persistence : Superblock is persistent

   Update Time : Thu Jun  3 16:19:26 2004
         State : clean, no-errors
Active Devices : 2
Working Devices : 2
Failed Devices : 0
 Spare Devices : 0

        Layout : left-symmetric
    Chunk Size : 128K

   Number   Major   Minor   RaidDevice State
      0       8        0        0      active sync   /dev/sda
      1       8       16        1      active sync   /dev/sdb
      2       0        0       -1      removed
          UUID : 8acbe990:b31287f7:32888f7d:f0edcf0c
        Events : 0.3887

Thats 2x 251Gb giving a 502Gb array - good

Add this to the config
cu:~# echo 'DEVICE /dev/sd[abcd]' >/etc/mdadm/mdadm.conf
cu:~# mdadm --detail --scan >>/etc/mdadm/mdadm.conf
cu:~# cat /etc/mdadm/mdadm.conf
DEVICE /dev/sd[abcd]
ARRAY /dev/md0 level=raid5 num-devices=3 
UUID=8acbe990:b31287f7:32888f7d:f0edcf0c
  devices=/dev/sda,/dev/sdb

Now, the new disk arrives...
cu:~# mdadm  /dev/md0 -a /dev/sdc
mdadm: hot added /dev/sdc

So it should be working fine...
cu:~# mdadm --detail /dev/md0
/dev/md0:
       Version : 00.90.01
 Creation Time : Thu Jun  3 16:19:26 2004
    Raid Level : raid5
    Array Size : 490234624 (467.52 GiB 502.00 GB)
   Device Size : 245117312 (233.76 GiB 251.00 GB)
  Raid Devices : 3
 Total Devices : 3
Preferred Minor : 0
   Persistence : Superblock is persistent

   Update Time : Thu Jun  3 16:23:21 2004
         State : clean, no-errors
Active Devices : 2
Working Devices : 3
Failed Devices : 0
 Spare Devices : 1

        Layout : left-symmetric
    Chunk Size : 128K

Rebuild Status : 0% complete

   Number   Major   Minor   RaidDevice State
      0       8        0        0      active sync   /dev/sda
      1       8       16        1      active sync   /dev/sdb
      2       0        0       -1      removed
      3       8       32        2      spare   /dev/sdc
          UUID : 8acbe990:b31287f7:32888f7d:f0edcf0c
        Events : 0.3888


Nope it's a spare?
Help - how do I make my array resilient?


Environment:
Debian testing
Linux cu.dgreaves.com 2.6.6 #1 Thu Jun 3 14:12:01 BST 2004 i686 GNU/Linux
mdadm - v1.5.0 - 22 Jan 2004

Thanks

David
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