heres a few errors mdadm might give you, which might not be as easy to find an explanation for.(might just be me)

root@xxxx:~# mdadm –add /dev/md1 /dev/sdj
mdadm: add new device failed for /dev/sdj as 2: Invalid argument

and from syslog

Aug 18 11:35:23 xxxx kernel: [ 3419.147467] md: sdj does not have a valid v0.90 superblock, not importing!
Aug 18 11:35:23 xxxx kernel: [ 3419.147472] md: md_import_device returned -22

to make a way to long, lame story short, i will post a few notes to self.

1) DO NOT EXPECT YOU CAN CREATE A 2-DISKS RAID-5 ARRAY A AND EXPAND IT LATER

2) DO NOT EXPECT YOU CAN CREATE A 3-DISK RAID-5 ARRAY WITH 1-MISSING DISK AND ADD IT LATER

ydadada, yes, im aware of it, you need at minimum 3 disks. dont ask me why i even tried, but mdadm DIDN’T complain at all, it just created it.

root@xxx:~# mdadm –detail /dev/md1
/dev/md1:
Version : 00.90.03
Creation Time : Sun Aug 17 18:52:55 2008
Raid Level : raid5
Array Size : 976759936 (931.51 GiB 1000.20 GB)
Used Dev Size : 976759936 (931.51 GiB 1000.20 GB)
Raid Devices : 2
Total Devices : 2
Preferred Minor : 1
Persistence : Superblock is persistent

Update Time : Mon Aug 18 11:23:27 2008
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 64K

UUID : e1962bdb:72c36721:46188f91:72750c0f (local to host xxxx)
Events : 0.4

Number  Major  Minor  RaidDevice State
0      8       1       0     active sync  /dev/sda1
1      8      17       1     active sync  /dev/sdb1