raid array metadata info (mdadm)
 

When we assemble a raid array, from where does it load configuration information for that array? I thought it refers to /etc/mdadm.conf file, but in my system, mdadm.conf file doesn't even contain all information. Still it is able to successfully assemble previously created device.

# cat /etc/mdadm.conf
DEVICE /dev/sd[bcdjkl]1
DEVICE /dev/loop[012345]
MAILADDR root
ARRAY /dev/md0 level=raid1 num-devices=2 UUID=8be34453:45298936:fae1028b:65b2510c


Above, the information about devices that comprises device md0 is not present (e.g. /dev/sda1, /dev/sda2 etc). Its not revealed by mdadm scan either.

# mdadm --detail --scan
ARRAY /dev/md0 level=raid1 num-devices=2 UUID=8be34453:45298936:fae1028b:65b2510c


Still it is able to assemble the device.

# mdadm --assemble /dev/md0
mdadm: /dev/md0 has been started with 2 drives.



How did it find the underlying devices of md0 while assembling it?

Regards,

You might want to check out this wiki regarding RAID superblock storage and format.


Hope this helps.

Regards,

Fordeck

Thanks for the reply.. that was helpful..

However, there is one more thing I am wondering... I know mdadm uses device mapper only at the backend.. so I decided to get all info about raid array I created with mdadm command through dmsetup command. However when I can still see array status in mdadm --scan command, I am not getting any info in dmsetup info or dmsetup table command. Why is it that, dmsetup should show information abuot mdadm created array right?

Regards,