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You have two devices of approximately same size, and you want the two to be mirrors of each other. Eventually you have more devices, which you want to keep as stand-by spare-disks, that will automatically become a part of the mirror if one of the active devices break.

Set up the /etc/raidtab file like this:

raiddev /dev/md0
raid-level 1
nr-raid-disks 2
nr-spare-disks 0
persistent-superblock 1
device /dev/sdb6
raid-disk 0
device /dev/sdc5
raid-disk 1

Ok, now we're all set to start initializing the RAID. The mirror must be constructed, eg. the contents (however unimportant now, since the device is still not formatted) of the two devices must be synchronized.

Issue the mkraid /dev/md0 command to begin the mirror initialization. Check out the /proc/mdstat file. It should tell you that the /dev/md0 device has been started, that the mirror is being reconstructed, and an ETA of the completion of the reconstruction.

Reconstruction is done using idle I/O bandwidth. So, your system should still be fairly responsive, although your disk LEDs should be glowing nicely. The reconstruction process is transparent, so you can actually use the device even though the mirror is currently under reconstruction. Try formatting the device, while the reconstruction is running. It will work. Also you can mount it and use it while reconstruction is running. Of Course, if the wrong disk breaks while the reconstruction is running, you're out of luck.

Autodetection allows the RAID devices to be automatically recognized by the kernel at boot-time, right after the ordinary partition detection is done. This requires several things: i) You need autodetection support in the kernel ii) You must have created the RAID devices using persistent-superblock iii) The partition-types of the devices used in the RAID must be set to 0xFD (use fdisk and set the type to "fd".