I recently had a client that needed to replace a hard drive in a Linux FakeRaid RAID5 array using dmraid. The process is very unintuitive:
dmraid -a y
This will activate the raid set and should start the rebuild process. In my case this did not persist after reboot, hence /dev/mapper did not show the partition table. The solution was to rebuild the initrd image:
uname -a mkinitrd /boot/initrd-2.6.18-128.1.6.el5.img.NEW 2.6.18-128.1.6.el5
uname -a was used to identify the correct kernel. Obviously you need to update grub with the new image. Reboot otherwise you will not see the partitions on the mapper device to mount.
For some reason the whole ext3 filesystem was corrupted. I had to recreate the filesystem and rebuild the data from scratch. Not sure if it is FakeRaid's fault, or whether I made a mistake. But it certainly did not boost my confidence in software based RAID.