Mirroring the OS should really be done up front at build time.
Doing it after-the-fact is dangerous and not very easy.
Before you begin, you can do this with SCSI, SATA, or IDE drives, BUT if you use IDE drives, you MUST set up both disks as MASTER disks on two different channels! [...]
Building a Linux iSCSI SAN
(Update: The newest FreeNAS is easier and does all the bits and pieces. Build a FreeNAS if you can)
Start with Ubuntu 9.04 64 Bit Server
Do a base install, mirror the OS drives if you want, but it's trivial to rebuild if done properly
Mirroring the OS [...]
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