I'm considering creating something like "hierarchical raid" and wondering whether it is possible using pure mdraid. Moreover, I'm going to boot from this device. I'm using Ubuntu Server 12.04 LTS with Grub2 bootloader.

Motivation behind doing that is: I have 4 x 1tb 7200rpm disks. Two are newer and faster (up to 200mb/sec) and other two are slower (up to 140mb/sec). I want to create RAID-0 device from them. When creating such RAID-0 directly from 4 hard disks, I get summary speed up to ~480mb/sec. That is roughly 4*120mb/sec, so RAID-0 works with speed of the slowest device. I have an idea to create a separate RAID-0 md0 device from 500gb partitions of slower hard disks. Theoretically, this md0 device will have speed 2*140=240~280mb/sec. After that, I'm going to add this md0 device to RAID-0 with faster disks, finishing with up to 3*200=600mb/sec. Stripe-width for this raid will be 2x times bigger than for underlying raid with slow disks.

Questions are:

• is it possible or I'm missing something?
• will that work as expected?
• can I boot from such consolidated raid device?
• any better ideas?
• any pitfalls?

I don't want to use fakeraid for consolidating slow disks for multiple reasons (portability, ability to customize parameters and so on).

PS Speed is needed for home virtualization server and just for experience/fun. Reliability is provided via regular automatic backups to a separate device.

PPS I considered also using different stripe-width for hard disks with different speed in single raid, but mdraid does not seem to support that.