Monday, 17 May 2010

RAID system that just works

On Tom's hardware, ratbat asks for a RAID that just works (no matter what).

His requirements are fairly simplistic:
  1. Minimum maintenance,
  2. Minimum downtime in case the drive fails,
  3. Simplest possible recovery,
  4. The setup has to boot from the RAID.

The only match for these criteria is RAID1 (mirror).

As you know, the RAID levels are (exotics excluded) RAID0, 1, 5, and 0+1. Each RAID level has its own strengths and weaknesses, but these are too complex to fit into this post. For more read on this, check RAID levels reference.

RAID0 is eliminated from the contest because it is not fault tolerant.

RAID5 fails the "Simplest possible recovery" requirement. To have a bootable RAID, the hardware controller is required. In case the controller dies, the recovery can get complicated, requiring RAID recovery software, which may be fairly tricky to operate properly.

So we are left with RAID1 (mirror) and RAID0+1.

RAID1 wins the contest because of its simplicity. If one of the drives dies, the mirror just continues to operate. if the controller dies, you can get any of the two identical drives, plug it into any compatible PC and have the data immediately available. You can even plug the drive into the mainboard (bypassing the dead RAID controller) and boot from it. The only thing you lose by doing so is redundancy.

Now we have to choose between hardware and software implementations of the RAID1.
On Windows, the RAID1 is only available in Server versions, so there's really not much choice. Either get a hardware controller, or use a controller integrated into the motherboard (if available).

Make sure to test your RAID once you have created it and placed the OS on it.
1. Unplug each drive in turn (make sure to power off before doing it) and check how the controller responds.
2. Unplug each drive in turn and attach it to the non-RAID mainboard port. Check if the system boots from that drive.
3. After each of these steps, the resynchronization of the array is needed, so it would take some time.

Once you are done with that, install the software as usual.

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