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Assuming I have enabled in BIOS a (presumably) hardware RAID in mirror aka RAID1 mode (as opposed to Software-one provided by the OS like Windows Storage Spaces), how is it actually ensuring data consistency between 2, and 3+ drives in the RAID 1 (mirror) configuration:

  • What happens if I replace one of the RAID 1 drives with an identical (same brand, model, capacity) one but different data (maybe empty or just garbage or different data)? How does it know which is the "good/trusty" drive it should read from to reconstruct RAID array? And will it even detect the change at all, or will it assume that nothing was changed and start spitting out garbage (especially when doing parallel reads for performance)?

    • Is it any different if a different drive model is used (but same capacity)?

  • Why is a special AHCI RAID driver needed for such a set up if data duplication (mirroring) is performed at a sub-OS level? And would some kind of messing up with the driver (like it does not load for example) completely break or damage RAID 1 array and lead to data loss?

  • What happens, and how it should be done, if I want to migrate the 2 drives from being attached to on Motherboard to another one? Do I lose my RAID setup, even if both drives at the point of migration contain identical data? Is there a way on the new system to tell BIOS/system in advance that these 2 drives come from RAID 1, they are synced, do not initialize them, simply remember the fact that they are in RAID 1 already?

  • Assuming there is a BIOS-level RAID1 available (say at the motherboard level), and we are talking about 2 similar drives (same tech, i.e. both SATA SSDs or both NVME SSDs or both HDDs) of the same capacity, when (if ever) would it make sense to use Software RAID (like Windows Storage Spaces) instead? Put another way, if available should hardware RAID 1 be always preferred?

Sorry for several related questions, but all of them have to do with RAID 1 data consistency insurance, and so I think deserve to be put together.

Fit Nerd
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1 Answers1

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Based on my limited experience with the various BIOS-level hardware RAID systems, I will take a moment to answer here.

  • How does it know about a new disk? As I understand it, the BIOS actually places data in the MBR identifying a disk as part of a RAID set. The new disk doesn't have that marker, so its contents are not RAID, so it gets overwritten.
  • Can you use a different drive model with the same capacity? Yes. I've mixed Seagate and WD drives of the same capacity in RAID sets.
  • Can you move RAID drives from one hardware RAID system to another? In my experience, no. The MBR information mentioned in my first point above varies from one RAID BIOS to another, so unless something has changed about standardization of that data block since I last dared to try this, the new RAID BIOS will not recognize that the drives you're dropping on it are a RAID set, or even contain valid data. Which is the reason for my last point:
  • When does it make sense to use software RAID instead? Well, in my experience, always. I have lost more data due to hardware RAID controllers failing and leaving me two loaded, mirrored, and useless disks than I have to all failures in the much longer time I've been using (specifically Linux mdadm) software RAID.
tsc_chazz
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