How Many Drives Can You Lose? RAID & RAIDZ Explained

Sourced from real discussions on r/homelab, r/zfs and r/DataHoarder, cross-checked against live PricePerGig listings.

When you’ve got a RAID array, you can afford for exactly one drive to fail without any issues; but depending on how you’ve set the RAID up, you are now in the ‘danger zone’ and urgently need to replace the down drive. If you’ve set up RAID with 2 redundant drives as explained below, then you still need to replace the drive, but you’re not at immediate risk of data loss, still, better get it replaced ASAP.

I remember when I first discovered RAID5, it was magical, the idea of adding practically as many drives as you like and keeping more than 50% of the storage, with redundancy, and read speedups, amazing. Then you realise writing data is painfully slow and SSDs take care of the speed way better. Still, RAID5 still has its use, and that is surviving a downed drive, and if you purchased all your drives new at the same time, you better get ready for another failure, soon.

The table above shows live prices for new CMR internal drives of 6TB and up on your marketplace, because more redundancy simply means buying a few more drives, and they are cheaper than you think.

What the levels actually mean

Let me translate the jargon into plain survival numbers.

• Mirror / RAID1. Two drives holding the same data. One can die, the other carries on. Simple and fast.
• RAID5 / RAIDZ1. Spreads data plus one parity drive across the array. Any one drive can fail. You lose the capacity of one drive to parity.
• RAID6 / RAIDZ2. Two parity drives. Any two drives can fail at once. The sweet spot for most home arrays.
• RAIDZ3. Three parity drives. Any three can fail. For big, precious arrays where downtime is not an option.

More parity means more safety, at the cost of a couple more drives. That is the whole trade.

Why RAIDZ2 is the home sweet spot

If you take one thing away, make it this.

For a serious home array, RAIDZ2 is usually the right call, not RAIDZ1.

Here is why. With RAIDZ1, one drive can fail and you are fine, but you are now running with zero safety net while you rebuild. Rebuilds hammer the remaining drives for hours, and that stress is exactly when a tired second drive likes to die. If it does, everything is gone.

RAIDZ2 means a second failure during that rebuild is a shrug, not a funeral. On big modern drives, that extra parity drive is cheap insurance.

Redundancy is not a backup

I have to say it, because people forget constantly.

RAID and RAIDZ keep your array running through drive failures. That is all.

They do nothing against an accidental delete, ransomware, a dodgy power supply frying everything, or a fire. All of those hit every drive at once.

So redundancy plus a real offsite backup. Always both, never just one.

How I would build it

The practical plan.

• Pick your capacity using the best price per TB on the live table above.
• Buy CMR drives, never SMR. SMR drives rewrite overlapping tracks, which crawls under array writes and turns a rebuild into an ordeal. The table above is filtered to CMR for exactly this reason.
• For four to eight drives, run RAIDZ2 so two can fail safely.
• For a tiny two drive box, a mirror is fine.
• Keep an offsite backup of anything irreplaceable, because RAID is not a backup.

Buy the extra parity drive. Future you, mid rebuild at 2am, will be very grateful.

The bottom line

How many drives you can lose comes down to your redundancy level: one for mirrors and RAIDZ1, two for RAIDZ2, three for RAIDZ3.

For most home arrays, RAIDZ2 is the sweet spot, because it survives the dangerous rebuild window. Buy the drives using the live price per TB above, add an offsite backup, and a dead drive becomes a non event instead of a disaster.

I’d recommend going with at least RAIDZ, and if you can stretch to ‘wasting’ the space for 2 drives, RAIDZ2, but given the current prices, affording any redundancy is a boon, so look into the best value drives you can afford, and remember you want CMR drives only for RAID, settling with cheaper SMR drives will kill your speed, even when just writing movies, not to mention resilvering.

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