For most surveillance servers, the answer is no more than 12 drives per RAID 6 set. Past 12 bays, the right build is RAID 60: two or three separate 12-drive RAID 6 sets striped together into one array, instead of one array spanning every drive in the chassis. That’s the standard Arxys builds to on every VideoX appliance with more than 12 bays.
The 16-bay is the one chassis that breaks this rule on purpose, and it’s worth knowing why before a client asks.1 It runs as a single 16-drive RAID 6 instead of splitting into smaller sets, because at 16 drives, two parity drives instead of four keeps more capacity usable and keeps the box priced where it needs to be. The rebuild is longer than a 12-drive set’s, but still short enough that we’re comfortable making that one exception.
Here’s the one line worth keeping in your back pocket for the next client walkthrough:
“The best balance of efficiency and protection is no more than 12 drives per RAID set. Anything bigger — 24-bay, 36-bay — gets split into two or three 12-drive RAID 6 sets striped together as RAID 60, instead of one giant array.”
Say that once in a walkthrough and you’ve said more about how the box actually protects footage than most spec sheets do in a page.
Why stripe width is the number that actually matters
RAID 6 gets sold as the fix for large drives, and it is, but only up to a point people don’t usually mention. RAID 6 survives two drive failures in the same set. What it doesn’t do is make the rebuild fast. Rebuild time is driven by how many drives share that rebuild, not by how many bays the chassis has.
A 12-drive RAID 6 set rebuilds in roughly the time it takes to fill one replacement drive. Stripe 24 or 36 drives into that same set instead, and every drive in it has to be read to rebuild the failed one — the array is exposed for two to three times as long, on the exact hardware, with the exact same RAID level. On genuinely large arrays (20-plus drives in one set), rebuild windows stretching past a full day aren’t rare, and a second failure inside that window is how “RAID 6” arrays still end up lost.
Splitting a 24 or 36-bay chassis into 12-drive sets under RAID 60 keeps every rebuild short and keeps the sets independent. If a drive dies in one set, the others keep recording and keep serving playback, untouched. Only that one 12-drive set rebuilds. It costs a bit of usable capacity, since each set carries its own two parity drives instead of sharing one pair across the whole chassis. That’s the trade: a few hours of capacity against days of exposure. Not a close call.
RAID terms, in plain English
You don’t need the engineering-class version of any of this to use it correctly in front of a client. Here’s the field version.
| Term | What it means, in one line |
|---|---|
| Parity | Extra data written alongside your footage that lets the array rebuild a drive that dies, without that drive’s original data. |
| RAID 6 | Two independent parity drives per set. Two drives can fail at the same time and the set keeps running. |
| Stripe / stripe width | How many drives share the same rebuild math. This is the number that actually drives rebuild time, not bay count. |
| Span | One complete RAID 6 set (12 drives, in our builds) inside a larger array. Each span fails and rebuilds on its own. |
| RAID 60 | Multiple RAID 6 spans striped together into one array the OS sees as a single volume. Full protection of RAID 6, in pieces small enough to rebuild fast. |
| XOR | The math the controller uses to calculate parity. You’ll never need to do it by hand — the controller has dedicated hardware for exactly this. |
What to say when the client keeps asking
These are the follow-ups that come up most in a walkthrough, with the answer that holds up.
“Isn’t that wasteful, more parity drives?”
Two parity drives per 12-drive set instead of two for the whole chassis, yes. That’s the price of keeping every rebuild short. Losing a few hours of usable capacity to buy back days of risk isn’t a trade most clients need convincing on once they hear it that way.
“What actually happens if a drive dies?”
Only that one 12-drive set rebuilds. The other one or two sets in the same chassis keep recording and keep serving playback, completely unaffected. Each set is its own independent fault domain.
“We’ve always used RAID 5, why change now?”
RAID 5 was fine when drives were 2TB. On today’s 16TB-plus drives, a single-parity rebuild can run well over a day, and a second failure during that window loses the array. RAID 6 is the floor now, not the upgrade.
“Does this get worse as drives get bigger?”
It does, which is exactly why the 12-drive cap isn’t arbitrary. Drive capacity keeps climbing every generation, so the math this policy is built around keeps getting less forgiving, not more.
RAID 6 for NVR’s with a 12drive RAID set is the best balance of data protection and rebuild time
RAID protects the drives. It doesn’t protect against a power failure.
One more piece worth having straight before a client asks, because it’s easy to conflate two things that solve different problems.
Every Arxys server with a hardware RAID controller ships with a CacheVault module. When the controller caches a write before it hits disk, CacheVault gives the controller just enough backup power to flush that cached write safely to flash the instant power drops, so an in-progress write survives a sudden outage instead of getting corrupted or lost. It protects the controller’s cache for the few seconds it needs, not the server.
A UPS is a separate, necessary layer on top of that. It keeps the whole server running long enough to shut down cleanly, or ride out a short outage entirely. CacheVault protects the write in flight. A UPS protects the server it’s flying through. Every Arxys deployment needs both — recommend a UPS on every job, full stop, and you’ve closed the one gap CacheVault was never designed to cover.
If a client is already feeling the pinch of rising drive prices covered in our recent piece on HDD cost trends, this is the other side of that same coin: the drives are getting more expensive and the stakes of losing an array to a bad rebuild are climbing right along with them. A 12-drive RAID 60 set and a UPS on every install is how that risk gets managed instead of just hoped away.
1 The 16-bay is the only Arxys chassis where we don’t apply the 12-drive cap. Every other capacity follows the doctrine above without exception.
