The Array, Parity & Mixed-Size Drives Explained

What "The Array" Is

In Unraid, "the array" simply means your collection of hard drives working together as one storage system. It is the main pool where your files live.

Picture a row of drawers in a filing cabinet. Each drawer is a hard drive. Together they make up the cabinet - that is your array. You do not have to think about which drawer a file is in; Unraid keeps track for you and shows it all as one big space.

• Data drives - the drawers that hold your actual files.
• Parity drive(s) - a protective spare, explained soon.
• A cache (optional) - a fast drive that speeds up saving files.

How Unraid Keeps Whole Files on Individual Disks

Here is something that makes Unraid special and safer than many systems: each of your files sits complete on a single drive. It is not chopped into pieces and spread across many drives.

In some storage systems, one photo might be split into fragments scattered over five drives. If two of those drives die, every file is ruined. Unraid does not do that. Your vacation video lives whole, start to finish, on one drive.

• A file is readable as long as its own drive is alive.
• If a drive you have no parity for ever failed, only that drive's files would be affected - not everything.
• You can even read a single drive on another computer in a pinch.

What Parity Is, in Plain Words

Parity is the clever trick that lets Unraid rebuild a dead drive. It sounds like magic, but the idea is simple arithmetic - the kind you could do on paper.

Imagine three numbers: 2, 3, and 5. You write down their total, 10, on a spare card. Now if you lose the "3," you can figure it back out: 10 minus 2 minus 5 gives you 3. The spare card did not store the 3, but it let you recover it. That spare card is parity.

One vs. Two Parity Drives

Unraid lets you use one parity drive or two. The difference is simple: it is how many drives can fail at the same time without losing data.

For most homes, one parity drive is a fine, sensible choice. If you have many drives, or your data is especially important to you, a second parity drive adds another layer of safety for the rare case of two drives failing close together.

Why Parity Must Be as Large as Your Biggest Drive

There is one firm rule about the parity drive's size: it must be at least as big as your largest data drive. Here is the plain reason why.

Remember the "total" card from the parity trick? To hold the total for the biggest possible data drive, the parity drive needs at least that much room. If your largest data drive is 12 terabytes, your parity drive must be 12 terabytes or bigger. It cannot be smaller.

• Buy your parity drive to match, or exceed, your biggest data drive.
• If you later add a data drive bigger than parity, you must upgrade parity first.
• It is perfectly fine for parity to be larger than your data drives.

What Parity Protects You From

Parity has one specific job, and it does it very well: it protects you from a hard drive physically dying. Hard drives are machines, and machines eventually wear out.

In each of these cases, you replace the dead drive with a healthy one, and Unraid rebuilds the exact contents onto it using parity. Your files come back as if nothing happened. That is the payoff for having parity.

• A data drive stops working - clicking, not spinning up, or vanishing.
• A drive develops enough faults that Unraid marks it as failed.
• With two parity drives, even two drives failing around the same time.

What Parity Does NOT Protect

This is the page I most want you to read twice. Parity guards against a drive dying - and nothing else. It is not a backup, and confusing the two is how people lose files.

Parity keeps your drives in sync. So if you delete a file, or something encrypts it, parity faithfully mirrors that change. There is no "undo" hiding in parity. For those dangers, you need a separate backup.

Why Mixing Drive Sizes Works

One of the friendliest things about Unraid is that your drives do not all have to match. You can mix a 4 TB, an 8 TB, and a 12 TB drive in the same array. Many systems will not let you do that.

This works because each data drive stands on its own, holding whole files. Unraid does not need them to be identical twins. It just needs the parity drive to be as big as the largest one. Beyond that, use whatever drives you have.

• You can start with drives you already own, of different sizes.
• You can buy the best-value drive of the day, whatever its size.
• You are never forced to replace everything at once.

Adding Capacity Later

Your storage needs will grow, and Unraid grows with you. Adding space later is straightforward - you do not rebuild everything from scratch.

Just remember the size rule: if a new data drive would be larger than your parity drive, upgrade the parity drive first. Growing your server is one of the nicer chores - it means your collection is thriving.

• Add another drive - slot in an empty drive and add it to the array for more room.
• Replace a drive with a bigger one - swap a smaller drive for a larger one, and Unraid rebuilds its contents onto the new, roomier drive.

The Parity Check - What It Is and When It Runs

Now and then Unraid runs a "parity check." This is a routine health exam that makes sure your parity still matches your data perfectly, so it will be ready if a drive fails.

A parity check takes a while - often several hours to a day, depending on your drives - but it works quietly in the background. A clean check with no errors is Unraid telling you all is well.

• It reads through every drive and confirms parity is correct.
• It usually runs on a schedule, often monthly, and you can set that.
• It can also run after an unclean shutdown, like a power cut.
• Your server keeps working normally while it runs.

How Long a Rebuild Takes

When a drive fails and you replace it, Unraid rebuilds its contents onto the new drive. It is fair to wonder how long you will be waiting for that.

A rebuild reads all your other drives to reconstruct the missing one, so the time depends mostly on how large the drives are. As a rough guide, expect somewhere from several hours to a day or more for large modern drives. Bigger drives take longer.

• Your files stay available during the rebuild, though things feel a bit slower.
• It is best to let the rebuild finish without interruptions like a reboot.
• Once done, your new drive holds exactly what the old one did.

How This Differs From Traditional RAID

You may hear the word "RAID" from techy friends. Unraid's approach is related but different in ways that happen to be friendlier for a home.

The short version: traditional RAID ties your drives tightly together for speed, but it is rigid and can fail dramatically. Unraid keeps drives more independent, which is more flexible and, for a home, often safer. It trades a little speed for a lot of peace of mind.

Your Data Is Still Readable Even If the Server Dies

Here is a comforting thought. Because each drive holds whole files, your data is not trapped inside the Unraid machine forever. If the server itself broke, the files on the drives are still there.

Say the server's motherboard died tomorrow. The hard drives themselves are fine - and each one holds complete, readable files. With the right adapter, a drive can be read on another computer to recover its contents. Your files are not locked to one dead box.

• You are never held hostage by a single failed piece of hardware.
• In a pinch, files can be pulled off individual drives.
• This is another reason the whole-file design is reassuring.

Choosing How Many Parity Drives

So, one parity drive or two? There is no single right answer - it depends on how many drives you have and how much you value the extra cushion.

Remember that each parity drive is a drive that protects rather than stores - it does not add to your usable space. So two parity drives means two drives not holding your files. For most homes, one parity strikes a fine balance.

Drive Health and SMART, Simply

Hard drives can often warn you before they fail, through a built-in self-check called SMART. Unraid reads these signals and tells you if a drive is looking shaky.

Think of SMART as a drive's own health report - a bit like a check-engine light. It tracks little warning signs, like parts of the drive going bad. Unraid watches these for you and flashes a warning if a drive starts to look unwell.

• Glance at your drives' health in the Dashboard now and then.
• Green and healthy means carry on - nothing to do.
• A warning is your early nudge to plan a replacement, not to panic.

What Happens the Moment a Drive Fails

Let us walk through it, so if it ever happens you will know exactly what is going on and that you are not in trouble.

During all this, your files stay reachable. The failed drive being "emulated" by parity means life goes on while you sort out a replacement. When the rebuild finishes, you are fully protected again.

• Unraid notices the drive has failed and marks it, usually with a red signal.
• Thanks to parity, the failed drive's files are still available - Unraid fills them in on the fly.
• You get a replacement drive, at least as large as the one that failed.
• You power down, swap in the new drive, and tell Unraid to rebuild.
• Unraid rebuilds the exact contents onto the new drive using parity.

Planning Your Drive Sizes

A little planning up front saves money and hassle later. Here is how to think about what drives to buy for a smooth, growable array.

A common, comfortable plan is one generous parity drive plus a couple of data drives to start, then adding data drives over time as you fill up. There is no need to build the whole thing at once.

• Make parity your biggest drive. Buy parity as large, or larger, than any data drive you will use.
• Leave room to grow. A parity drive bigger than your current data drives lets you add larger drives later.
• Buy for value. Since sizes can mix, pick whatever drive gives the best storage per dollar.
• Do not overbuy on day one. You can always add drives as your collection grows.

Common Myths

Let us clear up the misunderstandings I hear most. Getting these straight will keep your data safe and your expectations right.

The biggest and most dangerous myth, by far, is the first one. Please do not let parity lull you into skipping backups. Everything else here is about flexibility; that one is about not losing your memories.

What's in the 27-page PDF

• What "the array" is, in plain words
• What parity is and what it really protects
• The one thing parity does NOT do (this matters)
• Why mixing drive sizes works in Unraid
• What happens the moment a drive fails
• Planning your drives and clearing up common myths

Why I wrote this

I write these as printable handouts because a guide you can keep in a drawer beats a web page you have to find again. No jargon, no judgment, no rushing. If you'd rather have someone sit beside you and go through it together, that's a flat $99 first visit with the 30-day follow-up included — serving Portage County from Atwater, Ohio. — Bill