RAID Levels Explained

RAID (Redundant Array of Independent Disks) combines multiple physical drives into a single logical unit. But here's what matters: every RAID level is a bet. You're trading something—capacity, performance, money, complexity—to protect against something else. The question isn't "which RAID is best?" It's "which disaster am I most willing to accept?"

One thing RAID won't protect you from: yourself. Delete files accidentally, suffer ransomware, or lose the whole machine to fire—RAID watches silently. It guards against drive failure and nothing else. Backups are a separate problem.

RAID 0: The Lie

RAID 0 is false advertising. The R stands for Redundant, but RAID 0 has zero redundancy. It stripes data across drives—splitting files into chunks spread across all disks—which means reads and writes happen in parallel. Fast. Very fast.

But if any drive fails, everything is gone. Not just that drive's data—all of it. You've multiplied your failure risk by the number of drives.

What you get: All capacity (two 1TB drives = 2TB). Maximum speed.

What you lose: Everything, eventually. It's not if, it's when.

The bet: "I don't care about this data." Use it for scratch space, render caches, data you can regenerate. Never for anything you'd miss.

RAID 1: The Simple Answer

RAID 1 mirrors. Every write goes to every drive. Every drive holds identical data. If one dies, the others continue without interruption.

This is the RAID level you can explain to anyone in ten seconds: "It's a copy."

What you get: Simplicity. Reliability. Survives until you're down to one drive. Reads can be faster since you're reading from multiple copies.

What you lose: Half your capacity. Two 1TB drives give you 1TB usable. Four drives still give you 1TB. The math is brutal.

The bet: "I'll pay double for peace of mind." Good for operating system drives, critical configs, anywhere simplicity and reliability trump capacity.

RAID 5: The Calculated Risk

RAID 5 stripes data across three or more drives, but adds a twist: parity. Parity is mathematical redundancy—extra information distributed across drives that lets you reconstruct any single failed drive from what remains.

One drive dies? The array keeps running in degraded mode while you replace it, then rebuilds the missing data from parity.

What you get: Good capacity—you lose only one drive's worth to parity. Three 1TB drives give you 2TB. Ten drives give you 9TB.

What you lose: Write speed (calculating parity takes time). And here's the real cost: safety during rebuilds.

When a drive fails and you're rebuilding, the array reads every sector of every remaining drive. If another drive fails during this process—or if you discover a bad sector that's been silently lurking—all data is lost.

With modern 10TB+ drives, rebuilds take days. RAID 5 with large drives is a game of chicken with probability—you're betting you can rebuild before another drive dies.

The bet: "One drive failure is the most I'll face at once." Reasonable for smaller drives. Increasingly risky as drives grow larger.

RAID 6: The Safer Bet

RAID 6 is RAID 5 with double parity. Two drives can fail simultaneously and you still recover.

What you get: Survivability during rebuilds. When one drive dies and you're in that vulnerable multi-day rebuild window, another failure won't kill you.

What you lose: Two drives' worth of capacity. Four 1TB drives give you 2TB. Writes are slower than RAID 5 (more parity to calculate).

The bet: "Large drives scare me, and I want to sleep at night." For anything over 4TB drives, RAID 6 is increasingly the responsible choice.

RAID 10: The Expensive Answer

RAID 10 combines mirroring and striping. First, drives are paired into mirrors. Then the mirrors are striped together.

You get the speed of RAID 0 with the reliability of RAID 1. Reads and writes are fast because they're parallel across mirrors. Failures are survivable because each mirror has redundancy.

What you get: Best performance. Good reliability. Can survive multiple failures as long as both drives in a single mirror don't die together.

What you lose: Half your capacity, same as RAID 1. And you need at least four drives.

The bet: "I need speed AND reliability AND I'll pay for it." This is the database answer. The high-transaction answer. When downtime costs more than drives do.

RAID 50 and 60: The Enterprise Answer

These nest RAID 5 or 6 inside RAID 0—multiple parity arrays striped together.

RAID 50: Better performance than RAID 5 alone, can survive one failure per sub-array. Minimum six drives.

RAID 60: Better protection than RAID 6 alone, can survive two failures per sub-array. Minimum eight drives.

These exist for large arrays where you need both the capacity efficiency of parity RAID and performance that single arrays can't provide.

Hardware vs. Software RAID

Hardware RAID uses a dedicated controller card with its own processor and cache. The server CPU never touches RAID calculations. Better performance for parity RAIDs, but expensive, and you're locked to that vendor's hardware. If the controller dies, you need an identical replacement to read your data.

Software RAID uses the operating system—Linux mdadm, Windows Storage Spaces, ZFS. Free. Flexible. Your data isn't locked to specific hardware. Modern CPUs are fast enough that the performance penalty is negligible for most workloads.

ZFS deserves special mention: it provides RAID-like redundancy plus checksumming (detecting silent data corruption), snapshots, and self-healing. If you're building new storage, investigate ZFS before committing to traditional RAID.

How to Choose

The decision tree is simpler than it looks:

"I need speed and don't care about this data" → RAID 0

"I need simplicity and reliability" → RAID 1

"I need capacity efficiency with reasonable protection" → RAID 6 (RAID 5 only if drives are small)

"I need speed AND reliability and have budget" → RAID 10

"I'm building serious infrastructure" → Consider ZFS or talk to someone who does this professionally

What RAID Won't Save You From

• Accidental deletion (RAID faithfully deletes across all drives)
• Ransomware (RAID faithfully encrypts across all drives)
• Controller failure with hardware RAID
• Catastrophic events affecting the whole machine
• Silent data corruption (unless you're using ZFS or similar)
• Human error during RAID management

RAID is one layer of a complete data protection strategy. It handles drive failure. Backups handle everything else.

The Modern Reality

Three trends are reshaping RAID:

Drive sizes keep growing. A 20TB drive takes days to rebuild. During those days, you're exposed. This is pushing everyone toward RAID 6 or RAID 10, away from RAID 5.

SSDs change the math. No mechanical parts means different failure patterns. Some argue redundancy matters less; others point out that SSDs fail suddenly and completely. The jury's still out.

Cloud storage abstracts it away. AWS, Azure, and Google handle redundancy at the infrastructure level. If your data lives in the cloud, you're thinking about application-level reliability and backups, not RAID levels.

But for local storage—your database server, your NAS, your workstation with irreplaceable work—RAID remains the first line of defense against the drive that will, eventually, die.