Which RAID Level Should You Use? A Decision Guide by Drive Count

Updated: 2026-07-05· Last verified July 2026

"Which RAID should I use?" is the most-asked question in every NAS community, and the frustrating classic answer is "it depends". It does depend — but on surprisingly few things: how many drives you have, how big they are, whether they're the same size, and what you optimize for. This guide walks the decision by drive count, explains the reasoning instead of just declaring winners, and links every claim to numbers you can check yourself in the calculator.

2 drives: RAID 1, no debate

With two bays there is exactly one redundant choice: the mirror. RAID 1 gives you the capacity of one drive and survives the loss of the other, which is the whole point of a small NAS holding data you'd miss. On a Synology, pick SHR-1 instead of classic RAID 1 — with two equal drives it behaves identically, but it keeps the door open to mixed sizes when you inevitably upgrade one drive before the other. What you should not do with two drives is RAID 0: twice the space, and either drive's death takes everything. Our RAID 0 guide covers the rare cases where that trade makes sense; a home NAS isn't one of them.

3-4 drives: the RAID 5 sweet spot — with a size caveat

Three or four drives is where single parity earns its popularity. RAID 5 (and its ZFS cousin RAIDZ1, and SHR-1) gives you all-but-one drive of capacity and survives one failure — with 4×8 TB that's 24 TB usable out of 32 raw, a 75 % efficiency no mirror can touch. Check the exact numbers for your sizes in the RAID 5 vs RAID 6 comparison or directly in the calculator.

The caveat is drive size. During a rebuild, every surviving drive is read end to end; the bigger the drives, the longer that window and the higher the chance of a second failure or an unrecoverable read error before it closes — the two classic ways RAID 5 arrays die, as our rebuild-time guide quantifies. With today's 10-12 TB+ drives, that risk is no longer academic even at four drives. The practical rule: up to ~8 TB per drive, RAID 5 is defensible; beyond that, treat double parity as the default even in a 4-bay. You can test your own configuration's odds in the failure simulator — clicking one drive dead and reading the URE probability is more convincing than any rule of thumb.

5+ drives: double parity is the default

Every additional drive is another component that can fail and another multi-hour stretch added to rebuild reads. From five or six drives, RAID 6 / RAIDZ2 / SHR-2 stops being paranoia and becomes the baseline: you give up one more drive of capacity and gain the ability to lose a second drive — or hit a bad sector — while degraded. The math is stark: on large arrays, single parity turns every rebuild into a coin flip you didn't need to take, which is why our RAID 5 vs RAID 6 deep dive lands where every storage vendor's guidance does. Eight drives and up, or genuinely irreplaceable data, is where RAIDZ3 or a second pool enters the conversation.

Mixed sizes or grow-as-you-go: SHR and Unraid

Classic RAID has one rigid assumption: every drive contributes only as much as the smallest one. If your drives are 4, 8 and 12 TB, RAID 5 treats them all as 4 TB. Two systems break that assumption. Synology SHR stacks partition layers so mixed sizes are used efficiently, and Unraid goes further: every data drive is independent, parity lives on dedicated drives, and you grow the array one cheap drive at a time. For media servers that expand whenever a drive is on sale, that flexibility usually beats raw striped performance. The trade-offs (Unraid's single-drive write speed, SHR's Synology lock-in) are covered in the linked guides.

Performance first: RAID 10

If the array hosts VMs, databases or heavy random-write workloads, parity RAID's write penalty becomes the bottleneck, and RAID 10 — striped mirrors — is the level built for that job. You pay 50 % of raw capacity and get fast writes plus quick, low-stress rebuilds (only the mirror partner is read, not the whole array). For a pure file-and-media NAS, that capacity price is rarely worth it; see the RAID 10 vs RAID 6 comparison for where the line runs.

The part no RAID level fixes

Whatever you choose, RAID protects against exactly one thing: drive hardware failure. Deletion, ransomware, controller death, fire, and silent corruption pass straight through every level on this page — that's what backups and checksumming filesystems are for, and RAID is not a backup explains the division of labor. Decide your RAID level for availability and capacity; decide your backup strategy for survival.

Further reading

All RAID Types Explained: The Complete Guide for NAS & Homelab

RAID 5 vs RAID 6: Which Should You Actually Pick?

RAID 10 vs RAID 6: Which One Fits Your NAS?

SHR vs Traditional RAID: Why Synology Does It Differently

RAID Rebuild Time: How Long Does It Actually Take?