What RAID does, and what it does not
RAID spreads data across several drives so that, if one fails, the server keeps running and nothing is lost while you replace the drive. It is about availability: it prevents downtime.
What it does not do is protect you from deletion, ransomware or fire: if someone deletes a folder or a virus encrypts it, RAID copies the change to every drive instantly. That is why it does not replace backups, which follow the 3-2-1 rule: three copies, on two different media, with one off site or offline.
The levels side by side
| Level | Minimum drives | Usable space | Failures it survives | What for |
|---|---|---|---|---|
| RAID 1 (mirror) | 2 | half | 1 drive | System and small servers |
| RAID 5 | 3 | all but 1 | 1 drive | Files, on SSDs or small drives |
| RAID 6 | 4 | all but 2 | 2 drives | Files and backups on large drives |
| RAID 10 | 4 | half | 1 per mirror | Databases and virtual machines |
| ZFS RAIDZ2 | 4 | all but 2 | 2 drives | Like RAID 6, and it also checks the data |
An example with four 8 TB drives: RAID 10 or RAID 6 leave 16 TB usable; RAID 5 leaves 24 TB, but survives only one failure.
Why RAID 5 no longer suits large drives
When a drive fails, the array rebuilds by reading every other drive from start to finish. With drives of 8 TB or more, that rebuild takes hours or days, and meanwhile the remaining drives work flat out. In RAID 5, if a second failure or an unreadable sector appears before it finishes, the whole volume is lost.
That is why, with large hard drives, the prudent choice is RAID 6, RAIDZ2 or RAID 10, which survive a second problem during the rebuild. RAID 5 is still reasonable with SSDs or small drives, which rebuild quickly.
Hardware or software RAID
Hardware RAID is handled by a controller with its own processor. The operating system sees a single drive, and it is the usual choice with Windows Server. If the controller fails, you need another compatible one to read the drives.
Software RAID is handled by the system itself: ZFS in Proxmox or TrueNAS, mdadm in Linux or Storage Spaces in Windows. With today’s processors it costs almost no performance, and the drives can be read on any other machine with the same system.
ZFS goes a step further: it stores a checksum for every block, detects corrupted data and repairs it automatically from the good copy, and takes instant snapshots. For ZFS, the drives must reach it without RAID in between, connected to the board or to an HBA, a controller that only provides ports. It also uses free memory as cache: the more, the better, and better still if it is ECC.
SSDs, hard drives and hot spares
- System and virtual machines: two mirrored NVMe SSDs (RAID 1). If one fails, the server still boots.
- Files and backups: high-capacity hard drives in RAID 6, RAIDZ2 or RAID 10.
- Hot spare: a drive installed and idle that takes over automatically when another fails. It shortens the time at risk if nobody can replace the drive straight away.
- Hot-swap bays: the failed drive is replaced with the server running, without opening the case.
Which RAID to choose for your case
| Case | Recommendation |
|---|---|
| Office server with two drives | RAID 1 (mirror) |
| Company files on 4 to 8 large drives | RAID 6 or RAIDZ2 |
| Virtual machines and databases | RAID 10 or mirrored SSDs |
| Backup server | RAIDZ2 with ZFS, plus an off-site copy |
| Windows Server with a controller | Hardware RAID 1 or RAID 10 |
Our servers ship with the RAID set up and checked during the 24-hour test. On each server’s page you choose the controller: none, for software RAID (ZFS or Linux); an LSI 9300-8i HBA, for ZFS with more drives; or hardware RAID with an LSI 9440-8i for Windows Server.
Related hardware and guides
Frequently asked questions
Does RAID replace backups?
No. RAID keeps the server running when a drive fails, but it instantly copies any deletion or encryption. Backups must be kept separately: three copies, on two media, with one off site.
RAID 5 or RAID 6?
With hard drives of 8 TB or more, RAID 6: it survives two failures, and the rebuild is long. RAID 5 only with SSDs or small drives, which rebuild quickly.
Can I mix drives of different sizes?
You can, but RAID only uses as much of each drive as the smallest one offers. It is best to use identical drives, of the same model and NAS or server grade.
What happens if the RAID controller fails?
With hardware RAID you need another compatible controller to recover the volume. With software RAID or ZFS, the drives can be read on any machine with the same system.
How many drives do I need for ZFS?
For RAIDZ2, which survives two failures, at least four; from six upwards you make better use of the space. For a mirror, two.
Need a server with RAID?
Tell us how much data you keep and how you work: we will propose the drives, RAID and backups, with the server assembled and tested for 24 hours.