RAID and ZFS explained

With multiple drives in a NAS, you can make them work together so that your data remains safe if one fails. This is called RAID. Here’s what the variants mean and which one suits you.

The main types

• RAID 0 (striping): drives are combined for speed and space, without protection. If one drive fails, everything is gone. Not for important data.
• RAID 1 (mirror): two or more drives contain the same data. One drive may fail. You keep the space of one drive. Simple and safe for home use.
• RAID 5: minimum of three drives. One drive may fail; you lose the space of one drive. With large drives, recovery takes a long time, and during that time a second drive may fail.
• RAID 6: minimum of four drives. Two drives may fail. Wiser than RAID 5 with large drives.
• RAID 10: a combination of mirrors and striping. Fast and safe, but you lose half the space.

See what it yields for your drives with our RAID and ZFS calculator.

ZFS

ZFS is a file system and RAID solution in one. It is used by TrueNAS and is available in Proxmox. What makes it special:

• Checksums: ZFS detects when a file becomes silently corrupted (bitrot) and can restore it with the second copy.
• Snapshots: a snapshot is created in a second and costs almost no space. Useful against accidentally deleted files.
• Scrubs: a periodic check of all data (with zpool scrub).

Names: mirror is a mirror, RAIDZ1 resembles RAID 5, RAIDZ2 resembles RAID 6. ZFS requires sufficient memory; count on at least 8 GB for a home NAS.

Synology SHR and Unraid

• Synology SHR uses drives of different sizes efficiently, with the protection of RAID 5 or 6.
• Unraid lets you use drives separately with one or two drives for parity (protection). You can simply add a drive later.

RAID is not a backup

This is the most important thing you can remember. RAID protects against a failed drive. It does not protect against:

• a file you accidentally delete (from all drives too);
• a virus or ransomware;
• fire, burglary, or water damage;
• a failed controller or a mistake you make.

Therefore also make a backup in another location.

What do you choose?

• Two drives: a mirror (RAID 1 or ZFS mirror).
• Four drives or more: RAID 6 or RAIDZ2.
• Drives of different sizes: SHR or Unraid.
• Your files are important and you want the best protection: ZFS.