Compare layouts
Same drives, different layouts. This is usually the decision that matters most.
Power, save & share
RAID terms explained
Here is a brief explanation of the terms used on this online RAID calculator.
Striping
Data is split into chunks and written across several drives at once. It is fast because every drive works in parallel, but nothing is stored twice, so losing one drive loses everything. This is RAID 0.
Mirror
The same data is written to two or more drives. If one dies, another still has a complete copy. Simple and safe, but you only get the space of one drive per mirror. This is RAID 1.
Parity
Extra data calculated from your files, like the answer to a sum. If one number goes missing, you can work it out from the rest. Parity lets the array recreate a failed drive without keeping a full copy, so it costs less space than mirroring.
Single parity
One drive's worth of space holds parity, spread across all drives. The array survives one failed drive. RAID 5 and RAIDZ1 work this way.
Double parity
Two drives' worth of parity, so any two drives can fail. That includes a second drive dying while the first is being replaced, which is when it is most likely. RAID 6 and RAIDZ2 work this way; RAIDZ3 adds a third.
Striped mirrors
Drives are paired into mirrors, then data is striped across the pairs. You get the speed of striping and the quick recovery of mirrors, but only half the raw space. This is RAID 10.
Rebuild (resilver)
After you replace a failed drive, the array recreates its data by reading the other drives. ZFS calls this a resilver. Until it finishes, the array has less protection than usual.
ZFS vs. traditional RAID
Traditional RAID, from a hardware card or Linux's mdadm, only sees blocks of data and cannot tell if a block has gone bad. ZFS is a filesystem and RAID manager in one. It checksums every block, so it spots silent corruption and repairs it from the redundant copy. It only rebuilds the space actually in use, and a read error usually costs one file rather than the whole array. In return, it likes plenty of RAM, and growing a pool is less flexible: adding a single drive to an existing RAIDZ only arrived in OpenZFS 2.3.
URE and drive class
A URE (unrecoverable read error) is a spot on a drive that cannot be read back. Manufacturers rate how often this can happen. Consumer drives are usually rated at 1 error per 10¹⁴ bits read (about 12.5 TB). NAS and enterprise drives are rated at 1 per 10¹⁵ (about 125 TB). A rebuild reads a lot of data, so the calculator uses this rating to estimate the chance of hitting an error mid-rebuild. These are spec-sheet worst cases; real drives often do better.
How is the power draw estimated?
Each drive type uses a typical average draw for a drive that is spinning or active but not under heavy load:
- 3.5" HDD: 6.5 W
- 2.5" HDD: 2.5 W
- 2.5" SATA SSD: 2.0 W
- NVMe SSD: 4.0 W
The array total is that figure multiplied by the number of drives. The yearly figure assumes the drives run 24 hours a day, every day: watts × 8,760 hours ÷ 1,000 = kWh.
It counts the drives only. The CPU, motherboard, fans and power supply losses add more, while spinning drives down when idle lowers it. Treat it as a ballpark for comparing layouts.
Frequently Asked Questions
Here are the answers to common questions you might have:
Should I use RAID 5 or RAID 6?
RAID 6 is the safer choice for arrays built with large modern drives (8TB and above). RAID 5 tolerates only one drive failure, and rebuilding after a failure requires reading every surviving drive in full. A single unreadable sector during that window can compromise the rebuild. RAID 6 adds a second parity block, so the array survives a second failure or read error during that window. The cost is one extra drive's worth of capacity. RAID 5 remains reasonable for small arrays of small drives or non-critical data.
Is ZFS RAIDZ better than a ZFS mirror?
Neither is universally better. RAIDZ (especially RAIDZ2) gives you more usable capacity from the same drives, while mirrors give faster resilvers, better random I/O performance, and simpler expansion, since you can grow a pool by adding another mirror pair. For most home NAS builds with 4 to 10 drives where capacity matters, RAIDZ2 is the common recommendation; for performance-sensitive workloads or pools you expect to expand gradually, mirrors are often the better fit.
Is RAID a backup?
No. RAID protects against drive hardware failure and nothing else. Accidental deletion, ransomware, file corruption, controller failure, theft, and fire all affect every drive in the array simultaneously. RAID keeps your system available when a drive dies; a backup lets you recover data you have lost. You need both, and the standard advice is the 3-2-1 rule: three copies of your data, on two different types of media, with one copy off-site.
How much usable space do I get from RAID 5?
RAID 5 gives you the capacity of all drives minus one, based on the smallest drive in the array. Four 4TB drives yield 12TB usable; six 8TB drives yield 40TB usable. Note that your operating system reports capacity in TiB (binary units), so 12TB will appear as roughly 10.9TiB. The space is not missing; it is a difference in units.
Can I mix different drive sizes in RAID?
Yes, but every drive in the array is treated as if it were the size of the smallest drive, and the extra capacity on larger drives sits unused. Mixing a 4TB drive into an array of 8TB drives effectively turns them all into 4TB drives. If you must mix sizes, replacing the smallest drive is usually the cheapest way to reclaim capacity. Some systems (like Unraid or Synology SHR) handle mixed sizes more gracefully than standard RAID.
