Size printed on the drive label. Different sizes? The array uses the smallest one for every drive.
Advanced settings
Typical hard drive: 100–200 MB/s with no other load. SSD: 400 MB/s or more.
From the drive datasheet. It is a worst-case spec, so real drives usually do better.
Drives that can fail
Rebuild of one drive
Read error during rebuild
Checks for this setup
The same drives in every RAID level
4 drives of 8 TB each.
RAID 10 at a glance
| Minimum drives | 4 (an even number) |
|---|---|
| Usable space | half of all drives |
| Drives that can fail | 1 always, up to half if each hits a different pair |
| Read speed | Up to the sum of all drives |
| Write speed | Half the sum of all drives (2 operations per write) |
| Rebuild | Copy of one drive from its partner |
Common RAID 10 setups
Usable space for each number and size of drives. Every cell opens a page with the full numbers for that setup.
How RAID 10 works
Drives are paired into mirrors (RAID 1), and data is striped across the mirrors (RAID 0). Every block exists on exactly two drives.
The efficiency is always 50%, however many drives you add.
How many drives can fail?
One failure is always safe. A second failure is safe if it hits a different pair; it loses the array if it hits the partner of the first failed drive. With more drives, a random second failure is less likely to hit that one partner:
| RAID 10 of | Chance a random second failure is survived |
|---|---|
| 4 drives | 67% |
| 6 drives | 80% |
| 8 drives | 86% |
| 10 drives | 89% |
| 12 drives | 91% |
| 16 drives | 93% |
So RAID 10 with many drives often survives more than one failure, but only RAID 6 promises to survive any two.
Fast, gentle rebuilds
Replacing a drive means copying its partner, nothing more. The other drives are not read at all, the copy is sequential, and there is no parity to calculate. On large drives this is the biggest practical advantage of RAID 10 over parity RAID: the window of risk is shorter and only one drive is stressed.
When to choose RAID 10
- Databases, virtual machines and mail servers: many small random writes.
- SSD arrays where write speed and drive wear matter more than space.
- Four-drive NAS units where you value fast rebuilds over the extra parity of RAID 6.
RAID 10 vs RAID 0+1
RAID 0+1 does it the other way round: two striped sets, mirrored. It gives the same space, but one failed drive takes down a whole stripe set, and a second failure on the other side loses everything. RAID 10 is safer and rebuilds faster, which is why it is the one controllers and NAS systems offer.
Questions
Does RAID 10 need an even number of drives?
Standard RAID 10 does. With an odd number, the last drive is not used; keep it as a hot spare. Linux mdadm has its own RAID 10 mode that can use odd numbers.
How much space do four 8 TB drives give in RAID 10?
16 TB, which Windows shows as 14.6 TB.
RAID 10 or RAID 5?
RAID 10 writes faster and rebuilds faster and safer. RAID 5 gives more space. For large hard drives, compare RAID 10 with RAID 6 instead.
Is RAID 10 the same as RAID 1+0?
Yes, it is the same thing.