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
6 drives of 16 TB each.
RAID 6 at a glance
| Minimum drives | 4 |
|---|---|
| Usable space | (drives − 2) × size of the smallest drive |
| Drives that can fail | Any 2 |
| Read speed | Close to the sum of all drives but two |
| Write speed | Slower for small writes (6 operations per write) |
| Rebuild | Still protected while one drive rebuilds |
Common RAID 6 setups
Usable space for each number and size of drives. Every cell opens a page with the full numbers for that setup.
| Drives | 2 TB | 4 TB | 6 TB | 8 TB | 10 TB | 12 TB | 14 TB | 16 TB | 18 TB | 20 TB | 22 TB | 24 TB |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 drives | 4 | 8 | 12 | 16 | 20 | 24 | 28 | 32 | 36 | 40 | 44 | 48 |
| 5 drives | 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 | 60 | 66 | 72 |
| 6 drives | 8 | 16 | 24 | 32 | 40 | 48 | 56 | 64 | 72 | 80 | 88 | 96 |
| 7 drives | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 |
| 8 drives | 12 | 24 | 36 | 48 | 60 | 72 | 84 | 96 | 108 | 120 | 132 | 144 |
| 10 drives | 16 | 32 | 48 | 64 | 80 | 96 | 112 | 128 | 144 | 160 | 176 | 192 |
| 12 drives | 20 | 40 | 60 | 80 | 100 | 120 | 140 | 160 | 180 | 200 | 220 | 240 |
How RAID 6 works
RAID 6 adds two independent parity blocks to every stripe. The first (P) is the same XOR parity RAID 5 uses. The second (Q) is calculated a different way (Reed-Solomon code), so the array can rebuild any two missing blocks in a stripe. Both parity blocks rotate across all drives.
| Drives | Usable | Efficiency |
|---|---|---|
| 4 drives | 2 drives' worth | 50% |
| 5 drives | 3 drives' worth | 60% |
| 6 drives | 4 drives' worth | 67% |
| 8 drives | 6 drives' worth | 75% |
| 10 drives | 8 drives' worth | 80% |
| 12 drives | 10 drives' worth | 83% |
| 16 drives | 14 drives' worth | 88% |
Why two parity blocks matter
The danger in a parity array is the rebuild. It can take a day or more on large drives, and every surviving drive is read from start to end. With RAID 6, losing one drive still leaves one parity, so a read error or even a second failed drive during the rebuild does not lose data.
That is why RAID 6 (and its ZFS cousin RAIDZ2) is the usual answer for drives of 8 TB and more. Rebuild times for one drive at 150 MB/s:
| Drive size | Idle array (150 MB/s) | Busy array (50 MB/s) |
|---|---|---|
| 8 TB | 14 h | 44 h |
| 12 TB | 22 h | 2.8 days |
| 16 TB | 29 h | 3.7 days |
| 20 TB | 37 h | 4.6 days |
| 24 TB | 44 h | 5.6 days |
RAID 6 or RAID 10?
With four drives both give half the space. RAID 6 survives any two failures; RAID 10 survives two only if they hit different mirrors, but it writes faster and rebuilds by copying a single drive. From six drives up, RAID 6 gives clearly more space: six 16 TB drives give 64 TB in RAID 6 and 48 TB in RAID 10.
Pick RAID 6 for storage, archives and media. Pick RAID 10 for databases and virtual machines that write a lot of small blocks.
How many drives in one RAID 6?
A common rule is up to about 12 drives in one RAID 6 group. More drives mean more data read during each rebuild and more drives that could fail in that time. Larger arrays are usually split into two RAID 6 groups, which is RAID 60.
Questions
What is the minimum number of drives for RAID 6?
Four. With four drives you get the space of two.
How much space do six 16 TB drives give in RAID 6?
64 TB, which Windows shows as 58.2 TB. Two drives' worth (32 TB) holds parity.
Is RAIDZ2 the same as RAID 6?
It has the same space and the same protection against two failed drives. ZFS also checks every block with a checksum and rebuilds only the data actually stored, so a half-full pool rebuilds faster.
Is RAID 6 slower than RAID 5?
Small writes need six operations instead of four, so random writes are slower. Reads and large writes are about the same.
Is Synology SHR-2 the same as RAID 6?
With drives of the same size, yes: same space, any two drives can fail.