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 5 at a glance
| Minimum drives | 3 |
|---|---|
| Usable space | (drives − 1) × size of the smallest drive |
| Drives that can fail | Any 1 |
| Read speed | Close to the sum of all drives but one |
| Write speed | Slower for small writes (4 operations per write) |
| Rebuild | Reads every surviving drive in full |
Common RAID 5 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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 3 drives | 4 | 8 | 12 | 16 | 20 | 24 | 28 | 32 | 36 | 40 | 44 | 48 |
| 4 drives | 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 | 60 | 66 | 72 |
| 5 drives | 8 | 16 | 24 | 32 | 40 | 48 | 56 | 64 | 72 | 80 | 88 | 96 |
| 6 drives | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 |
| 7 drives | 12 | 24 | 36 | 48 | 60 | 72 | 84 | 96 | 108 | 120 | 132 | 144 |
| 8 drives | 14 | 28 | 42 | 56 | 70 | 84 | 98 | 112 | 126 | 140 | 154 | 168 |
How RAID 5 works
RAID 5 stripes data across all drives and adds one parity block to every stripe. Parity is the XOR of the data blocks in that stripe, so any one missing block can be calculated from the others. The parity block moves to a different drive in each stripe, so no single drive does all the parity work (a fixed parity drive is RAID 4).
Because parity always costs one drive, RAID 5 gets more efficient as you add drives:
| Drives | Usable | Efficiency |
|---|---|---|
| 3 drives | 2 drives' worth | 67% |
| 4 drives | 3 drives' worth | 75% |
| 5 drives | 4 drives' worth | 80% |
| 6 drives | 5 drives' worth | 83% |
| 8 drives | 7 drives' worth | 88% |
| 10 drives | 9 drives' worth | 90% |
| 12 drives | 11 drives' worth | 92% |
The rebuild problem
When a drive fails, the array keeps running, but it has no protection left. The new drive is filled by reading every block on every surviving drive and calculating the missing data. On a four-drive array of 16 TB drives, that is 48 TB that must be read without a single unreadable sector.
Drive makers quote how often a sector cannot be read: about 1 per 1014 bits for desktop drives and 1 per 1015 for NAS and enterprise drives. These are worst-case specs, and real drives usually do better, but they show how the risk grows. Chance of at least one read error during a rebuild of a four-drive RAID 5:
| Array | Read during rebuild | Desktop drives (10¹⁴) | NAS drives (10¹⁵) |
|---|---|---|---|
| 4 × 4 TB | 12 TB | 62% | 9.2% |
| 4 × 8 TB | 24 TB | 85% | 17% |
| 4 × 12 TB | 36 TB | 94% | 25% |
| 4 × 16 TB | 48 TB | 98% | 32% |
| 4 × 20 TB | 60 TB | 99% | 38% |
| 4 × 24 TB | 72 TB | over 99% | 44% |
What happens on a read error depends on the system. Some hardware controllers stop the rebuild. Linux mdadm, ZFS and most NAS systems continue and lose only the affected file or block. Either way, nothing is left to fix it. That is the reason most people move to RAID 6 once drives reach about 8 TB.
Why RAID 5 writes are slower
Changing one small block means reading the old data and the old parity, then writing the new data and the new parity: four operations instead of one. This is the RAID 5 write penalty. Large sequential writes that fill whole stripes avoid most of it, so RAID 5 is fine for media libraries and backups, and weaker for databases and virtual machines.
When RAID 5 is still a good choice
- Drives of about 4 TB and smaller, where rebuilds finish in hours.
- SSD arrays: SSDs rebuild fast and rarely hit read errors.
- Three to five drives, with a separate backup of anything you cannot replace.
Avoid it for large hard drives, for more than about eight drives in one group, and for the only copy of important data.
Questions
What is the minimum number of drives for RAID 5?
Three. With three drives you get the space of two.
How much space do four 8 TB drives give in RAID 5?
24 TB, which Windows shows as 21.8 TB. One drive's worth (8 TB) holds parity.
Is RAID 5 dead?
No, but it fits fewer cases than it used to. With small drives and SSDs it is fine. With large hard drives, RAID 6 is the safer choice because the rebuild takes long and reads a lot of data.
Can I add a drive to RAID 5 later?
Linux mdadm and most NAS systems can grow a RAID 5 by one drive. The reshape can take a day or more, and the array is under stress the whole time, so make a backup first.
Is Synology SHR-1 the same as RAID 5?
With drives of the same size, SHR-1 gives the same space and protection as RAID 5. With mixed sizes, SHR uses more of the larger drives.