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
2 drives of 8 TB each.
RAID 1 at a glance
| Minimum drives | 2 |
|---|---|
| Usable space | size of the smallest drive |
| Drives that can fail | All but one |
| Read speed | Up to the sum of all drives (depends on the system) |
| Write speed | Same as one drive |
| Rebuild | Copy of one drive |
Common RAID 1 setups
Usable space for each number and size of drives. Every cell opens a page with the full numbers for that setup.
How RAID 1 works
Every write goes to all drives in the mirror. Each drive holds a complete copy of the data, so the array survives until the last drive fails. Reads can come from any copy, and good implementations such as Linux mdadm and most NAS systems spread reads across the drives.
A two-drive mirror is the most common setup. Three-way mirrors are used where downtime is expensive: they still have a full copy while one drive is being replaced.
Rebuilding a mirror
When a drive is replaced, the system copies the surviving drive onto the new one from start to end. There is no parity to calculate, so it is the simplest rebuild there is, but it still takes as long as writing one full drive.
In a two-drive mirror the surviving drive is the only copy during that time. If it hits an unreadable sector, that part of the data cannot be rebuilt. The chance depends on the size of the drive:
| Mirror | Read during rebuild | Desktop drives (10¹⁴) | NAS drives (10¹⁵) |
|---|---|---|---|
| 2 × 4 TB | 4 TB | 27% | 3.1% |
| 2 × 8 TB | 8 TB | 47% | 6.2% |
| 2 × 12 TB | 12 TB | 62% | 9.2% |
| 2 × 16 TB | 16 TB | 72% | 12% |
| 2 × 20 TB | 20 TB | 80% | 15% |
| 2 × 24 TB | 24 TB | 85% | 17% |
A three-way mirror removes that risk, because the third drive still holds the sector.
When RAID 1 is the right choice
- Two-bay NAS units. With two drives, RAID 1 is the only level with protection.
- Boot drives in servers and workstations.
- Small offices that want something simple: either drive can be read on its own.
With four or more drives, RAID 6 or RAID 10 give more space for the same number of drives.
A mirror is not a backup
RAID 1 copies everything at once, including mistakes. A deleted file, a file encrypted by ransomware or a corrupted folder is gone from both drives a moment later. Keep a separate backup.
Questions
Can I read a RAID 1 drive on another computer?
Usually yes for software mirrors (Linux mdadm, Synology, QNAP, Windows), because each drive is a full copy. Hardware RAID cards can store their own metadata, so a drive may need the same type of controller.
Does RAID 1 make my computer faster?
Reads can be faster if the system reads from both drives. Writes are as fast as one drive.
Can I turn RAID 1 into RAID 5 later?
Many NAS systems, including Synology, can convert a two-drive RAID 1 to RAID 5 when you add a third drive. The conversion takes many hours. Make a backup first.
What happens if both drives fail?
The data is lost. That is why drives from different batches and a separate backup matter.