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
8 drives of 8 TB each.
RAID 50 at a glance
| Minimum drives | 6 |
|---|---|
| Usable space | (drives − number of groups) × size of the smallest drive |
| Drives that can fail | 1 per group |
| Read speed | High |
| Write speed | Better than one large RAID 5 |
| Rebuild | Reads only the drives in the affected group |
How RAID 50 works
The drives are split into groups called spans. Each span is a RAID 5, and data is striped across the spans like RAID 0. This calculator uses two spans of equal size, which is the most common setup; controllers such as Broadcom MegaRAID allow more.
Why split a RAID 5 into spans?
A rebuild only reads the drives in the affected span, so it reads half as much data as a single RAID 5 of the same size, and the other span keeps full speed. Writes are also faster because each parity calculation covers fewer drives.
The weakness is the same as RAID 5: while one span rebuilds, a read error or a second failure in that span loses data. With large drives, RAID 60 is the safer version.
When RAID 50 is used
Mostly on hardware RAID controllers in servers with 8 to 24 drives of moderate size, where capacity and write speed both matter. On NAS systems and with ZFS, the equivalent is a pool of several RAIDZ1 groups.
Questions
What is the minimum number of drives for RAID 50?
Six: two RAID 5 groups of three drives.
How many drives can fail in RAID 50?
One per group is always safe. Two in the same group lose the array.
RAID 50 or RAID 6?
RAID 6 survives any two failures; RAID 50 does not. RAID 50 writes faster. For large drives, RAID 6 or RAID 60 is safer.