RAID
From Sysadmin
A Redundant Array of Independent Disks is a broad term describing several different strategies provide additional performance or reliability to hard disk drives.
Performance and Redundancy
This table lists all RAID types that are commonly implemented.
| RAID Type | Mode | Capacity | Fault Tolerance | 2 Disks | 3 Disks | ||
|---|---|---|---|---|---|---|---|
| Read | Write | Read | Write | ||||
| RAID 0 | Normal | 100% | --- | 200% | 200% | ? | ? |
| RAID 1 | Normal | ? | ? | ? | ? | ? | ? |
| Degraded | ? | --- | ? | ? | ? | ? | |
| RAID 5 | Normal | ? | Error Detection | --- | --- | ? | ? |
| Degraded | ? | --- | --- | --- | ? | ? | |
| RAID 6 | Normal | ? | Error Correction | --- | --- | ? | ? |
| Degraded | ? | Error Detection | --- | --- | ? | ? | |
| Double Degraded | ? | --- | --- | --- | ? | ? | |
| RAID 01 | Normal | ? | ? | ? | ? | ? | ? |
| Degraded | ? | ? | ? | ? | ? | ? | |
| RAID 10 | Normal | ? | ? | ? | ? | ? | ? |
| Degraded | ? | ? | ? | ? | ? | ? | |
| RAID 50 | Normal | ? | ? | ? | ? | ? | ? |
| Degraded | ? | ? | ? | ? | ? | ? | |
| RAID 60 | Normal | ? | ? | ? | ? | ? | ? |
| Degraded | ? | ? | ? | ? | ? | ? | |
Capacity: The percentage of the capacity of a single drive within the array that is available for storage.
Performance: The read and write performance for the array as a total compared to each individual drive.
Simultaneous Drive Failure
Failure of two drives in an array is not as rare as it should be. Common causes include:
- Failure of one without anyone noticing (they notice when the second one fails)
- Failure due to a common environmental effect (eg, heat, water)
- Failure as the drives were manufactured together an have a similar MTBF. It is now generally recommended to use dissimilar HDDs in RAID, in contrast to older advice.
Not a Backup Strategy
RAID is not a backup strategy as it is neither offsite nor offline.
A widely held belief is that RAID is a replacement for a proper backup strategy. This is commonly expressed as We don't need to backup, we use RAID 1! This is absolutely not true. RAID is not a backup strategy.
There are many failure modes that RAID will not protect you from. These include: user error, sysadmin error, failure of the RAID controller, destruction of the server, and destruction of the building holding the server.
