Adaptec SCSI RAID 2120S/2200S Software User’s Guide
The cost of this redundancy is inefficient use of capacity, because all data is written to both drives, only half of the total capacity is available.
RAID-1 offers no write performance advantage over a single drive, but read performance benefits from being able to share the load between two drives.
RAID-5
A minimum of three drives is required to create a RAID-5. Like a RAID-0, data is striped across the drives, however in the case of a RAID-5, the capacity of one drive is used to store parity information. The controller generates this parity data every time data is written to the array, and it is distributed in stripes across all the drives.
In the event of a drive failure, the contents of the failed drive can be rebuilt from the data and parity on the remaining drives.
RAID-5
Drive | Drive | Drive | Drive |
Data 0 | Data 1 | Data 2 | Parity 0 |
Data 3 | Data 4 | Parity 1 | Data 5 |
Data 6 | Parity 2 | Data 7 | Data 8 |
Parity 3 | Data 9 | Data 10 | Data 11 |
Using parity minimizes the capacity cost of redundancy. Since only one drive is used to store parity, the worst case of a three-drive RAID-5 only loses one third of the total capacity, for arrays with more drives the lost capacity is smaller.
RAID-5 write performance is limited by the need to generate parity data for every write. Read performance is good because the load is spread equally across all the drives.
