RAID (Redundant Arrays of Independent Disks)
RAID, or “Redundant Arrays of Independent Disks” is a technique which makes use of a combination of multiple disks instead of using a single disk for increased performance, data redundancy or both. The term was coined by David Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987.
Why data redundancy?
Data redundancy, although taking up extra space, adds to disk reliability. This means, in case of disk failure, if the same data is also backed up onto another disk, we can retrieve the data and go on with the operation. On the other hand, if the data is spread across just multiple disks without the RAID technique, the loss of a single disk can affect the entire data.
Key evaluation points for a RAID System
- Reliability: How many disk faults can the system tolerate?
- Availability: What fraction of the total session time is a system in uptime mode, i.e. how available is the system for actual use?
- Performance: How good is the response time? How high is the throughput (rate of processing work)? Note that performance contains a lot of parameters and not just the two.
- Capacity: Given a set of N disks each with B blocks, how much useful capacity is available to the user?
RAID is very transparent to the underlying system. This means, to the host system, it appears as a single big disk presenting itself as a linear array of blocks. This allows older technologies to be replaced by RAID without making too many changes in the existing code.
Different RAID levels
RAID-0 (Stripping)
- Blocks are “stripped” across disks.
- In the figure, blocks “0,1,2,3” form a stripe.
- Instead of placing just one block into a disk at a time, we can work with two (or more) blocks placed into a disk before moving on to the next one.
Evaluation:
- Reliability: 0
There is no duplication of data. Hence, a block once lost cannot be recovered. - Capacity: N*B
The entire space is being used to store data. Since there is no duplication, N disks each having B blocks are fully utilized.
RAID-1 (Mirroring)
- More than one copy of each block is stored in a separate disk. Thus, every block has two (or more) copies, lying on different disks.
What about the other RAID levels?
