RAID Redundant Arrays of Independent Disks

1. RAIDRedundant Arrays of Independent Disks Courtesy of Satya, 15-412 Fall ‘99

2. Motivation for Disk Arrays • Typical memory bandwidths  150 MB/sec • Typical disk bandwidths  10 MB/sec • Result: I/O-bound applications limited by disk bandwidth • (not just by disk latency!) • Two common disk bandwidth-limited scenarios • Scientific applications: one huge I/O-hungry app • Servers: many concurrent requests with modest I/O demands

3. S S S Solution: Exploit Parallelism • Stripe the data across an array of disks • many alternative striping strategies possible • Example: consider a big file striped across N disks • stripe width is S bytes • hence each stripe unit is S/N bytes • sequential read of S bytes at a time s0,0 s0,1 s0,2 • • • s0,N-1 s1,0 s1,1 s1,2 • • • s1,N-1 s2,0 s2,1 s2,2 • • • s2,N-1 • • • • • Disk 0 Disk 1 Disk 2 Disk N-1 S0,2 • • • S0,N-1 S0,1 S0,0 • • • S1,2 S1,N-1 S1,1 S1,0 S2,2 S2,N-1 S2,1 • • • S2,0 • • • • • • • • • • • •

4. Performance Benefit • Sequential read or write of large file • application (or I/O buffer cache) reads in multiples of S bytes • controller performs parallel access of N disks • aggregate bandwidth is N times individual disk bandwidth • (assumes that disk is the bottleneck) Disk 0 Disk 1 Disk 2 Disk N-1 • • • x MB/s x MB/s x MB/s x MB/s S k,1 S k,0 S k,2 S k,N-1 Array Controller Nx MB/s Sk

5. N concurrent small read or write requests • randomly distributed across N drives (we hope!) • common in database and Web server environments Disk 0 Disk 1 Disk 2 Disk N-1 • • • x MB/s x MB/s x MB/s x MB/s req 1 req 2 req 0 req N-1 Array Controller Nx MB/s N independent requests

6. Reliability of Disk Arrays • As number of disks grows, chances of at least one failing increases • Reliability of N disks = (reliability of 1 disk) / N • suppose each disk has MTTF of 50,000 hours • (roughly 6 years before any given disk fails) • then some disk in a 70-disk array will fail in (50,000 / 70) hours • (roughly once a month!) • Large arrays without redundancy too unreliable to be useful • “Redundant Arrays of Independent Disks” (RAID)

7. RAID Approaches • Many alternative approaches to achieving this redundancy • RAID levels 1 through 5 • hot sparing allows reconstruction concurrently with accesses • Key metrics to evaluate alternatives • wasted space due to redundancy • likelihood of “hot spots” during heavy loads • degradation of performance during repair

8. RAID Level 1 D1 B1 D2 B2 D3 B3 • Also known as “mirroring” • To read a block: • read from either data disk or backup • To write a block: • write both data and backup disks • failure model determines whether writes can occur in parallel • Backups can be located far way: safeguard against site failure

9. RAID Levels 2 & 3 D1 D2 D3 P • These are bit-interleaved schemes • In Raid Level 2, P contains memory-style ECC • In Rail Level 3, P contains simple parity • Rarely used today • • •

10. D0,2 P0 D0,1 D0,0 D1,2 P1 D1,1 D1,0 D2,2 P2 D2,1 D2,0 P3 D3,2 D3,1 D3,0 RAID Level 4 • Block-interleaved parity • Wasted storage is small: one parity block for N data blocks • Key problem: • parity disk becomes a hot spot • write access to parity disk on every write to any block D0,0D0,1D0,2=P0

11. D0,0 D0,1 D0,2 P0 D1,0 D1,1 P1 D1,3 D2,0 P2 D2,2 D2,3 P3 D3,1 D3,2 D3,3 RAID Level 5 • Rotated parity • Wastage is small: same as in Raid 4 • Parity update traffic is distributed across disks D0,0D0,1D0,2=P0

12. 3 1 2 4 Fault-free Read Fault-free Write  D D D P D D D P Degraded Read Degraded Write   D D D P D D D P RAID 5 Actions