1. Storage 2: RAID Learning Objectives –To understand the technology drivers leading to RAID arrays –To understand the principles of common RAID configurations –To understand performance and reliability consequences of common RAID configurations –To understand the limitations of RAID failure mode operation and recovery COMP25212 1

2. Technology Trends 1980: Hard disk state of art: 1Gbytes in 14” removable drive 1980’s – most PCs used floppy disks 1990’s – most PCs used hard disks over 1 decade, hard disk sales volumes increased x100 First effect: price reduction Second effect: PCs became drivers of disk technology Disk capacity and costs exceed Moore’s Law COMP25212 2

3. 1985-1995 (approx) Two distinct markets for disk drives: –Mainframe/minicomputer/servers (14 inch, high capacity, expensive) –PC (8 inch to 5.25 inch to 3.5 inch; lower capacity, cheap(er)) “How do we build server-class storage using PC-class components?” COMP25212 3

4. RAID in c.1990 PC disks are small => use lots of them PC disks are unreliable – how can we use lots to increase reliability Clue is in the “R” – “Redundant” Use “Redundancy” to provide some error immunity We’ve already seen this in “Disk Mirroring” – see Storage 1 What else can we do? COMP25212 4

5. A Diversion into Marketing A Diversion into Marketing 1985: Redundant Array of Inexpensive Disks COMP25212 5

6. A Diversion into Marketing A Diversion into Marketing 2010: Redundant Array of Inexpensive XXXXXX Individual Disks Don’t expect “inexpensive”!!! COMP25212 6

7. RAID 0 Striping –(as per previous lecture) COMP25212 7 All images © Colin M.L. Burnett

8. RAID 1 Mirroring – (as in previous lecture) COMP25212 8

9. RAID 2 (Bitwise) Hamming Code – not widely used COMP25212 9

10. RAID 3 Striping of (eg) bytes, with parity Can operate correctly with signalled errors COMP25212 10

11. RAID 4 Striping of blocks, with parity Performance issues: 1-block write: R-M-W Excessive traffic - Disk 3 COMP25212 11

12. RAID 5 Parity is distributed across multiple drives Distributes R-M-W COMP25212 12

13. RAID 6 Multiple Redundancy COMP25212 13

14. On Failure – An Entire Disk (RAID 2-6) Operate in degraded mode (every OS read needs to read every disk) Replace drive (hot-swap?) Rebuild array – how long? –Sequentially –On live system? Failure during rebuild? COMP25212 14

15. Where to Implement RAID? a)in OS b)in Device Interface (RAID controller) –OS-independent –but how to provide User Interface? COMP25212 15

16. For Next Time Does RAID implemented on disk controller suggest ways of building better systems? For the larger scale systems… COMP25212 16