1. COMP25212 ARRAY OF DISKS Sergio Davies Sergio.Davies@manchester.ac.uk Feb/Mar 2014COMP25212 – Storage 2

2. Problems With Disks They are (were?) too small –Fixed (multiple disks), fixed (striping) They sometimes fail –Fixed (mirroring), worsened (multiple/striping) They are too slow –Fixed (mirroring/striping) 2 Feb/Mar 2014COMP25212 – Storage 2

3. 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 3 Feb/Mar 2014COMP25212 – Storage 2

4. 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 4 Feb/Mar 2014COMP25212 – Storage 2

5. 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?” 5 Feb/Mar 2014COMP25212 – Storage 2

6. 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? 6 Feb/Mar 2014COMP25212 – Storage 2

7. A Diversion into Marketing A Diversion into Marketing 1985: Redundant Array of Inexpensive Disks 7 Feb/Mar 2014COMP25212 – Storage 2

8. A Diversion into Marketing A Diversion into Marketing 2010: Redundant Array of Inexpensive - Independent Disks Don’t expect “inexpensive”!!! 8 Feb/Mar 2014COMP25212 – Storage 2

9. RAID 0 Striping –(as per previous lecture) 9 All images © Colin M.L. Burnett Feb/Mar 2014COMP25212 – Storage 2

10. RAID 1 10 Mirroring –(as per previous lecture) Feb/Mar 2014COMP25212 – Storage 2

11. RAID 2 (Bitwise) Hamming Code – not widely used 11 Feb/Mar 2014COMP25212 – Storage 2

12. RAID 3 Striping of (eg) bytes, with parity Can operate correctly with signalled errors 12 Feb/Mar 2014COMP25212 – Storage 2

13. RAID 4 Striping of blocks, with parity Performance issues: 1-block write requires R-M-W Excessive traffic on Disk 3 13 Feb/Mar 2014COMP25212 – Storage 2

14. RAID 5 Parity is distributed across multiple drives Distributes R-M-W 14 Feb/Mar 2014COMP25212 – Storage 2

15. RAID 6 Multiple Redundancy 15 Feb/Mar 2014COMP25212 – Storage 2

16. RAID combinations Each RAID configuration has its own advantages and disadvantages; Can we combine the advantages by combining RAID configurations? Multiple RAID layers, or nested RAID 16 Feb/Mar 2014COMP25212 – Storage 2

17. RAID 01 (0+1) Balancing performance and redundancy Mirroring couple of striping disks [ image from: http://www.linux-mag.com/id/7928/ ] http://www.linux-mag.com/id/7928/ 17 Feb/Mar 2014COMP25212 – Storage 2

18. RAID 10 (1+0) Striping between mirrored couples of disks Same as before(?) [ image from Wheart available at: http://commons.wikimedia.org/wiki/File:RAID_10.svg ] http://commons.wikimedia.org/wiki/File:RAID_10.svg 18 Feb/Mar 2014COMP25212 – Storage 2

19. RAID 10 vs 01 [ image from Paulish available at: http://en.wikipedia.org/wiki/File:Raid_ 0%2B1.svg ] http://en.wikipedia.org/wiki/File:Raid_ 0%2B1.svg [ image from Paulish available at: http://commons.wikimedia.org/wiki/Fi le:RAID_10_6Drives.svg ] http://commons.wikimedia.org/wiki/Fi le:RAID_10_6Drives.svg 19 Feb/Mar 2014COMP25212 – Storage 2 Let’s check the combinations of faults which may happen

20. RAID 50 High performance and error correction 20 Feb/Mar 2014COMP25212 – Storage 2 [ image from: http://blog.iweb.com/en/2010/05/an-overview-of-raid-technology/4283.html ]http://blog.iweb.com/en/2010/05/an-overview-of-raid-technology/4283.html

21. RAID 160 [ image from: http://www.linux-mag.com/id/7932/ ]http://www.linux-mag.com/id/7932/ 21 Feb/Mar 2014COMP25212 – Storage 2

22. 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? 22 Feb/Mar 2014COMP25212 – Storage 2

23. Where to Implement RAID? a)in OS b)in Device Interface (RAID controller) –OS-independent –but how to provide User Interface? 23 Feb/Mar 2014COMP25212 – Storage 2

24. For Next Time Does RAID implemented on disk controller suggest ways of building better systems? For the larger scale systems… 24 Feb/Mar 2014COMP25212 – Storage 2