1. Copyright © Curt Hill, 2003-2010 RAID What every server wants!

2. Copyright © Curt Hill, 2003-2010 History I In the late part of the 1970s and there were two types of disks: –Hard –Floppy The hard drives were only used on mainframes or minis –Quite expensive The floppys were mostly used on personal computers In about 1984 hard drives started appearing on IBM personal computers

3. Copyright © Curt Hill, 2003-2010 History II Now there were two types of hard drives: –Professional Large 100-500 M Expensive –Amateur Small: 5-50 M Inexpensive

4. Copyright © Curt Hill, 2003-2010 History III At this point economics takes charge The large disks were a small market, typically 10s of thousands The small disks were mass market, typically 10s of millions What happens to the prices?

5. Copyright © Curt Hill, 2003-2010 History IV Someone gets a bright idea: –Should I buy one 500 M professional disk for $50,000 or 10 50M amateur disks at $100 each? RAID is born: Redundant Array Inexpensive Disks –Later Redundant Array Independent Disks There might even be a performance improvement if I can access the array independently However, we need some type of controller to treat the 10 disks as if it were one

6. Copyright © Curt Hill, 2003-2010 Issues Redundancy –If the small is less reliable or if the data is so important we need multiple copies for safety Speed –Increase speed by writing part of the data to one drive and another part to another at the same time Versions –There have been many, termed Levels –Currently level 0 through level 6

7. Copyright © Curt Hill, 2003-2010 Redundancy Multiple copies allows one disk to crash without losing data Two forms: –Mirroring AKA Shadowing, Duplexing –Error Correction Codes

8. Copyright © Curt Hill, 2003-2010 Mirroring Write all the data twice –Once to a disk and its mirror –This is done simultaneously, so no extra delay A read only has to wait for the faster of the two If a disk crashes, no rebuild is needed since the other disk may just be copied High disk overhead, needs two disks to store one disk worth of data Can read different parts of the two at same time to increase speed

9. Copyright © Curt Hill, 2003-2010 Error Correction Code Even more history and background is needed here See the ECC.ppt presentationECC.ppt Many of the EC codes were studied by Hamming of Bell Labs –Thus known as Hamming codes

10. Copyright © Curt Hill, 2003-2010 ECC Instead of mirroring which requires double disk space use an ECC Conceptually: –The eight bit data placed on eight separate drives and the four bit ECC on another –Any one disk that fails may be recreated from the rest

11. Copyright © Curt Hill, 2003-2010 Speed Speed requires parallelism –Do two things at once With a mirrored disk read the front half from one and the back half from the other Transfer time cut in half This leads to a more general approach: stripes

12. Copyright © Curt Hill, 2003-2010 Stripes Cut the data into stripes If you have N disks –Partition into N pieces –Read or write N pieces at a time Best if each piece goes to a separate controller

13. Copyright © Curt Hill, 2003-2010 Controllers Controllers may be hardware or software or both Some levels make a software controller a problem

14. Copyright © Curt Hill, 2003-2010 RAID Levels Originally specified with six levels –0-5 Some of these are seldom used and new ones have been added

15. Copyright © Curt Hill, 2003-2010 Level 0 Striped disk array Minimum of two disks No redundancy or error checking –If a drive is lost all the data is lost –Only RAID that does not protect data Real RAID or not?

16. Copyright © Curt Hill, 2003-2010 Level 0 - Striping A1A2A3A4 B1B2B3B4 C1C2C3C4 Four stripes Each block of data is written in four pieces No redundancy

17. Copyright © Curt Hill, 2003-2010 Level 1 Mirroring Minimum of two disks 100% redundancy –A rebuilt disk is just a copy, not computed Simple controller Cannot be expanded on the fly

18. Copyright © Curt Hill, 2003-2010 Level 1 - Mirroring AAEE BBFF CCGG Four disks, each is a mirror of another Each block of data is written twice Need twice as much space

19. Copyright © Curt Hill, 2003-2010 Level 2 Striped disk at bit level ECC provides the redundancy Minimum of seven disks for storing 4 bit word Not commercially viable Number of parity disks is proportional to log of number of data disks Not very flexible

20. Copyright © Curt Hill, 2003-2010 Level 2 – Striping with ECC A1A2A3A4 B1B2B3B4 C1C2C3C4 Four stripes of data protected by three of ECC ECC is computed on the fly EAx EBx EAzEAy EByEBz ECxECyECz

21. Copyright © Curt Hill, 2003-2010 Level 3 Striped disk array with one ECC disk Each block is striped across disks One disk may fail without diminishing throughput Minimum of three disks High data rates Controller should be in hardware, not just software

22. Copyright © Curt Hill, 2003-2010 Level 3 – Striping with ECC A1A2A3A4 B1B2B3B4 C1C2C3C4 Four stripes of data protected by three of ECC ECC is computed on the fly ECCA ECCB ECCC

23. Copyright © Curt Hill, 2003-2010 Level 4 Similar to level 3 except –Blocks are not subdivided –Different block are written to different disks Minimum of three disks Controller is complex –Should be hardware Not easy to rebuild in case of failure

24. Copyright © Curt Hill, 2003-2010 Level 5 Striped disk array with distributed ECC blocks Each disk stores both data and ECC –A disk stores ECC data for other disks Minimum of three disks Any single disk failure will result in no loss of data Most complex controller design

25. Copyright © Curt Hill, 2003-2010 Level 5 – Striping with Distributed ECC A1 A2 A3 A4 B1 B2 B3 B5 C1 C2 C5 C4 Four stripes of data Each ECC protects other four No disks are only data or only ECC ECC5 ECC4 ECC3 ECC2 ECC1D1 D5 D4 D3 E2 E3 E4 E5

26. Copyright © Curt Hill, 2003-2010 Level 6 Striped disk array –Similar to five except two independent ECC schemes –ECC blocks distributed among data disks Minimum of four disks May have multiple disk failures without loss of data

27. Copyright © Curt Hill, 2003-2010 Level 6 – Striping with Two Distributed ECCs A1 A2 A3 B1 B2 B3 C1 C3 C2 Four stripes of data Two types of ECC each of which protects a different group No disks are only data or only ECC ECC4 ECC3 ECC2 ECC1 D3 D2 D1 ECCb ECCc ECCdECCa

28. Copyright © Curt Hill, 2003-2010 Others Several combinations of these exists 0 + 1 –Mirroring a striped disk 10 –Striping a mirrored disk 53 –Level 0 and 3 –Level 0 whose stripes are level 3 arrays

29. What is coming? As disk arrays get larger the likelihood of a double failure becomes significant Although no product yet, it seems likely that a triple parity scheme is inevitable Copyright © Curt Hill, 2003-2010