T111

1. T111 RAID (not the fly spray…)

2. RAID • Redundant Array of Inexpensive Disks • HDDs are the main type of storage used in computing • They hold vast amounts of Data for their size • They are relatively cheap but… • They are relatively slow

3. Question… • What is better? • 1 x 1TB HDD • 2 x 512GB HDD

4. RAID • One HDD is limited to: • One read / write at a time • One interface • Multiple HDDs working together can overcome these limitations and more! • Disks used in a RAID array must be the same size, otherwise they will all be reduced to the size of the smallest disk

5. Performance • When a file or several files are shared across two or more disks, they can be accessed / read / written faster • Imagine a large video file 3GB in size: • 1 disk can copy the file in 3 units of time • 2 disks can copy the file in 1.5 units of time • 3 disks can copy the file in 1 unit of time • 4 disks can copy the file in… ? • This is how RAID improves performance (in theory…)

6. Reliability • Disks are prone to failure • The more disks you have – the more likely it is that one will fail • If however, you have two disks that both have a copy of exactly the same data – you have halved your chance of loosing that data due to disk failure

7. Management • Windows assigns a drive letter to each HDD installed in the computer • If you have 10 HDDs then you have 10 different drive letters to manage • If your 10 HDDs are all 90% full you cannot store a file that would take up 11% of one of your HDDs • RAID can combine these disks to appear as one

8. RAID 0 • RAID is defined in levels • Level 0 RAID uses two or more disks, which spread files across all the disks in the array RAID 0 Array HDD1 HDD2 One HDD without RAID

9. RAID 0 • Good for performance • The read / write performance will increase for every disk added to the array • If one of the disks fail – all data is lost! Bye bye data…

10. RAID 0 • High performance • High risk – low reliability • Good for storage of temporary files where performance needs to be high but reliability does not • Would the page file benefit from this?

11. RAID 1 • Also called Disk Mirroring • Requires two disks • All data is written twice – once to each disk • At any time, there are two copies of all the data e.g.

12. RAID 1 • Performance is similar to having one disk • Write performance is slightly slower as everything has to be written to both disks – albeit simultaneously • Read performance can be slightly faster as there are two copies to read from • High reliability – if one disk fails, not only is there another copy of the data but the computer can carry on unaffected • Low efficiency – one of the disks is essentially wasted as two 500GB disks will be seen as one 500GB disk • Good for when reliability needs to be high • When read performance needs to be higher than write performance e.g. the operating system

13. RAID 5 • Similar to RAID 0 except that another disk is used as Parity • Minimum of 3 disks

14. RAID 5 • Uses Parity to add redundancy to the array • If the first disk fails, it can be remade using the other two disks • But how?! • Using XOR (Exclusive Or)

15. XOR • You do not need to know how XOR works… • But you do need to know the rules • The symbol for XOR is ⊕ X, Y and Z are three numbers. If X ⊕ Y = Z then Y ⊕ X = Z and X ⊕ Z = Y and Z ⊕ Y = X etc..

16. XOR • This rule is easily extended so that if W is also some number then: If X ⊕ Y ⊕ Z = W then X ⊕ Y = W ⊕ Z and X ⊕ Z = W ⊕ Y and Z ⊕ Y = W ⊕ X and Z = W ⊕ X ⊕ Y and X = W ⊕ Z ⊕ Y etc.. • Given that 0x45⊕0x76 = 0x33 what is 0x76 ⊕ 0x33 ?

17. RAID 5 • RAID 5 uses XOR to add redundancy to an array • In RAID 5 the parity is spread across the disks

18. RAID 5 • Disk 2 has failed… • This is how the RAID controller will recover your data: Block A: read directly from disk 1. Block B: read block A from disk 1 and (A ⊕ B) from disk 3, calculate B = A ⊕ (A ⊕ B) Block C: read directly from disk 1. Block D: read directly from disk 3. Block E: read block E F from disk 1 and F from disk 3, calculate E = (E ⊕ F) ⊕ F Block F: read directly from disk 3. • If you want to save new data B’, this is what the RAID controller does: • Read Block A from disk 1 and calculate new (A ⊕ B)’ = A ⊕ B’ and write this to disk 3.

19. RAID 5 • Read performance is better than a single disk as all disks can be read simultaneously • Write performance will be poor because we need to write the data onto one disk, calculate the parity and then store the parity on another disk • Reliability • One disk can fail without data loss • Read and write performance will be terrible as a simple read may need to be calculated using the parity of the other disk • Efficiency • The equivalent of one disk in the array will be used for parity, this means that you loose one disk worth of storage

20. RAID 5 • Good all rounder • Lower wastage of disk space (compared to RAID 1) • Fair performance • Your H: drive probably uses RAID 5 or similar • Not so good for operating system drives as the OS often crashes when one disk fails due to the poor performance

21. Disk Reads / Writes • The number of times a disk has to be read or written for a given file affects the performance of the operation • The performance of the operation can be increased by performing these reads / writes at the same time on different disks

22. RAID Quiz • If you have four 500GB drives in a RAID 5 array, how big is the logical drive as seen by the operating system? • What about five 500GB drives?