1. Disk Failures Skip

2. Index 13.4 Disk Failures 13.4.1 Intermittent Failures 13.4.2 Organizing Data by Cylinders 13.4.3 Stable Storage 13.4.4 Error- Handling Capabilities of Stable Storage 13.4.5 Recovery from Disk Crashes 13.4.6 Mirroring as a Redundancy Technique 13.4.7 Parity Blocks 13.4.8 An Improving: RAID 5 13.4.9 Coping With Multiple Disk Crashers

3. Intermittent Failures If we try to read the sector but the correct content of that sector is not delivered to the disk controller Controller will check good and bad sector If the write is correct: Read is performed Good sector and bad sector is known by the read operation

4. CheckSum Read operation that determine the good or bad status

5. How CheckSum perform Each sector has some additional bits Set depending on the values of the data bits stored in each sector If the data bit in the not proper we know there is an error reading Odd number of 1: bits have odd parity(01101000) Even number of 1: bit have even parity (111011100) Find Error is the it is one bit parity

6. Stable Storage Deal with disk error Sectors are paired and each pair X showing left and right copies as Xl and Xr It check the parity bit of left and right by subsituting spare sector of Xl and Xr until the good value is returned

7. Error-Handling Capabilities of Stable Storage Since it has XL and XR, one of them fail we can still read other one Chance both of them fail are pretty small The write Fail, it happened during power outage

8. Recover Disk Crash The most serious mode of failure for disks is “head crash” where data permanently destroyed. The way to recover from crash, we use RAID method

9. Mirroring as a Redundancy Technique it is call Raid 1 Just mirror each disk

10. Raid 1 graph

11. Parity Block It often call Raid 4 technical read block from each of the other disks and modulo-2 sum of each column and get redundant disk disk 1: 11110000 disk 2: 10101010 disk 3: 00111000 get redundant disk 4(even 1= 0, odd 1 =1) disk 4: 01100010

12. Raid 4 graphic

13. Parity Block- Fail Recovery It can only recover one disk fail If it has more than one like two disk Then it can’t be recover us modulo-2 sum

14. An Improvement Raid 5

15. Coping with multiple Disk Crash For more one disk fail Either raid 4 and raid 5 can’t be work So we need raid 6 It is need at least 2 redundant disk

16. Raid 6

17. Reference http://www.definethecloud.net/wp- content/uploads/2010/12/325px- RAID_1.svg_.pnghttp://www.definethecloud.net/wp- content/uploads/2010/12/325px- RAID_1.svg_.png http://en.wikipedia.org/wiki/RAID