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Disk Failures

Disk Failures

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Disk Failures

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  1. Disk Failures Presented by Timothy Chen Spring 2013

  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_.png • http://en.wikipedia.org/wiki/RAID