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Chapter 38 Redundant Arrays of Inexpensive Disks (RAIDs)

Chapter 38 Redundant Arrays of Inexpensive Disks (RAIDs). Byungjun Kim(bjkim@archi.snu.ac.kr) School of Computer Science and Engineering Seoul National University. Outline. Introduction About RAIDs Variant levels of RAIDs RAID level 0 : Striping RAID level 1 : Mirroring

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Chapter 38 Redundant Arrays of Inexpensive Disks (RAIDs)

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  1. Chapter 38Redundant Arrays of Inexpensive Disks(RAIDs) Byungjun Kim(bjkim@archi.snu.ac.kr) School of Computer Science and Engineering Seoul National University

  2. Outline • Introduction • About RAIDs • Variant levels of RAIDs • RAID level 0 : Striping • RAID level 1 : Mirroring • RAID level 4 : Saving space with parity • RAID level 5 : Rotating parity • RAIDs comparison • Other interest RAIDs issues • Summary

  3. Introduction RAID Redundant Arrays of Inexpensive(independent) Disks Source : http://en.hdyo.org/you/questions (Retrieved on 2015/06/01) Source : http://gizmodo.com/tag/raid (Retrieved on 2015/06/01) Source : https://www.backuprun.com/blog/raid-is-not-backup (Retrieved on 2015/06/01) • When we use a disk, • We sometimes wish it to be faster • We sometimes wish it to be larger • We sometimes wish it to be morereliable

  4. RAIDs From Wikipedia RAID is a data storage virtualization technology that combines multiple disk drive components into single logical unit for the purposes of data redundancy or performance improvement Aggregation • Performance • Parallelism • Capacity • Multiple disk drive components • Reliability • Mirroring • Striping with parity

  5. RAIDs Source : http://www.sisense.com/technology/ (Retrieved on 2015/06/01) • A RAID provides advantages transparently • A RAID can replace a disk without changing a software • A RAID looks like a big, fast, reliable disk

  6. How To Evaluate A RAID Source : http://www.seagate.com/kr/ko/manuals/network-storage/business-storage-nas-os/raid-modes/ (Retrieved on 2015/06/01) • Evaluating each RAID design along three axes • Capacity • A set of N disks • Without redundancy : N • Reliability • Some form of redundancy • Performance

  7. Evaluating RAID Performance • Two types of performance metrics • Single-request latency • How much parallelism can exist during a single logical I/O operation • Steady-state throughput • Total bandwidth of many concurrent requests • Two types of workloads • Sequential (S) • Spending little time seeking and waiting for rotation • Spending most of its time transferring data • Random (R) • The opposite of a sequential type

  8. RAID Level 0 : Striping RAID 0 Stripe Block 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 DISK 0 DISK 1 DISK 2 DISK 3 • RAID 0 spreads blocks of the array across the disks • Only one block is placed on each disk before moving on to the next • The stripe consists of four blocks of data RAID 0 : Striping

  9. RAID Level 0 : Striping RAID 0 Chunk size : 2 blocks 0 2 4 6 1 3 5 7 8 10 12 14 9 11 13 15 DISK 0 DISK 1 DISK 2 DISK 3 • Two blocks on each disk before moving on to the next disk • The chunk size of the RAID array is two blocks • The stripe consists of eight blocks of data • RAID 0 : Striping • A different chunk size ver.

  10. RAID Level 0 : Striping • Capacity • N disks • Reliability • Any disk failure will lead to data loss • Performance

  11. RAID Level 1 : Mirroring RAID 1 0 0 1 1 2 2 3 3 DISK 0 DISK 1 • The RAID updates both copies of the data RAID 1 : Mirroring

  12. RAID Level 1 : Mirroring RAID 10 RAID 01 RAID 0 RAID 1 RAID 1 RAID 1 RAID 1 RAID 0 RAID 0 2 2 0 0 1 1 2 2 0 1 0 1 5 5 3 3 4 4 5 5 3 4 3 4 8 8 6 6 7 7 8 8 6 7 6 7 11 11 9 9 10 10 11 11 9 10 9 10 DISK 4 DISK 5 DISK 0 DISK 1 DISK 2 DISK 3 DISK 2 DISK 5 DISK 0 DISK 1 DISK 3 DISK 4 Different ways to place block copies across the disks

  13. RAID Level 1 : Mirroring • Capacity • N / 2 disks • Reliability • It can tolerate the failure of any one disk • It can tolerate up to N / 2 failures depending on which disks fail • Performance

  14. RAID Level 1 : Mirroring RAID 1 Request : Read block 0 ~7 RAID 0 0 0 1 1 0 1 2 3 2 2 3 3 0 1 0 1 2 3 4 5 6 7 4 4 5 5 2 3 4 5 6 7 8 9 10 11 4 5 6 6 7 7 12 13 14 15 6 7 DISK 0 DISK 1 DISK 2 DISK 3 DISK 0 DISK 1 DISK 2 DISK 3 Performance

  15. RAID Level 1 : Mirroring RAID 1 6 0 0 0 0 1 1 2 2 3 3 Disk 0 4 4 4 5 5 6 6 7 7 4 2 DISK 0 DISK 1 DISK 2 DISK 3 Performance

  16. RAID Level 4 : Saving Space With Parity P = D0 ⊕ D1 ⊕ D2 ⊕ D3 RAID 4 0 1 2 3 P0 4 5 6 7 P1 8 9 10 11 P2 12 13 14 15 P3 DISK 0 DISK 1 DISK 2 DISK 3 DISK 4 • Write requests update the data block and the parity block RAID 4 : Saving space with parity

  17. RAID Level 4 : Saving Space With Parity Large RAIDs require a high number of reads to compute parity RAID 4 0 1 2 3 P0 4 5 6 7 P1 8 9 10 11 P2 12 13 14 15 P3 DISK 0 DISK 1 DISK 2 DISK 3 DISK 4 • Random write • Additive parity • Read in all of the other data blocks in the stripe in parallel (block 0, 2 and 3) • XOR those with new block 1 • Write the new data and parity to disks in parallel

  18. RAID Level 4 : Saving Space With Parity New 1 0 Old 1 2 3 P0 1. Read 2. Read 3. Write 4. Write 0 New 1 2 3 P0 Source : 컴퓨터 구조 강의 슬라이드 (Retrieved on 2015/06/01) • Random write • Subtractive parity

  19. RAID Level 4 : Saving Space With Parity The read and write to data could happen in parallel The parity disk prevents any parallelism RAID 4 0 1 2 3 P0 4 5 6 7 P1 The parity disk two I/Os (one read, one write) per logical I/O 8 9 10 11 P2 12 13 14 15 P3 DISK 0 DISK 1 DISK 2 DISK 3 DISK 4 Small-write problem

  20. RAID Level 4 : Saving Space With Parity • Capacity • (N -1) disks • Reliability • It can tolerate 1 disk and no more • Performance

  21. RAID Level 5 : Rotating Parity RAID 5 0 1 2 3 P0 5 6 7 P1 4 10 11 P2 8 9 15 P3 12 13 14 P4 16 17 18 19 DISK 0 DISK 1 DISK 2 DISK 3 DISK 4 • Eliminates the parity disk bottleneck in RAID 4 • Parallel service of write requests is possible RAID 5 : Rotating parity

  22. RAID Level 5 : Rotating Parity • Capacity • (N -1) disks • Reliability • It can tolerate 1 disk and no more • Performance

  23. RAID Level 5 : Rotating Parity RAID 5 can utilize all of the disks RAID 4 VS RAID 5

  24. RAID Level 5 : Rotating Parity RAID 5 RAID 5 still generates 4 total I/O operations 0 1 2 3 P0 5 6 7 P1 4 10 11 P2 8 9 15 P3 12 13 14 P4 16 17 18 19 All writes can proceed in parallel DISK 0 DISK 1 DISK 2 DISK 3 DISK 4 RAID 4 VS RAID 5

  25. RAID Comparison : A Summary • RAID capacity, reliability and performance

  26. Other Interesting RAID Issues Source : http://www.seagate.com/kr/ko/manuals/network-storage/business-storage-nas-os/raid-modes/ (Retrieved on 2015/06/01) RAID level 6 : Tolerating multiple disk faults Hot spare

  27. Summary • About RAIDs • Variant RAIDs levels • RAID level 0 : Striping • RAID level 1 : Mirroring • RAID level 4 : Saving space with parity • RAID level 5 : Rotating parity • RAID comparison

  28. Appendix. Chunk Size Source : http://www.dailymail.co.uk/femail/article-2098861/Dont-use-Photoshop-make-models-slimmer (Retrieved on 2015/06/01) Source : http://www.apaxsoftware.com/database-management/ (Retrieved on 2015/06/01) • The one of the keys to increase RAID performance • To get good performance, the user must have a reasonable chunk size • A small chunk size VS A big chunk size • The parallelism : A small chunk size wins! • The positioning time : A big chunk size wins! • What is reasonable chunk size? • It depends on average I/O request size

  29. Appendix. RAID Mapping Problem How does the RAID know which physical disk and offset to access? Disk = A % Num_of_disks Offset = A / Num_of_disks RAID 0 A request for block 14 0 1 2 3 Disk = 14 % 4 = 2 Offset = 14 / 4 = 3 4 5 6 7 8 9 10 11 12 13 14 15 14 DISK 0 DISK 1 DISK 2 DISK 3 • Given a logical block to read or write • Example

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