Overview of Redundant Disk Arrays
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Some slides on the original design of RAID, a Redundant Array of Inexpensive Disks. Demonstrates the tradeoffs between the varying RAID levels and gives some historical context.
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Overview of Redundant Disk Arrays
- 1. Andrew Robinson University of Michigan <[email protected]> Redundant Arrays of Inexpensive Disks (RAID) What a cool idea!
- 2. Authors • David A Patterson • Garth Gibson • Randy H Katz Officially published in 1988.
- 3. Overview • What is RAID? • Why bother? • What is RAID, really? • How well does it work? • How’s it holding up?
- 4. What is RAID? • Take a bunch of disks and make them appear as one disk. • Put data on all of them • Use all at once to gain performance • Duplicate data to gain reliability • Buy cheap disks to gain dollars
- 5. This seems like a lot of work… why bother?
- 6. Let’s go back to 1987
- 7. CPUs and Memory kept getting faster… • Exponential growth everywhere! • CPU Performance: 1.4X increase per year – More transistors – Better architecture • Memory Performance: 1.4-2X increase per year – Invention of caches – SRAM technology
- 8. … but disks did not. • It’s hard to make things spin exponentially faster every year (they tend to fly apart). • Disk seek time improved at a rate of approximately 7% a year. • Caching had been employed to buffer I/O activity, this works reasonably well for predictable workloads.
- 9. Slow I/O Makes Slow Computers • Amdahl’s Law describes the impact of only improving some pieces, while leaving others. 1 S= S – The effective speedup F – Fraction of work in faster mode (1- f ) + f / k K – Speedup while in faster mode
- 10. …really slow. • If applications spend 10% of their time in I/O, when computers are 10 times faster, they will only appear 5% faster. Something needed to be done.
- 11. What should we do? • Single Large Expensive Disks (SLED) are not improving fast enough. • Larger memory or solid state drives weren’t practical • Small personal hard drives are emerging… can we do something with those?
- 14. Why didn’t someone do this before? • Standards like SCSI have finally allowed drive makers to integrate features seen in traditional mainframe controllers.
- 15. There is a problem… • A hundredfold increase in number of disks means a hundredfold increase decrease in total reliability MTTFSingleDisk MTTFDiskArray = nDisks
- 16. that’s all really nice, but what is RAID, really?
- 17. A couple levels… a single idea • RAID manages the tradeoff between performance and reliability • RAID comes in levels (RAID1 to RAID5) • These levels represent points in the performance reliability space
- 18. Groups, Disks, and Check Disks • RAID organizes disks into groups of reliability • Some of the disks in a group store error correcting data D = Total disks with data G = Disks in a group C = Number of check disks in a group
- 19. Metrics • Useable Storage – Percent of storage that holds data, excluding parity information • Performance – Tough to make one number: – Reads, Writes, and Read-Modify-Write Access Patterns – Sequential and Random Data Distribution
- 20. RAID1 – The Naive Approach • Mirroring of all data • To read: – Use either disk • To write: – Send to both disks simultaneously • Minor read performance increase.
- 21. Evaluation Pros Cons • Reads can occur • Useable storage is cut in simultaneously half • Seek times can improve • All other performance with special controllers metrics are left the same • Predictable performance Alright for large sequential jobs and transaction processing jobs
- 22. RAID2 – Bit Level Striping • Uses Hamming Code for Error Detection • Requires many check disks – For 10 data disks, 4 check disks – For 25 data disks, 5 check disks • Can detect errors, and determine the at-fault disk
- 23. RAID2 - Visually
- 24. Evaluation Pros Cons • Better useable storage, 71% • Dismal small random data for G=10, 83% for G=25 access performance: 3-9% of RAID1 or SLED Good for large sequential jobs, bad for transaction processing systems.
- 25. RAID3 – Byte Level Striping • Simpler parity error correction • Only a single check disk required for error detection • Cannot determine which disk failed, but that’s usually pretty obvious • Transfers of large continuous blocks is good
- 26. RAID3
- 27. Evaluation Pros Cons • Even better useable • Small random data access storage, 91% for G=10, 96% performance: Just as bad as for G=25 RAID2 Even better for large sequential jobs, bad for transaction processing systems.
- 28. What is parity? • Parity is calculated as an XOR of the data blocks. • XOR is reversible: – 1011 (A1) XOR 1100 (A2) => 0111 (AP) “parity” – 0111 (AP) XOR 1011 (A1) => 1100 (A2) – 0111 (AP) XOR 1100 (A2) => 1011 (A1) • This makes error detection and reconstruction possible!
- 29. RAID4 - Block Level Striping • Like RAID3, but more parallelly • Interleave data at sector level rather than bit level • Allows for servicing of multiple block requests by different drives • Still keeps all the parity information on a single drive
- 30. RAID4
- 31. Evaluation Pros Cons • Finally better small random • Small writes, and read- access. Reads are fast! write-modifies are still slow. Good for large sequential jobs, still not great for transaction processing systems.
- 32. RAID5 – Block Level Striping with Distributed Parity • Instead of checksums on a single disk, we distribute them across all disks. • Allows us to support multiple writes per group
- 33. RAID5
- 34. Evaluation Pros Cons • Really good usable storage • Slightly worse write • Finally decent small random performance, data must be data access performance written to two disks across the board! simultaneously Finally, a system that works well for both applications!
- 35. sounds complicated, how well does it work?
- 36. As a Whole • RAID has many different levels that achieve different tradeoffs in reliability and performance • Almost all of them, for some (or many) use cases will outperform a SLED for the same cost.
- 37. Read-Modify-Write Per Disk Performance
- 38. wow, raid sounds awesome, how’s it holding up?
- 39. Arriving back in 2012 now…
- 40. RAID has held up remarkably well • Data centers around the world use RAID technology. • The small, inexpensive disk is the de facto standard of storage • The ideas developed for RAID have been applied to many not-RAID things
- 41. Some open questions • What will become of RAID as new, super fast storage mediums start to become cost effective? • How does it fit in with massive internet-scale storage farms?
- 42. Take Aways • RAID offers significant advantage over SLED for the same cost – RAID5 offers 10x improvement in performance, reliability, and power consumption while reducing size of array. • RAID allows for modular growth (add more disks) • Cost effective option to meet challenge of exponential growth in processor and memory speeds
- 43. References • “A Case for Redundant Arrays of Inexpensive Disks” by David A Patterson, Garth Gibson, and Randy H Katz • “RAID: A Personal Recollection of How Storage Became a System” by Randy H Katz • Slides by David Luo and Ramasubramanian K. • Images generously borrowed from Wikipedia <http://en.wikipedia.org/wiki/RAID>
- 44. Thank you!
Editor's Notes
- #2: ----- Meeting Notes (1/21/12 13:53) -----Invented around 1987.
- #3: ----- Meeting Notes (1/21/12 13:53) -----Patterson - BerkeleyGibson – Currently at CMUKatz - Berkeley
- #30: Exploits clever XOR trick to not require reading data off of all the disks to recalculate parity.Each small write requires 2 disks and 4 accesses, 2 reads and 2 writes.Each small read requires only 1 access.