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Enhanced Availability With RAID

Enhanced Availability With RAID. CC5493/7493. RAID. Redundant Array of Independent Disks RAID is implemented to improve: IO throughput (speed) and Availability of a file system. RAID Implementation. Software – often criticized as not being a true RAID implementation.

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Enhanced Availability With RAID

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  1. Enhanced Availability With RAID CC5493/7493

  2. RAID • Redundant Array of Independent Disks • RAID is implemented to improve: • IO throughput (speed) and • Availability of a file system.

  3. RAID Implementation • Software – often criticized as not being a true RAID implementation. • Hardware – A special RAID controller is required.

  4. RAID: Stripe • The stripe takes on two meanings within the context of a RAID system: • Stripe width (number of independent drives) • Stripe size (storage block size) Both stripe width and stripe size are adjusted to enhance IO throughput.

  5. RAID Stripe Width • Stripe width refers to the number of disks used in parallel for IO transfers to and from the array.

  6. Raid Stripe Size • Stripe size refers to the size of the storage units organized on the disk surface. • The stripe size is adjusted to optimize the speed of the IO transfers.

  7. Common RAID Types • RAID-0 • RAID-1 • RAID-1+0, RAID-0+1 • RAID-5 • RAID-6

  8. RAID-0 • AKA disk striping • Does not provide redundancy • Degrades data availability, reduces MTF • Improves IO throughput (average IO transfer rate improves)

  9. RAID-0 • Ideal for temporary storage requiring fast data access. -Engineering/Scientific calculations on large data volumes. However, the data is a redundant temporary copy.

  10. RAID-1 • AKA mirroring • Requires two independent disk devices • The first disk stores the data • The second disk is an image of the first • Can double the overall read throughput

  11. RAID-1 • width = 1

  12. RAID-1 Advantages • Improves data availability. • Dual-channel controller allows for two simultaneous read operations. • Allows for error detection on read. • Administrative advantages for service on one drive while the other remains available. • Fault tolerance is one drive.

  13. RAID-1 Disadvantages • Writes have a slight performance penalty compared to no RAID. • Doubles the cost of storage. • Storage efficiency = 50%

  14. RAID-1 • Ideal for data that is read more often than written: • Some database information that is not updated often. • Web Server information (lots of reads, few writes)

  15. RAID-1+0 • Enhances IO throughput and data availability. • Requires 2(n+1) separate disk devices, where n = 1, 2, 3, 4, … • Minimum of 4 disks required (n=1)

  16. RAID-1+0 Width=2

  17. RAID-1+0 • Width = 4

  18. RAID-1+0 • RAID-1+0 has a higher fault tolerance compared to RAID-0,1, & 5. • Storage efficiency is 50%

  19. RAID-0+1 • Requires the same hardware as RAID-1+0, but less fault tolerant. • However, there is better read throuthput from RAID-0+1 compared to RAID-1+0.

  20. RAID-0+1 • Duplicate RAID-0 arrays. Allows simultaneous reads

  21. RAID-5 • RAID-5 enhances • IO data throughput • Data availability • Parity information enhances availability • Requires a minimum of 3 independent disk devices.

  22. Parity Information • Based on the logical exclusive-or operation.

  23. RAID-5 Configuration • Stripe Width = 4

  24. RAID-5 • The most common implementation of RAID. • Ideal for a disk-server providing general storage. • A good balance between reliability and speed. • Often implemented using high quality disk drives (SCSI, 15k-rpm, high MTF)

  25. RAID-5 Limitations • Overhead occurs during writes due to the parity calculation and parity write. • Storage efficiency is not 100% due to the parity storage requirements. storage efficiency = (n-1)/n, where n = number of drives.

  26. RAID-5 (S)ATA Limitations • Large capacity (S)ATA drives are more likely to contain bad blocks. • After a disk failure, the bad blocks make it impossible to rebuild the array from the remaining drives.

  27. RAID-6 • Contains two sets of parity. • Tolerates two simultaneous disk failures. • A better solution for (S)ATA arrays where each disk has a large capacity (multiple TB).

  28. Stripe Width = 6

  29. RAID-6 • Higher availability at the cost of greater IO overhead due to complex parity calculations and storage. • Storage efficiency = (n-2)/n • Becoming more popular for large storage capacity (S)ATA arrays

  30. RAID-6 Disadvantages • More expensive to implement due to extra parity information • Slower write operations compared to other RAID-5

  31. RAID Disk Swapping • Hot Swap • Warm Swap • Cold Swap

  32. Hot Swap • The ability to swap out a failed disk from a RAID array without an interruption of service from the array. • Performance will be slower due to the operations required to rebuild the new replacement disk.

  33. Warm Swap • The array is not accessible while a drive is being serviced, but the system does not need to be shut down.

  34. Cold Swap • System must be shutdown to service the array.

  35. Spare Disk: Hot Spare • Some RAID controllers can be configured to immediately recover from a disk failure if a hot-spare disk is connected to the controller at all times.

  36. RAID Disk Failure and Performance • When a failed disk is replaced in an array, there is a performance hit as the new disk must be re-populated with the required data for the complete array.

  37. RAID Summary • RAID-0 : for temporary storage only • RAID-1 : ideal for disk services that provide mostly read operations like data base services and web services. • RAID-5 : general purpose disk-server • RAID-6 : for very large data requirement environments (multiple T-Bytes).

  38. RAID Summary • RAID 1+0 : general purpose disk server where RAID-5 & 6 are not adequate. • Better fault tolerance • More IO throughput

  39. Other? • RAID 1+1, mirror a mirrored RAID-1 • Triples the cost of storage • Excellent fault tolerance. • Excellent read throughput. • Writes will suffer

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