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RAID: Data Protection

2. RAID. RAID: a redundant array of inexpensive disks using multiple fixed-disk drives, high speed controllers and special software drivers to control the safety of your data and improve the performance of fixed-disk system.All commercial systems use Small Computer Systems Interface (SCSI pronounc

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RAID: Data Protection

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    1. 1 EC/MIS 619 RAID: Data Protection

    2. 2 RAID RAID: a redundant array of inexpensive disks using multiple fixed-disk drives, high speed controllers and special software drivers to control the safety of your data and improve the performance of fixed-disk system. All commercial systems use Small Computer Systems Interface (SCSI pronounced scuzzy)

    3. 3 RAID levels of 1 or higher protect your data by spreading it on multiple disks then calculating and storing parity bit information. This redundancy allows one drive to fail without causing the array itself to fail. RAID 1 increases disk subsystem performance by distributing data across several drives, allowing the same data to be retrieved from many locations - depending on which is closer to the read head(s).

    4. 4 RAID Levels Several level of RAID exists. RAID 0 increases read and write performance but does not provide data protection. RAID was initially designed for mainframe and microcomputers. Until recently its deployment was limited by price. Plummeting disk price has made RAID more common of late. Disk drives provide two functions: read and write. RAID may be chosen to optimize these functions.

    5. 5 RAID 0 High performance, zero redundancy array. Isnt truly RAID at all. Data is lost if one drive fails It strips blocks of data across multiple disks to improve subsystem throughput. RAID 0 is used for applications needing the highest possible read and write rates.

    6. 6 RAID 0 with 2 disks

    7. 7 RAID 0 is important because the same stripping mechanism used in RAID is used to improve performance in other RAID levels. RAID 0 is inexpensive because no additional disk space is needed for parity data it uses simple algorithms that dont add much overhead or require a dedicated processor. RAID 0 uses stripping to store data: data blocks are alternately written to different physical drives that make up the logical drive used by the array.

    8. 8 Each read request is directed to the individual drive on which multiple blocks multiple reads request are generated serially stripping allows data transfer to occur in parallel. Overall read time is significantly reduced. Efficiency is influenced by size data blocks

    9. 9 RAID 1 with 2 disks

    10. 10 RAID 1 We avoid loosing data by making copies of it. RAID 1 provides 100% redundancy:- if a disk is lost in array1, theres another drive with an exact duplicate of the failed drive's contents. RAID 1 offers the highest level of redundancy (but at the highest cost).

    11. 11 Mirroring- each drive has a twin drive. Write functions take place simultaneously. Disadvantages: twice as many disk needed for storage slow writes - because of overheads introduces with the need to write to twin drives and maintain coherency of their contents. Advantages: duplicates means data loss is less likely faster reads - reads can be made from drive whose head is closest to the data.

    12. 12 Duplexing: similar to mirroring but adds a second host adapter to control the second set of drives. Introduces cost of second adapter Duplexing with two cards eliminates the host adapter as a single point of failure. In a large server the cost of duplicating every disk gets very expensive. Cost are reasonable for smaller systems

    13. 13 Read Performance: is faster than that of stand alone drive. Offers two read alternatives: Circular queue or round-robin scheduling: reads are alternated between two physical drive with each drive handling every second request. Geometric, regional, or assigned circular scheduling: overcomes slow reads by giving two drives the responsibility to cover only half of the physical drives, thus head positioning time is minimized.

    14. 14 RAID 1 Write Performance: is more problematic. Data has to be written twice (like going through the checkout line twice) This double write that facilitates the high level of data safety. Overall RAID 1 performance: for most server environments, reads greatly outnumber writes. Therefore any factor that benefits read performance at the expense of write performance will improve server performance most of the time.

    15. 15 Secondly, writing to two drives does not cut write performance in half. Performance is cut by 10-20%. Write request are generated serially, but the actual writes are performed in parallel. The extra time for the second write has little impact on total time.

    16. 16 RAID 2 Distributes data at the bit level Uses multiple bits to store parity data and uses a large number of individual drives Large amount of processing overhead makes it unsuitable for database servers. RAID 2 is useful for special purpose servers such as digital video servers.

    17. 17 RAID 3 Distributes data at the byte level Dedicates one drive for storage of parity bits In a 4 disk system, the first three disks stores data and the fourth handles the parity bits. RAID 3 is optimized for special purpose servers such as digital video servers and is inappropriate for random access areas such as PC LANs.

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    19. 19 RAID 4 Similar to RAID 3, except it stripes data at the block rather than byte level - providing better read performance than RAID 3. The small chunk size of RAID 3s means every read requires participation from every disk in the array. As such RAID 3 is referred to as being synchronized or coupled RAID 4 larger chunk size means single disk may be accessed (unsynchronized or decoupled) RAID 4 is not for PC LANs.

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    21. 21 Most common RAID for PC LANs Parity data is striped across all disks taking up the equivalent of one disk. The equivalent of one drive is unavailable to the operating system. RAID 5 is optimized for transaction-processing activity, in which users frequently read and write small amounts of data.

    22. 22 RAID 5 read performance: does not need to access parity information unless one or more strips are unreadable. Both data and parity bits are optimized for sequential reads. Allows parallel read and write, attaining better performance on random reads. Matches RAID 0 on sequential reads.

    23. 23 RAID 5 write performance: writing is more problematic. Write requires 4 disk operations: one read for data and another for existing parity information. Parity information is recalculated based on the read and pending write. Two writes are performed (one for data, one for parity info). This compares to one operation for RAID 0. Guard against system failure after data but before parity information is written.

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    25. 25 Stacked RAID RAID arrays are seen as a single logical disk by the operating system. This allows us to stack arrays, using one level to control an array of arrays (individual disks are replaced by arrays operating at the same or different levels)

    26. 26 This allows us to gain the benefits of multiple levels without the disadvantages. High performance RAID is visible while lower performance RAID providing redundancy is hidden E.g. RAID 0/1 - combines RAID 0 stripping with RAID 1 redundancy.

    27. 27 Array Selection (failure) RAID 0 failure results in data loss RAID 1 - data is read from mirrored drive RAID 3 - parity drive failure results in no data loss. If data drive fails, reads of other drive plus parity drive is used to reconstruct the data. RAID 5 - failure results in loss of some data and some parity information. Reading of other drives is used to reconstruct the data.

    28. 28 Specifying RAID Implementation Decision is made based on: the type of data to be stored (large vs. small writes) importance of performance vs. safety of data. Cost e.g. RAID 0 offers high performance; RAID 1 offers redundancy; RAID 3 good performance and safety but is poor for PC LANs; Stacked RAID offers good performance and safety but is expensive.

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