TECHNICAL SEMINAR PRESENTATION RAID TECHNOLOGY RAID TECHNOLOGY TECHNICAL SEMINAR PRESENTATION BY BISWAJIT PATTANAIK CS200117270 INSTRUCTED BY MR. A. RAHMAN

INTRODUCTION In order to increase the Size, Speed, Performance, Availability of a life hard disk various effective technologies have been explored out of which the RAID has been described in this term paper briefly. RAID combines multiple inexpensive disk drives into an array of disk drives to obtain performance, capacity and reliability that exceeds that of a single large drive. The array of drives appears to the host computer as a single logical drive .

The two-decade-old principles of large data storage and management have grown and evolved into new forms, yet the basic advantages of dependability, availability, and protection remain key factors in RAID technology’s enduring value.

ABOUT RAID Introduction of the concept took place in a 1988 UC Berkeley publication entitled A Case for Redundant Arrays of Inexpensive Disks. The intention of this technology is to allow users and operations to take advantage of the large amount of space by storing important data in more secure multiple (redundant) locations, and yet access it as easily and quickly as a single PC-based disk drive.

A major challenge is the difficulty of providing access to the stored information and the ability to update (I/O for In and Out) at speeds similar to those of single disk systems. The calculation of this access velocity is referred to as throughput. One basic solution to this difficulty lies in distributing the burden of transfer among all the disks in the array, thus reducing the workload borne by any single disk

DISK MIRRORING The term mirroring refers to the storage of two copies of the same data located on separate hard drives within an array, or within different arrays. While inputting the data, the system writes the information simultaneously to both locations. A major part of RAID technology’s redundancy advantages, mirroring allows continued operation of the system in the event of one drive or array failing. Mirroring offers a distinct advantage to data safety concerns, especially in environments where availability is critical.

DISK STRIPING This is a method of combining multiple drives into one logical storage unit. Striping partitions the storage space of each drive into stripes, which can be as small as one sector (512 bytes) or as large as several megabytes. These stripes are then interleaved in a rotating sequence, so that the combined space is composed alternately of stripes from each drive. To maximize throughput for the disk subsystem, the I/O load must be balanced across all the drives so that each drive can be kept busy as much as possible..

This situation allows all drives to work concurrently on different I/O operations, and thus maximize the number of simultaneous I/O operations that can be performed by the array.

RAID LEVELS These original levels are assigned designations as RAID Levels 0 through 6. In addition to the original RAID levels, development has led to the establishment of Hybrid RAID levels. These actually combine the original 7 levels’ features. Hybrid RAID Levels are numbered with 2 digits

RAID 0 RAID 0 requires a minimum of 2 drives to implement Striped disk array without fault tolerance RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive

RAID 1 For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair. RAID Level 1 requires a minimum of 2 drives to implement

RAID 2 Each bit of data word is written to a data disk drive (4 in this example: 0 to 3). Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors.

RAID 3 The data block is subdivided ("striped") and written on the data disks. Stripe parity is generated on Writes, recorded on the parity disk and checked on Reads. RAID Level 3 requires a minimum of 3 drives to implement

RAID 4 Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads. RAID Level 4 requires a minimum of 3 drives to implement

RAID 5 Each entire data block is written on a data disk; Parity for blocks in the same rank is generated on Writes, Recorded in a distributed location and checked on Reads. RAID Level 5 requires a minimum of 3 drives to implement

RAID 6 RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (two-dimensional parity) Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives; RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures

RAID 10 RAID 10 is implemented as a striped array whose segments are RAID 1 arrays RAID 10 has the same fault tolerance as RAID level 1 RAID 10 has the same overhead for fault-tolerance as mirroring alone RAID Level 10 requires a minimum of 4 drives to implement

RAID 50 RAID 50 should really be called "RAID 03" because it is implemented as a striped (RAID level 0) array whose segments are RAID 3 arrays RAID 50 has the same fault tolerance as RAID 3 as well as the same fault tolerance overhead RAID Level 50 requires a minimum of 5 drives to implement   

RAID 0+1 RAID 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays RAID 0+1 has the same fault tolerance as RAID level 5 RAID 0+1 has the same overhead for fault-tolerance as mirroring alone RAID Level 0+1 requires a minimum of 4 drives to implement

CONCLUSION RAID technology allows users and operations to take advantage of the large amount of space by storing important data in more secure multiple locations, and yet access it as easily and quickly as a single PC-based disk drive. Performance, Availability, Capacity, & Cost are the major considerations to be taken into account while judging the suitability of any RAID configuration with respect to individual application needs.