 Stands for Redundant Array of Independent Disks.  It’s a technology that enables greater levels of performance, reliability and/or large volumes when dealing with data. How By concurrent use of two or more ‘hard disk drives’. Mirroring, Stripping (of data) and Error correction techniques combined with multiple disk arrays give you the reliability and performance.

The term "RAID" was first defined by David Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in Marketers representing industry RAID manufacturers later attempted to reinvent the term to describe a redundant array of independent disks as a means of disassociating a low-cost expectation from RAID technology.

Mirroring provides high reliability, but it is expensive. Striping provides high data-transfer rates, but it does not improve reliability. Numerous schemes to provide redundancy at lower cost by using the idea of disk striping combined with ``parity'' bits have been proposed. These schemes have different cost-performance trade-offs and are classified according to levels called RAID levels

RAID 0 a.It splits data among two or more disks. b.Provides good performance. c.Lack of data redundancy means there is no fail over support with this configuration. d.In the diagram to the right, the odd blocks are written to disk 0 and the even blocks to disk 1 such that A1, A2, A3, A4, … would be the order of blocks read if read sequentially from the beginning.

RAID 1 a.RAID1 is ‘data mirroring’. b.Two copies of the data are held on two physical disks, and the data is always identical. c. Twice as many disks are required to store the same data when compared to RAID 0. d.Array continues to operate so long as at least one drive is functioning.

RAID 5 a. RAID 5 is an ideal combination of good performance, good fault tolerance and high capacity and storage efficiency. b. An arrangement of parity and CRC to help rebuilding drive data in case of disk failures. a. “Distributed Parity” is the key word here.

RAID 10 a.Combines RAID 1 and RAID 0. b.Which means having the pleasure of both - good performance and good failover handling. c.Also called ‘Nested RAID’.

Software based RAID: Software implementations are provided by many Operating Systems. A software layer sits above the disk device drivers and provides an abstraction layer between the logical drives(RAIDs) and physical drives. Server's processor is used to run the RAID software. Used for simpler configurations like RAID0 and RAID1.

Implementations (contd…) Hardware based RAID: A hardware implementation of RAID requires at least a special- purpose RAID controller. On a desktop system this may be built into the motherboard. Processor is not used for RAID calculations as a separate controller present. A PCI-bus-based, IDE/ATA hard disk RAID controller, supporting levels 0, 1, and 01.

Write-ahead log scheme requires stable storage. To implement stable storage: Replicate information on more than one nonvolatile storage media with independent failure modes. Update information in a controlled manner to ensure that we can recover the stable data after any failure during data transfer or recovery.

Disk write has 1 of 3 outcomes 1.Successful completion - The data were written correctly on disk 2.Partial failure - A failure occurred in the midst of transfer, so only some of the sectors were written with the new data, and the sector being written during the failure may have been corrupted 3.Total failure - The failure occurred before the disk write started, so the previous data values on the disk remain intact

If failure occurs during block write, recovery procedure restores block to consistent state System maintains 2 physical blocks per logical block and does the following: 1.Write to 1 st physical 2.When successful, write to 2 nd physical 3.Declare complete only after second write completes successfully

Typical storage hierarchy is presented as: Primary Storage - main memory Secondary Storage - internal disk storage Tertiary Storage - Removable Media Removable Media includes: Floppy disk Optical and Magneto-optic disk Tape This form of storage is usually slower but offers portability and greater permanence or is more affordable

Thin flexible disk coated with magnetic material, enclosed in a protective plastic case. Most floppies hold about 1 MB; similar technology is used for removable disks that hold more than 1 GB. Removable magnetic disks can be nearly as fast as hard disks, but they are at a greater risk of damage from exposure.

Records data on a rigid platter coated with magnetic material. Laser heat is used to amplify a large, weak magnetic field to record a bit. Laser light is also used to read data (Kerr effect). The magneto-optic head flies much farther from the disk surface than a magnetic disk head, and the magnetic material is covered with a protective layer of plastic or glass; resistant to head crashes.

The data on read-write disks can be modified over and over. WORM (“Write Once, Read Many Times”) disks can be written only once. Thin aluminum film sandwiched between two glass or plastic platters. To write a bit, the drive uses a laser light to burn a small hole through the aluminum; information can be destroyed by not altered. Very durable and reliable.

Compared to a disk, a tape is less expensive and holds more data, but random access is much slower. Tape is an economical medium for purposes that do not require fast random access, e.g., backup. Large tape installations typically use robotic tape changers. stacker – library that holds a few tapes silo – library that holds thousands of tapes A disk-resident file can be archived to tape for low cost storage.