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Premium member Presentation Transcript RAID IN DBMS : RAID IN DBMS 3/14/2013 1 C.B.Anand kumar AP/CSE SMKFITRAID: RAID The data storage requirements of some applications (in particular web, database, and multimedia applications) have been growing so fast that a large number of disks are needed to store their data even though disk drive capacities have been growing very fast. Storing data’s in disk are inexpensive 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 2RAID: RAID A variety of disk organization techniques collectively called redundant arrays of independent disks(RAID) have been proposed to achieve improved performance and reliability Since disk storage is cost inexpensive it is also called as REDUNDANT ARRAY OF INEXPENSIVE DISK. 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 3Improvement of reliability via redundancy: Improvement of reliability via redundancy At least one disk out of set of N disks will fail much higher than the chance that a specific single disk will fail. Suppose the MTTF of the disk is 100,000 hrs or slightly over 11 years, then the MTTF of some disk in an array of 100 disks will be 100,000/100=1000 hrs (or) around 42 days. Its ok 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 4Cont…: Cont… MTTF the average time the disk is expected to run continuously without failure. If we store only one copy of the data, then each disk failure will result in loss of significant amount of data such a high frequency of data loss is unacceptable. 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 5Cont..: Cont.. The solution to the problem of reliability is to introduce redundancy. The simplest (but most expensive) approach to introducing redundancy is to duplicate every disk. This technique is called mirroring or sometimes shadowing. However mirrored disk systems offer much higher reliability than do single disk systems mirrored disk systems with mean time to data loss of about 500,000 to 1000,000 hrs or 55 to 110 years, are available today 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 6Improving of performance via parallelism: Improving of performance via parallelism 1. Distribute files (f1 D1, f2 D2, ....) or 2. Distribute parts of files (“striping”) block striping sector striping ...... bit striping 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 7RAID LEVELS: RAID LEVELS Mirroring provides high reliability, but it is expensive. Striping provides high data transfer rates, but does not improve reliability. Various alternative schemes aim to provide redundancy at lower cost by combining disk striping with “parity bits”. These schemes are classified into RAID levels. 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 8RAID LEVELS: RAID LEVELS RAID Level 0 : Block striping; non-redundant. Used in high-performance applications where data loss is not critical. RAID Level 1 : Mirrored disks with block striping Offers good write performance. Popular for applications such as storing log files in a database system. 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 9RAID LEVELS: RAID LEVELS RAID Level 2 : Memory-Style Error-Correcting-Codes (ECC) with bit striping. RAID Level 3 : Bit-Interleaved Parity a single parity bit is enough for error correction, not just detection, since we know which disk has failed When writing data, corresponding parity bits must also be computed and written to a parity bit disk To recover data in a damaged disk, compute XOR of bits from other disks (including parity bit disk) 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 10RAID LEVELS: RAID LEVELS RAID Level 3 (Cont.) Faster data transfer than with a single disk, but fewer I/Os per second since every disk has to participate in every I/O. Subsumes Level 2 (provides all its benefits, at lower cost). RAID Level 4 : Block-Interleaved Parity ; uses block-level striping, and keeps a parity block on a separate disk for corresponding blocks from N other disks. When writing data block, corresponding block of parity bits must also be computed and written to parity disk To find value of a damaged block, compute XOR of bits from corresponding blocks (including parity block) from other disks. 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 11RAID LEVELS: RAID LEVELS RAID Level 4 (Cont.) Provides higher I/O rates for independent block reads than Level 3 Provides high transfer rates for reads of multiple blocks than no-striping Before writing a block, parity data must be computed Can be done by using old parity block, old value of current block and new value of current block (2 block reads + 2 block writes) Parity block becomes a bottleneck for independent block writes since every block write also writes to parity disk 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 12RAID LEVELS: RAID LEVELS RAID Level 5 : Block-Interleaved Distributed Parity ; partitions data and parity among all N + 1 disks, rather than storing data in N disks and parity in 1 disk. E.g., with 5 disks, parity block for n th set of blocks is stored on disk ( n mod 5) + 1, with the data blocks stored on the other 4 disks. 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 13RAID LEVELS: RAID LEVELS RAID Level 5 (Cont.) Higher I/O rates than Level 4. Block writes occur in parallel if the blocks and their parity blocks are on different disks. Subsumes Level 4: provides same benefits, but avoids bottleneck of parity disk. RAID Level 6 : P+Q Redundancy scheme; similar to Level 5, but stores extra redundant information to guard against multiple disk failures. Better reliability than Level 5 at a higher cost; not used as widely. 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 14PowerPoint Presentation: 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 15PowerPoint Presentation: 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 16Reference books: Reference books Abraham Silberschatz , Henry F. Korth , S. Sudharshan , “Database System Concepts”, Fifth Edition, Tata McGraw Hill, 2006 3/14/2013 C.B.Anand kumar AP/CSE SMKFIT 17 You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.