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1. RAID STRUCTURE Sanika C MCA S1

2. RAID (Redundant Arrays of Independent Disks) Having a large number of disks in a system presents opportunities for improving the rate at which data can be read or written, if the disks are operated in parallel. this setup offers the potential for improving the reliability of data storage, because redundant information can be stored on multiple disks. Thus, failure of one disk does not lead to loss of data. A variety of disk-organization techniques, collectively called redundant arrays of independent disks (RAID), commonly used to address the performance and reliability issues. In the past, RAIDs composed of small, cheap disks were viewed as a cost - effective alternative to large, expensive disks. Today, RAIDs are used for their higher reliability and higher data-transfer rate, rather than for economic reasons. Hence, the I in RAID, which once stood for “inexpensive,” now stands for “independent.”

3.  Improvement of Reliability via Redundancy Redundancy store extra information that is not needed normally, but that can be used in the event of failure of a disk to rebuild the lost information. Mirroring (or Shadowing): The simplest approach to introduce redundancy is to duplicate every disk. Logical disk consists of 2 physical disks. Every write is carried out on both disks. If one of the disk fails, the data can be read from the other. Data will be lost only if the second disk fails before the first disk is repaired. Mean time to data loss depends on mean time to failure and mean time to repair.

4. Improvement in performance via parallelism Two main goals of parallelism in a disk system are Load balance multiple small accesses to increase throughput. Parallelize large accesses to reduce response time. Improve transfer rate by striping data across multiple disks. Bit-level striping – Split the bits of each byte across multiple disks Block level striping – blocks of a file are striped across multiple disks

5. RAID LEVELS RAID LEVEL 0 : Non redundant block striping. Used in high performance applications where data lost is not critical Without redundancy RAID LEVEL 1: Mirrored disks with block striping Offers best write performance

6. 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, as well as for detection. Faster data transfer than with a single disk, but fewer I/Os per second since every disk has to participate in every I/O.

7. RAID Level 4: Block interleaved parity Uses block level striping. Provides higher I/O rates for independent reads than level 3 Block read goes to a single disk, so blocks stored on different disks can be read in parallel. Provides higher rates for reads of multiple blocks than no striping. Before writing a block, parity data must be computed. More efficient for writing large amount of data sequentially.

8. RAID Level 5: Block interleaved distributed parity. Distributed data and parity among all N+1 disks, rather than storing data in N disks and parity in disk. Higher I/O rates than level 4. Block writes occur in parallel if the blocks and their parity blocks are on different disks.

9. 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, but not used as widely. RAID 1+0 and 1/0 Combination of RAID levels 0 and 1 Very high reliability combined with high performance. High I/O rates are achieved by striping RAID 1 segments.

10. Choice of RAID level Factors in choosing RAID level Performance during failure Performance during rebuild of failed disk RAID 0 is used only when data safety is not important Level 1 is preferred for all other applications .

11. RAID structures are able to recover data even if one of the tapes in an array is damaged. When applied to broadcast of data, a block of data is split into short units and is broadcast along with a parity unit. If one of the units is not received for any reason, it can be reconstructed from the other units. Commonly, tape-drive robots containing multiple tape drives will stripe data across all the drives to increase throughput and decrease backup time. Extensions

12. RAID alone does not prevent or detect data corruption or other errors, just disk failures Solaris ZFS adds checksums of all data and metadata Can detect and correct data and metadata corruption pools Filesystems with a pool share that pool. use and release space like malloc() and free() memory allocate / release storage Problems with RAID

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