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Explain various RAID levels.

Mumbai University > Information Technology > Sem5 > Operating System

Marks: 5M

Year: May 15

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RAID 0:

  • In a RAID 0 configuration, data is striped across the HDDs in a RAID set. It utilizes the full storage capacity by distributing strips of data over multiple HDDs in a RAID set.

  • To read data, all the strips are put back together by the controller.

  • The stripe size is specified at a host level for software RAID and is vendor specific for hardware RAID.

  • Figure below shows RAID 0 on a storage array in which data is striped across 5 disks. When the number of drives in the array increases, performance improves because more data can be read or written simultaneously.

  • RAID 0 is used in applications that need high I/O throughput. However, if these applications require high availability, RAID 0 does not provide data protection and availability in the event of drive failures.

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RAID 1:

  • In a RAID 1 configuration, data is mirrored to improve fault tolerance. A RAID 1 group consists of at least two HDDs.

  • As explained in mirroring, every write is written to both disks, which is transparent to the host in a hardware RAID implementation.

  • In the event of disk failure, the impact on data recovery is the least among all RAID implementations. This is because the RAID controller uses the mirror drive for data recovery and continuous operation. RAID 1 is suitable for applications that require high availability.

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RAID 3:

  • RAID 3 stripes data for high performance and uses parity for improved fault tolerance. Parity information is stored on a dedicated drive so that data can be reconstructed if a drive fails.

  • RAID 3 always reads and writes complete stripes of data across all disks, as the drives operate in parallel. There are no partial writes that update one out of many strips in a stripe.

  • Figure illustrates the RAID 3 implementation. RAID 3 provides good bandwidth for the transfer of large volumes of data.

  • RAID 3 is used in applications that involve large sequential data access, such as video streaming.

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RAID 4:

  • Similar to RAID 3, RAID 4 stripes data for high performance and uses parity for improved fault tolerance.

  • Data is striped across all disks except the parity disk in the array. Parity information is stored on a dedicated disk so that the data can be rebuilt if a drive fails. Striping is done at the block level.

  • Unlike RAID 3, data disks in RAID 4 can be accessed independently so that specific data elements can be read or written on single disk without read or write of an entire stripe. RAID 4 provides good read throughput and reasonable write throughput.

RAID 5:

  • RAID 5 is a very versatile RAID implementation. It is similar to RAID 4 because it uses striping and the drives (strips) are independently accessible.

  • The difference between RAID 4 and RAID 5 is the parity location. In RAID 4, parity is written to a dedicated drive, creating a write bottleneck for the parity disk. In RAID 5, parity is distributed across all disks.

  • The distribution of parity in RAID 5 overcomes the write bottleneck.

  • Figure illustrates the RAID 5 implementation. RAID 5 is preferred for messaging, data mining, and medium-performance media serving, and relational database management system (RDBMS) implementations in which database administrators (DBAs) optimize data access.

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RAID 6:

  • RAID 6 works the same way as RAID 5 except that RAID 6 includes a second parity element to enable survival in the event of the failure of two disks in a RAID group.

  • Therefore, a RAID 6 implementation requires at least four disks.

  • RAID 6 distributes the parity across all the disks. The write penalty in RAID 6 is more than that in RAID 5; therefore, RAID 5 writes perform better than RAID 6.

  • The rebuild operation in RAID 6 may take longer than that in RAID 5 due to the presence of two parity sets.

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