R.A.I.D. Copyright © 2005 by James Hug Redundant Array of Independent (or Inexpensive) Disks

2 ► In 1987, Patterson, Gibson and Katz at the University of California Berkeley, published a paper entitled "A Case for Redundant Arrays of Inexpensive Disks (RAID)". This paper described various types of disk arrays, referred to by the acronym RAID. The basic idea of RAID was to combine multiple small, inexpensive disk drives into an array of disk drives which yields performance exceeding that of a Single Large Expensive Drive (SLED). Additionally, this array of drives appears to the computer as a single logical storage unit or drive. ► Five types of array architectures, RAID-1 through RAID-5, were defined by the Berkeley paper, each providing disk fault-tolerance and each offering different trade-offs in features and performance. In addition to these five redundant array architectures, it has become popular to refer to a non-redundant array of disk drives as a RAID-0 array.

3 Redundant Array of Independent (or Inexpensive) Disks ► Striping Spreading out blocks of each file across multiple disk drives. ► Advantages:  I/O performance is greatly improved by spreading the I/O load across many channels and drives. ► Disadvantages:  Not a "True" RAID because it is NOT fault-tolerant.  The failure of just one drive will result in all data in an array being lost.  Should never be used in mission critical environments.

4 Redundant Array of Independent (or Inexpensive) Disks ► Mirroring Duplicating blocks of each file across several disk drives. ► Advantages:  100% redundancy of data means no data is lost and no rebuild is necessary in case of a disk failure, just a copy to the replacement disk. ► Disadvantages:  Twice the Read transaction rate of single disks, same Write transaction rate as single disks.

5 Redundant Array of Independent (or Inexpensive) Disks ► Level 0 -- Striped Disk Array without Fault Tolerance:  Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance.  If one drive fails then all data in the array is lost.  Requires a minimum of two (2) hard disks.

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7 ► Level 1 -- Mirroring and Duplexing:  Data is written to two duplicate disks simultaneously. This way if one of the disk drives fails, the system can instantly switch to the other disk without any loss of data or service.  Disk mirroring is used commonly in mission critical systems where it's critical that the data be accessible at all times.  Requires a minimum of two (2) hard disks.

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9 ► Level A Mirror of Stripes:  Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them.  Used for both replicating and sharing data among disks.  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.  Requires a minimum of four (4) hard disks

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11 Redundant Array of Independent (or Inexpensive) Disks ► Level 2 -- Error-Correcting Coding (ECC):  Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.

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13 Redundant Array of Independent (or Inexpensive) Disks ► Level 3 -- Bit-Interleaved Parity:  Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.

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15 Redundant Array of Independent (or Inexpensive) Disks ► Level 4 -- Dedicated Parity Drive:  A commonly used implementation of RAID, Level 4 provides block- level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.

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17 Redundant Array of Independent (or Inexpensive) Disks ► Level 5 -- Block Interleaved Distributed Parity:  Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance.  Level 5 is one of the most popular implementations of RAID used in corporate file server installations.

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19 Redundant Array of Independent (or Inexpensive) Disks ► Level 6 -- Independent Data Disks with Double Parity:  Provides block-level striping with parity data distributed across all disks.

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21 Redundant Array of Independent (or Inexpensive) Disks ► Level A Stripe of Mirrors:  Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.

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23 Redundant Array of Independent (or Inexpensive) Disks ► Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4. ► RAID S: EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.

24 Redundant Array of Independent (or Inexpensive) Disks ► Most RAID systems are managed at the hardware level and are invisible to the operating system. ► A RAID. system requires the appropriate hard disk controller (card). ► Some motherboards include Level 1, Level 0, and/or Level 1+0. ► There are RAID. systems for SATA, IDE/ATA, and SCSI hard drives. ► Motherboards that have RAID capabilities also have the standard disk controller, which can be used conventionally.

25 Redundant Array of Independent (or Inexpensive) Disks ► Hard drives in a RAID. system should be matched for size and speed. ► In un-matched systems the smaller/slower hard drive will be the controlling factor. ► Naturally with more hard drives (minimum of two) the more expensive the computer. ► In a striped system the storage capacity is twice the smallest hard drive. ► In a mirrored system the storage capacity is the same as the smallest hard drive.