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

R.A.I.D.R.A.I.D.

Copyright © 2005 by James Hug

RRedundant edundant AArray of rray of IIndependent ndependent (or (or IInexpensive) nexpensive) DDisksisks

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RRedundant edundant AArray of rray of IIndependent ndependent

(or (or IInexpensive) nexpensive) DDisksisks► In 1987, Patterson, Gibson and Katz at the University of California In 1987, Patterson, Gibson and Katz at the University of California

Berkeley, published a paper entitled "A Case for Redundant Arrays of Berkeley, published a paper entitled "A Case for Redundant Arrays of Inexpensive Disks (RAID)" . This paper described various types of Inexpensive Disks (RAID)" . This paper described various types of disk arrays, referred to by the acronym RAID. The basic idea of RAID disk arrays, referred to by the acronym RAID. The basic idea of RAID was to combine multiple small, inexpensive disk drives into an array was to combine multiple small, inexpensive disk drives into an array of disk drives which yields performance exceeding that of a Single of disk drives which yields performance exceeding that of a Single Large Expensive Drive (SLED). Additionally, this array of drives Large Expensive Drive (SLED). Additionally, this array of drives appears to the computer as a single logical storage unit or drive. appears to the computer as a single logical storage unit or drive.

► Five types of array architectures, RAID-1 through RAID-5, were Five types of array architectures, RAID-1 through RAID-5, were defined by the Berkeley paper, each providing disk fault-tolerance defined by the Berkeley paper, each providing disk fault-tolerance and each offering different trade-offs in features and performance. In and each offering different trade-offs in features and performance. In addition to these five redundant array architectures, it has become 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 popular to refer to a non-redundant array of disk drives as a RAID-0 array. array.

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(or (or IInexpensive) nexpensive) DDisksisks► StripingStriping

Spreading out blocks of each file across multiple disk drives.Spreading out blocks of each file across multiple disk drives.

►Advantages:Advantages:

I/O performance is greatly improved by spreading the I/O I/O performance is greatly improved by spreading the I/O load across many channels and drives.load across many channels and drives.

►Disadvantages:Disadvantages:

Not a "True" RAID because it is NOT fault-tolerant.Not a "True" RAID because it is NOT fault-tolerant.

The failure of just one drive will result in all data in an The failure of just one drive will result in all data in an array being lost.array being lost.

Should never be used in mission critical environments.Should never be used in mission critical environments.

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(or (or IInexpensive) nexpensive) DDisksisks►MirroringMirroring

Duplicating blocks of each file across several disk drives.Duplicating blocks of each file across several disk drives.

►Advantages:Advantages:

100% redundancy of data means no data is lost and no 100% redundancy of data means no data is lost and no rebuild is necessary in case of a disk failure, just a copy to rebuild is necessary in case of a disk failure, just a copy to the replacement disk.the replacement disk.

►Disadvantages:Disadvantages:

Twice the Read transaction rate of single disks, same Twice the Read transaction rate of single disks, same Write transaction rate as single disks.Write transaction rate as single disks.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 0Level 0 -- -- Striped Disk Array without Fault Striped Disk Array without Fault

Tolerance: Tolerance:

Provides data striping (spreading out blocks of each file Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance.improves performance but does not deliver fault tolerance.

If one drive fails then all data in the array is lost. If one drive fails then all data in the array is lost.

Requires a minimum of two (2) hard disks.Requires a minimum of two (2) hard disks.

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(or (or IInexpensive) nexpensive) DDisksisks

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(or (or IInexpensive) nexpensive) DDisksisks►Level 1Level 1 -- -- Mirroring and Duplexing:Mirroring and Duplexing:

Data is written to two duplicate disks simultaneously. This Data is written to two duplicate disks simultaneously. This way if one of the disk drives fails, the system can instantly way if one of the disk drives fails, the system can instantly switch to the other disk without any loss of data or service.switch to the other disk without any loss of data or service.

Disk mirroring is used commonly in mission critical Disk mirroring is used commonly in mission critical systems where it's critical that the data be accessible at all systems where it's critical that the data be accessible at all times. times.

Requires a minimum of two (2) hard disks.Requires a minimum of two (2) hard disks.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 0+1Level 0+1 ---- A Mirror of Stripes: A Mirror of Stripes:

Not one of the original RAID levels, two RAID 0 stripes are Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them.created, and a RAID 1 mirror is created over them.

Used for both replicating and sharing data among disks.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 fault tolerance as RAID level 5.

RAID 0+1 has the same overhead for fault-tolerance as RAID 0+1 has the same overhead for fault-tolerance as mirroring alone. mirroring alone.

Requires a minimum of four (4) hard disksRequires a minimum of four (4) hard disks

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(or (or IInexpensive) nexpensive) DDisksisks►Level 2Level 2 -- Error-Correcting Coding (ECC): -- Error-Correcting Coding (ECC):

Not a typical implementation and rarely used, Level 2 Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.stripes data at the bit level rather than the block level.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 3Level 3 -- Bit-Interleaved Parity: -- Bit-Interleaved Parity:

Provides byte-level striping with a dedicated parity disk. Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple Level 3, which cannot service simultaneous multiple requests, also is rarely used.requests, also is rarely used.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 4Level 4 -- Dedicated Parity Drive: -- Dedicated Parity Drive:

A commonly used implementation of RAID, Level 4 A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks. parity disk can create write bottlenecks.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 5Level 5 -- Block Interleaved Distributed -- Block Interleaved Distributed

Parity: Parity:

Provides data striping at the byte level and also stripe Provides data striping at the byte level and also stripe error correction information. This results in excellent error correction information. This results in excellent performance and good fault tolerance. performance and good fault tolerance.

Level 5 is one of the most popular implementations of RAID Level 5 is one of the most popular implementations of RAID used in corporate file server installations.used in corporate file server installations.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 6Level 6 -- Independent Data Disks with -- Independent Data Disks with

Double Double Parity: Parity:

Provides block-level striping with parity data distributed Provides block-level striping with parity data distributed across all disks.across all disks.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 10Level 10 -- A Stripe of Mirrors: -- A Stripe of Mirrors:

Not one of the original RAID levels, multiple RAID 1 mirrors Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.are created, and a RAID 0 stripe is created over these.

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(or (or IInexpensive) nexpensive) DDisksisks►Level 7:Level 7: A trademark of Storage Computer A trademark of Storage Computer

Corporation that adds caching to Levels 3 or Corporation that adds caching to Levels 3 or 4. 4.

►RAID SRAID S:: EMC Corporation's proprietary EMC Corporation's proprietary striped parity RAID system used in its striped parity RAID system used in its Symmetrix storage systems.Symmetrix storage systems.

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(or (or IInexpensive) nexpensive) DDisksisks► MostMost RAID systems are managed at the hardware RAID systems are managed at the hardware

level and are invisible to the operating system.level and are invisible to the operating system.► A RAID. system requires the appropriate hard disk A RAID. system requires the appropriate hard disk

controller (card).controller (card).► Some motherboards include Level 1, Level 0, and/or Some motherboards include Level 1, Level 0, and/or

Level 1+0. Level 1+0. ► There are RAID. systems for SATA, IDE/ATA, and There are RAID. systems for SATA, IDE/ATA, and

SCSI hard drives.SCSI hard drives.► Motherboards that have RAID capabilities also have Motherboards that have RAID capabilities also have

the standard disk controller, which can be used the standard disk controller, which can be used conventionally. conventionally.

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(or (or IInexpensive) nexpensive) DDisksisks► Hard drives in a RAID. system Hard drives in a RAID. system should beshould be matched matched

for size and speed.for size and speed.► In un-matched systems the smaller/slower hard In un-matched systems the smaller/slower hard

drive will be the controlling factor.drive will be the controlling factor.► Naturally with more hard drives (minimum of two) Naturally with more hard drives (minimum of two)

the more expensive the computer.the more expensive the computer.► In a striped system the storage capacity is twice the In a striped system the storage capacity is twice the

smallest hard drive.smallest hard drive.► In a mirrored system the storage capacity is the In a mirrored system the storage capacity is the

same as the smallest hard drive.same as the smallest hard drive.

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

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