Compaq 108164-003 - ProLiant - 800 White Paper page 14

ECG007.0897
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HITE APER
(cont.)
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RAID 5 - Distributed Data Guarding
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RAID 4 - Data Guarding
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RAID 1 - Disk Mirroring
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RAID 0 - No Fault Tolerance Support
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The Compaq Smart-2 Array Controller is needed to support hardware striping and all levels of
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fault tolerance support. Features offered by the Compaq Smart-2 Controllers that are not found
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with Fast-Wide SCSI-2 Controllers are:
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Support for RAID 0, RAID 1, RAID 4 and RAID 5 Hardware
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Striping and Fault Tolerance
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Dual Fast-Wide SCSI-2 channels on a single board support up to
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14 drives (7 per channel)
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Support for multiple logical drives per drive array
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Removable Array Accelerator - battery-backed 4 MB Read/Write
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cache with Error Checking and Correcting (ECC)
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Read-ahead caching
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Online Capacity Expansion and Disk Drive Upgrades
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Fault Management Features
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RAID 5 - Distributed Data Guarding
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RAID 5 is also referred to as distributed data guarding because it uses parity data to guard against
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the loss of data. The parity data is distributed or striped across all the drives in the array. RAID 5
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provides very good data protection because if a single drive fails, the parity data and the data on
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the remaining drives is used to reconstruct the data on the failed drive. With Compaq Smart-2
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. . controller technology, this reconstruction process allows the failed drive to be replaced while the
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system continues to operate at a slightly reduced performance. RAID 5 also offers good
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. . performance because spreading the parity across all the drives allows more simultaneous read
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operations.
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The usable disk space when using RAID 5 depends on the total number of drives in the array. If
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there are three drives, 67 percent of the disk space is usable for data, with the remainder being
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used to support fault tolerance. If there are fourteen drives, 93 percent of the disk would be
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. . available. The tests that follow used seven drives.
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RAID 1 - Drive Mirroring
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RAID 1 is also referred to as drive mirroring. This is typically the highest performance fault
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tolerance method. RAID 1 is the only option for fault tolerance if no more than two drives are
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selected. Drive mirroring works as its name implies, storing two sets of duplicate data on a pair of
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disk drives. Therefore, RAID 1 always requires an even number of disk drives. From a cost
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standpoint, RAID 1 is the most expensive because 50 percent of the drive capacity is used for fault
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tolerance.
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. . If a drive fails, the mirror drive provides a backup copy of the data and normal system operation
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is not interrupted. A system with more than two drives may be able to withstand multiple drive
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failures as long as the failed drives are not mirrored to one another.
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