HP Microsoft Windows Server 2003 for Itanium-based Systems Probability of logical drive failure

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Probability of logical drive failure

In this section
Factors involved in logical drive failure	56

Factors involved in logical drive failure

The probability that a logical drive will fail depends on the RAID-level setting and on the number and type of physical drives in the array. If the logical drive does not have an online spare, the following results apply:

•A RAID 0 logical drive fails if only one physical drive fails.

•A RAID 1+0 logical drive fails if any two failed physical drives are mirrored to each other.

•The maximum number of physical drives that can fail without causing failure of the logical drive is n/2, where n is the number of hard drives in the array. In practice, a logical drive usually fails before this maximum is reached. As the number of failed physical drives increases, it becomes increasingly likely that the newly failed drive is mirrored to a previously failed drive.

•The minimum number of physical drive failures that can cause the logical drive to fail is two. This situation occurs when the two failed drives are mirrored to each other. As the total number of drives in the array increases, the probability that the only two failed drives in an array are mirrored to each other decreases.

•A RAID 5 logical drive fails if two physical drives fail.

•A RAID 6 (ADG) logical drive fails when three physical drives fail.

At any given RAID level, the probability of logical drive failure increases as the number of physical drives in the logical drive increases. This principle is illustrated more quantitatively in the graph ("Probability of logical drive failure vs. number of drives in array" on page 57). The data for this graph is calculated from the MTBF value for a typical physical drive, assuming that no online spares are present. If an online spare is added to any of the fault-tolerant RAID configurations, the probability of logical drive failure is further decreased.

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Contents HP Array Configuration Utility User Guide Audience assumptions Contents Diagnosing array problems Features and system requirements Installing ACUGetting started This sectionComparison of ACU execution modes Overview for using ACUSetting the execution mode for Microsoft Windows Opening ACU in Local Application mode Choosing an operating modeOpening ACU in Browser mode Http//servername2301Opening ACU through Systems Insight Manager Select Device QueriesTypical Standard mode screen GUI operating modesTypical Configuration Wizards mode screen Completing the configuration process Typical Express Configuration mode screenPossible tasks in Standard Configuration mode Configuring a new controllerUsing Standard Configuration mode Using Express Configuration mode Using the configuration wizards Creating an arrayClick Finish Click Create an array, and then click BeginConfiguring a new controller Creating a logical drive Click Create a logical drive, and then click BeginConfiguring a new controller Choices available after opening ACU Modifying an existing configurationModifying a configuration using Standard Configuration mode Modifying a configuration using Express mode Modifying a configuration using the Configuration wizardsClear Configuration Click Express Configuration, and then click BeginCreate an array Controller settingsClick Controller Settings, and then click Begin Create a logical drive Delete arrays Click Delete logical drives, and then click Begin Delete logical drivesExpand Array Extend logical drive Migrate a logical driveClick Extend logical drive, and then click Begin Click Migrate a logical drive, and then click BeginSpare management Selective Storage PresentationRA4x00 controllers Click Spare Management, and then click BeginMSA and Smart Array Cluster storage controllers Configuring switches Click ACU Switch ConfigurationSplitting a mirrored array Recombining a split, mirrored arrayModifying an existing configuration Command line syntax Scripting in ACUIntroduction to scripting in ACU Operating modesSample custom input script Cpqacuxe -i FilenameScript file options Description of option categories in ACU scriptingControl category Controller categoryAction MethodClearConfigurationWithDataLoss ControllerLicenseKey, DeleteLicenseKey RAIDArrayIDSurfaceScanDelay Array categoryRebuildPriority, ExpandPriority SSPStateLogical Drive category OnlineSpareDrive LogicalDriveSize SectorsStripeSize ArrayAcceleratorError reporting SSPAdaptersWithAccessConnectionName HostModeACU scripting error messages 0-9, !, @, #, *, , , ,, -, , +, , ., /, and space. The ID Current configuration Running the CLI Using the Command Line InterfaceOverview of the ACU CLI Command variable CLI syntaxKeyword abbreviations Target variableTypical procedures Example commandHiding warning prompts Querying a deviceSample scenario SyntaxExample Example commandsModifying the controller chassis name Using Selective Storage Presentation Sample scenarioModifying connection names Managing host modes connection profilesDeleting target devices Identifying devicesExpanding an array Extending a logical driveTarget modify raid=01+0156adg? ss=8163264128256default? Managing spare drivesMigrating a logical drive Target modify size=#max? forcedChanging the controller cache ratio Changing the Rebuild Priority settingChanging the Expand Priority setting Enabling or disabling the drive cache Changing the surface scan delay timeRe-enabling a failed logical drive Setting the target Enabling or disabling the array acceleratorDisabling a redundant controller Typical scenarioRescanning the system = rescanProbability of logical drive failure Factors involved in logical drive failureProbability of logical drive failure Drive arrays and fault-tolerance methods Drive arraysDrive arrays and fault-tolerance methods Fault-tolerance methods Hardware-based fault-tolerance methodsDisadvantages RAID 0-No fault toleranceAdvantages RAID 1+0-drive mirroring RAID 5-distributed data guarding RAID 6 ADG-Advanced Data GuardingComparing the hardware-based RAID methods RAID RAID 6 ADGAlternative fault-tolerance methods Selecting a RAID methodDiagnosing array problems Diagnostic toolsServer Diagnostics Post messagesAcronyms and abbreviations SSP Index Index