Compaq EK-SMCPO-UG. C01 manual Efficient Write and Read Algorithms, Write-Back Caching

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2-10RAID Array 3000 Pedestal Storage Subsystem Hardware User’s Guide

Efficient Write and Read Algorithms

Standard RAID write operations that involve parity, such as those in RAID levels 4 and 5, require multiple, time-consuming steps:

1.Read data from the parity drive.

2.Read existing data from the target data drives.

3.Exclusive-or the old parity, old data, and new data to generate new parity data.

4.Write the new parity data to the parity drive.

5.Write the new data to the target data drives.

The controller uses several techniques to streamline write operations and significantly improve performance. All the techniques use the controller’s on- board cache, which can contain up to 64 MB of memory in the form of standard 72-pin, 60 ns SIMMs.

NOTE: The controller will not operate without at least one 4 MB SIMM installed in its cache. Also, it will not operate without either a backup or a UPS connected to the controller. Without a backup, data stored in the cache, but not yet written to the disk drives, would be lost in the event of a power interruption.

Write-Back Caching

When the host sends data to be written to a redundancy group, the controller stores the data in its cache and immediately reports to the host that it has completed the write. The controller eventually writes the data to the disk drives when the write can be done most efficiently, or when the controller must flush the cache to make room for other data or to prepare for a shutdown.

Write-back caching makes the host more responsive to the user, since the host does not have to wait for a lengthy RAID write before proceeding to another task.

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Contents Compaq StorageWorks Page Contents RAID Array Controller List of Figures RAID 0 write List of Tables Page About This Guide Maintenance Related Documents Text Conventions Symbols in Text This is a test Rack Stability Compaq Website Product Overview Product DescriptionRAID Array 3000 pedestal enclosure drives optional Compaq Part No RA3000 pedestal subsystem with one controller, 230Pedestal Features Pedestal CabinetPedestal front panel major components RAID Array Controller Pedestal ComponentsStorageWorks Building Blocks SBBs Pedestal Power Supplies UPSEnvironmental Monitor Unit EMU EMU circuit board locationShows the Pedestal rear panel components UltraSCSI bus port and default Scsi ID assignments UltraSCSI BusesUltraSCSI bus configuration switch Connecting the Pedestal to a Host System Single Host, Single Adapter Cabling ItemÂDescriptionSingle Host, Dual Adapter/Dual Controller Cabling DescriptionHost System Dual Host, Single Adapter/Single Controller Cabling Specifications Pedestal Technical SpecificationsPedestal Physical and Power Specification Physical SpecificationsDimension Power SpecificationsRAID Array Controller Controller OverviewPage Controller Features Bridging the gap between the host and the pedestalUnits created from storagesets, partitions, and disk drives Controller Specifications SpecificationsController Specifications Controller Reset and LED Indicators LED/Reset Switch InterfaceEnvironmental NameFlexible RAID Set Configuration Shows the front panel of the controllerPerformance Enhancements Custom ComponentsEfficient Write and Read Algorithms Write-Back CachingWrite Gathering Write On TopRAID Levels Supported RAID Levels SupportedPedestal RAID Set Restrictions Max. No. * of DrivesRAID Shows a diagram of a RAID 0 write Diagram of a RAID 1 write RAID 0+1Function RAID 0+1 ExampleDrives Selected Diagram of a RAID 4 write Page Diagram of a RAID 5 write Redundant Operation Just a Bunch of Drives JbodEnvironmental InitializationMessage Passing FailoverBackup Power Management Response to Various AC Power ConditionsCondition ResponseVoltage Monitoring System VoltageAcceptable System Voltage Levels Termination VoltageTemperature Monitoring External TemperatureAcceptable External Temperature Levels Supply VoltageBoard Temperature Acceptable Board Temperature LevelsPage Maintenance Pedestal Status and Power LEDs SBB Status LEDs Shows the Pedestal status LEDsDisk Drive SBB Status LEDs Activity LED Fault LED IndicationController LEDs EMU Error Reporting EMU Error ConditionsReplacing Components FRUs Removing the Pedestal Door Replacing an SBBReplacing an SBB Replacing a Power Supply Replacing a power supplyReplacing the RAID Array Controller Removing the controller from the pedestalReplacing the EMU Board Tools requiredRemove standoffs from UPS and external fault connectors Differential/Wide UltraSCSI Bus Replacing the UPSReconfiguring the Scsi Bus Scsi Bus Length and External CablesAssigned Slot Device Addresses in the Pedestal Rate Bus LengthMaintenance Shows how to remove the screw and panel Close the side panels and replace the cover screw Replacing the Controller Memory Cache Modules 11. Remove controller12 shows how to release the locking clips 14. Install replacement Simm Expansion Pedestal Option Expansion Pedestal Cabinet Shows the Expansion PedestalExpansion Pedestal Option Shows the rear panel power supplies Expansion Pedestal ComponentsEMU Reconfiguring Base Pedestal UltraSCSI Bus Remove the side cover from the base pedestal Figure Disconnect Scsi cable Connect Scsi jumper Remove connector knockout plate 10. Connect Scsi cable Set the bus configuration switch to 7 Figures 4-11 13. Reconfigured Scsi bus addresses 14. Connect Scsi cable between pedestals 15. Power cable connections 16. Transfer drives from base to expansion pedestal Second Controller Option Second Controller Option Kit Second controller pedestal slot locationInstalling the Upgrade Saving Existing ConfigurationUpdating Firmware Using Swcc Updating FirmwareUpdating Firmware Using Serial Interface Firmware update dialog boxShutting Down the RA3000 Installing Two SIMMs into Second Controller Insert Simm into connectorReplacing Existing Controller Remove controller from top slotRestoring the Configuration Restoring configuration to new controller10. Restored configuration example Updating Firmware on Second ControllerInstalling Original Controller Configuring the RA3000 for the Active Mode Operation Configuring the Dual Controllers Connecting to Your Dual Controller Storage System Verifying the Controller Operating Parameters