Compaq EK-SMCPO-UG. C01 Efficient Write and Read Algorithms

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

Compaq Confidential – Need to Know Required

Writer: Bob Young Project: RAID A rray 3000 Pedestal Storage Subsystem Hardware User’s Guide Comments:

Part Number: EK-SMCPO-UG. C01 File Name: c-ch2 RAID Array Controller.doc Last Saved On: 12/4/00 1:51 PM

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. With out a backup, data stored in the cac he, but not yet wr itten to the di sk

drives, would be lost in the event of a power interruption.

Write-Back Caching

When the host send s data to be writt en to a redundancy g roup, the control ler

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 cont roller

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.

Contents

Main Compaq StorageWorks Page Contents About This Guide Chapter 1 Product Overview Chapter 2 RAID Array Controller Chapter 3 Maintenance Chapter 4 Expansion Pedestal Option Chapter 5 Second Controller Option List of Figures Page List of Tables Page About This Guide Purpose and Scope Intended Audience Document Structure Page Related Documents Table 1 Related Documents Text Conventions Symbols in Text Symbols on Equipment xiv RAID Array 3000 Pedestal Storage Subsystem Hardware User's Guide Compaq Confidential Need to Know Required This is a test Rack Stability Getting Help Compaq Technical Support Compaq Website Compaq Authorized Reseller Chapter 1 Product Overview Product Description Page Product Overview 1-3 Table 1-1 RAID Array 3000 Pedestal Part Numbers and Model Descriptions Pedestal Features Pedestal Cabinet Page Pedestal Components StorageWorks Building Blocks (SBBs) RAID Array Controller Pedestal Power Supplies UPS Environmental Monitor Unit (EMU) Product Overview 1-9 Figure 1-5 shows the Pedestal rear panel components. Figure 1-5. Pedestal rear panel components UltraSCSI Buses Product Overview 1-11 Figure 1-8. Slot locations and SCSI ID addresses Figure 1-7. UltraSCSI bus configuration switch Connecting the Pedestal to a Host System Product Overview 1-13 Figure 1-9 shows the Single host, Single Adapter Cabling diagram. Host System Table 1-2 Single Host, Single Adapter Cabling 1-14 RAID Array 3000 Pedestal Storage Subsystem Hardware Users Guide Figure 1-10 shows the Single host, Dual Adapter/dual Controller Cabling diagram. Table 1-3 Single Host, Dual Adapter/Dual Controller Cabling Product Overview 1-15 Table 1-4 Single Host, Single Adapter/Dual Controller (Y-cable connection cabling) 1-16 RAID Array 3000 Pedestal Storage Subsystem Hardware Users Guide Figure 1-12 shows the Dual host, Single Adapter/single Controller Cabling diagram. Table 1-5 Dual Host, Single Adapter/Single Controller Cabling Product Overview 1-17 Specifications Table 1-6 Pedestal Technical Specifications 1-18 RAID Array 3000 Pedestal Storage Subsystem Hardware Users Guide Table 1-7 Pedestal Physical and Power Specification Chapter 2 RAID Array Controller Controller Overview Page Controller Features Page Table 2-1 Controller Specifications Page Controller Reset and LED Indicators Table 2-2 LED/Reset Switch Interface Flexible RAID Set Configuration Performance Enhancements Custom Components Efficient Write and Read Algorithms Write-Back Caching Write Gathering Write On Top RAID Levels Supported Table 2-3 RAID Levels Supported Table 2-4 Pedestal RAID Set Restrictions RAID 0 Page RAID 1 RAID 0+1 Table 2-5 RAID 0+1 Example RAID 4 Page RAID 5 Just a Bunch of Drives (JBOD) Redundant Operation Initialization Message Passing Failover Environmental Backup Power Management Table 2-6 Response to Various AC Power Condition s Voltage Monitoring Temperature Monitoring Page Page Chapter 3 Maintenance Pedestal Status and Power LEDs SBB Status LEDs Table 3-1 Disk Drive SBB Status LEDs Controller LEDs EMU Error Reporting EMU Error Conditions Replacing Components (FRUs) Removing the Pedestal Door Replacing an SBB Page Replacing a Power Supply Replacing the RAID Array Controller Replacing the EMU Board Replacing the UPS Differential/Wide UltraSCSI Bus Table 3-2 SCSI Bus Length and External Cables Table 3-3 Assigned Slot Device Addresses in the Pedest al Reconfiguring the SCSI Bus Page Page Maintenance 3-17 Replacing the Controller Memory Cache Modules Page Page Chapter 4 Expansion Pedestal Opt ion Product Description Expansion Pedestal Cabinet Page Expansion Pedestal Components Page Reconfiguring Base Pedestal UltraSCSI Bus Page Page Page Page Page Page Expansion Pedestal Option 4-15 UPS, and the AC power source (Figure 415). Chapter 5 Second Controller Option Second Controller Option Kit Installing the Upgrade Saving Existing Configuration Updating Firmware Updating Firmware Using SWCC Updating Firmware Using Serial Interface Shutting Down the RA3000 Installing Two SIMMs into Second Controller Replacing Existing Controller Restoring the Configuration Updating Firmware on Second Controller Installing Original Controller Configuring a Dual Controller Installation for a Single Serial Port Configuring the RA3000 for the Active Mode of Operation Configuring the Dual Controllers Connecting to Your Dual Controller Storage System Verifying the Controller Operating Parameters