RAID Technology Overview HP Smart Array RAID Controllers HP Part Number: J6369-90050 Published: September 2007 Edition: 1
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Table of Contents About This Document ........................................................................................................9 Intended Audience.................................................................................................................................9 New and Changed Documentation in This Edition...............................................................................9 Publishing History............................................................................
List of Figures 1-1 1-2 1-3 2-1 2-2 2-3 2-4 2-5 A-1 Physical Disks Added to System ..................................................................................................13 Physical Disks Configured into a Logical Drive (L1) ...................................................................13 Data Striping (S1-S4) of Data Blocks B1-B12 ................................................................................14 Data Striping (S1-S4) of Data Blocks B1-B12 ..................................
List of Tables 2-1 2-2 Summary of RAID Methods..........................................................................................................19 Choosing a RAID Method.............................................................................................................
About This Document This document provides an overview of Redundant Array of Independent Disks (RAID) technology. The information in this document applies to HP Integrity and HP 9000 servers equipped with HP Smart Array RAID controllers, running any supported operating system. The latest version of this document can be found online at: http://docs.hp.com/en/netcom.
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1 Introduction to RAID Technology This chapter provides an overview of RAID technology and descriptions of the different RAID levels that are supported by HP Smart Array Controllers.
Overall, RAID has three main attributes that are exploited in some way by all five original RAID configurations and by most other RAID configurations that have been defined since the 1987 study.
Performance and Data Redundancy Increasing Logical Drive Performance Without an array controller, connecting extra physical disks to a system increases the total storage capacity. However, it has no effect on the efficiency of read/write operations, because data can only be transferred to one physical disk at a time (see Figure 1-1).
Figure 1-3 Data Striping (S1-S4) of Data Blocks B1-B12 For data in the logical drive to be readable, the data block sequence must be the same in every stripe. This sequencing process is performed by the Smart Array Controller, which sends the data blocks to the physical disk, writing the heads in the correct order. In a striped array, each physical disk in a logical drive contains the same amount of data.
2 Smart Array Controller Supported RAID Configurations This chapter provides details about each of the RAID levels that are supported by HP Smart Array Controllers.
Figure 2-2 Disk Drive Mirroring of P1 onto P2 (RAID 1) The advantages of RAID 1 are as follows: • • No data loss or interruption of service if a disk fails. Fast read performance — data is available from either disk. The disadvantages of RAID 1 are as follows: • High cost — 50% of disk space is allocated for data protection, so only 50% of total disk drive capacity is usable for data storage. RAID 1+0: Disk Mirroring and Striping RAID 1+0 requires an array with four or more physical disks.
The advantages of RAID 1+0 are as follows: • • Highest read and write performance of any fault-tolerant configuration. No loss of data as long as no of failed disks are mirrored to any other failed disk (up to half of the physical disks in the array can fail). The disadvantages of RAID 1+0 are as follows: • • Expensive — many disks are needed for fault tolerance. Only 50% of total disk capacity usable for data storage.
RAID ADG is most useful when data loss is unacceptable but cost must also be minimized. The probability that data loss will occur when arrays are configured with RAID ADG is less than when they are configured with RAID 5. For more information, see Appendix A (page 21). Figure 2-5 Advanced Data Guarding, Showing Parity Information (Px,y and Qx,y) The advantages of RAID ADG are as follows: • • • High read performance. High data availability—any two disks can fail without loss of critical data.
Summary of RAID Methods Table 2-1 summarizes the important features of the different RAID configurations that are supported by the Smart Array Controllers. The decision chart in Table 2-2 (page 19) can help you determine which option is best for your computing environment.
A Logical Drive Failure Probability This appendix discusses the probability of logical drive failure. RAID Level and Probability of Drive Failure The probability that a logical drive will fail depends on the RAID level setting. • A RAID 0 logical drive fails if only one physical disk fails.
Figure A-1 Relative Probability of Logical Drive Failure 22 Logical Drive Failure Probability
Glossary array A set of physical disks configured into one or more logical drives. Arrayed disks have significant performance and data protection advantages over non-arrayed disks. array capacity expansion See capacity expansion. Auto-Reliability Monitoring (ARM) Also known as surface analysis. A fault management feature that scans physical disks for bad sectors. Data in the faulty sectors remaps onto good sectors. Also checks parity data consistency for disks in RAID 5 or RAID ADG configurations.
Redundant Array of Independent Disks (RAID) A form of fault-tolerant storage control. See Chapter 1: “Introduction to RAID Technology” (page 11). Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T.) Technology co-developed by HP and the physical disk industry that provides warning of imminent disk failure. The self-monitoring routines are customized for each specific disk type and have direct access to internal performance, calibration, and error measurements. spare See online spare.
Index A N ADGSee RAID ADG, 17 advanced data guarding (RAID ADG), 17 array defined, 12 physical limitations of, 14 no fault tolerance (RAID 0), 15 B block of data, defined, 13 C comparison of different RAID methods, 19 D data block, defined, 13 data protection methods RAID, 14 data stripes, defined, 13 distributed data guarding (RAID 5), 17 drive arraySee array, 12 drive mirroring (RAID 1+0), 15 F fault tolerance description of methods, 14 fault toleranceSee also RAID methods, 14 features of RAID meth
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