User guide
Table Of Contents
- MSA1000 User Guide
- Contents
- About this Guide
- Chapter 1: Introduction
- Chapter 2: Operating System Specific Information
- Chapter 3: MSA1000 Setup and Sample Configurations
- Chapter 4: Operation and Management
- Chapter 5: Array Configuration Utility (ACU)
- Installing the ACU on the Server
- Accessing the ACU
- Description of Screen Regions
- Configuring a New Controller
- Modifying an Existing Controller
- Probability of Logical Drive Failure
- Chapter 6: Command Line Interface (CLI)
- CLI Overview
- CLI Setup
- Help Commands
- Display Commands
- Array Controller Configuration Commands
- LUN Management Commands
- Server Connection Commands
- Selective Storage Presentation/Access Control List Commands
- Appendix A: Regulatory Compliance Notices
- Appendix B: Electrostatic Discharge
- Appendix C: Specifications
- Appendix D: Hard Drive Arrays
- Appendix E: Recovering from Hard Drive Failure
- Appendix F: Controller Display Messages
- Appendix G: Recovery ROM and ROM Cloning
- Appendix H: SCSI ID Assignments
- Index
Array Configuration Utility (ACU)
114 Modular SAN Array 1000 User Guide
Probability of Logical Drive Failure
The probability that a logical drive will fail depends on the RAID level setting. 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 drives increases, it becomes increasingly
unlikely that a newly failed drive is not mirrored to a previously failed
drive.
—The minimum number of physical drive failures that can cause the
logical drive to fail is two, if the two drives happen to be mirrored to each
other. The probability that this will happen decreases as the total number
of hard drives in the array increases.
■ A RAID 5 logical drive fails if two physical drives fail.
■ A RAID 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.
The graph in Figure 57 provides more quantitative information. The data for this
graph is calculated from the mean time between failure (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.
230941-005_MSA1000_UG.book Page 114 Thursday, April 17, 2003 5:53 PM