2    Planning a High-Performance and High-Availability Configuration

A high-performance configuration is one that will rapidly respond to the demands of a normal workload, and also handle an increase in the workload. A high-availability configuration provides protection against single points of failure.

This chapter describes how to perform the following tasks:

• Identify a resource model for your workload (Section 2.1)

• Identify performance and availability goals (Section 2.2)

• Choose high-performance system hardware (Section 2.3)

• Choose high-performance disk storage hardware (Section 2.4)

• Choose how to manage your disk storage (Section 2.5)

• Choose a high-availability configuration (Section 2.6)

2.1    Identifying a Resource Model for Your Workload

Before you can plan or tune a configuration, you must identify a resource model for your workload. That is, you must determine if your applications are memory-intensive or CPU-intensive, and how they perform disk and network I/O. This information will help you to choose the configuration and tuning guidelines that are appropriate for your workload.

For example, if a database server performs large sequential data transfers, choose a configuration that provides high bandwidth. If a application performs many disk write operations, you may not want to choose a RAID 1 (mirrored) configuration.

Use Table 2-1 to help you determine the resource model for your workload and identify a possible configuration solution for each model.

Table 2-1:  Resource Models and Possible Configuration Solutions

2.2    Identifying Performance and Availability Goals

Before you choose a configuration, you must determine the level of performance and availability that you need. In addition, you must account for cost factors and plan for future workload expansion.

When choosing a system and disk storage configuration, you must be sure to evaluate the configuration choices in terms of the following criteria:

• Performance

  You must determine an acceptable level of performance for the applications and users. For example, you may want a real-time environment that responds immediately to user input, or you may want an environment that has high throughput.

• Availability

  You must determine how much availability is needed. Some environments require only highly available data. Other environments require you to eliminate all single points of failure.

• Cost

  You must determine the cost limitations for the environment. For example, solid-state disks provide high throughput and high bandwidth, but at a high cost.

• Scalability

  A system that is scalable is able to utilize additional resources (for example, additional CPUs) with a predictable increase in performance. Scalability can also refer to the ability of a system to absorb an increase in workload without a significant performance degradation. Be sure to include in your plans any potential workload increases and, if necessary, choose a configuration that is scalable.

After you determine the goals for your environment, you can choose the system and disk storage configuration that will address these goals.

2.3    Choosing System Hardware

Different systems provide different configuration and performance features. A primary consideration for choosing a system is its CPU and memory capabilities. Some systems support multiple CPUs, fast CPU speeds, and very-large memory (VLM) configurations.

Because very-large database (VLDB) systems and cluster systems usually require many external I/O buses, another consideration is the number of I/O bus slots in the system. Some systems support I/O expansion units for additional I/O capacity.

Be sure that a system is adequately scalable, which will determine whether you can increase system performance by adding resources, such as CPU and memory boards. If applicable, choose a system that supports RAID controllers, high-performance network adapters, and cluster products.

Table 2-2 describes some hardware options that can be used in a high-performance system, and the performance benefit for each option.

Table 2-2:  High-Performance System Hardware Options

For detailed information about hardware performance features, see the Compaq Systems & Options Catalog.

The following sections describe these hardware options in detail.

2.3.1    CPU Configuration

To choose a CPU configuration that will meet your needs, you must determine your requirements for the following:

• Number of CPUs

  Only certain types of systems support multiprocessing. If your environment is CPU-intensive or if your applications can benefit from multiprocessing, you may want a system that supports multiple CPUs.

  Depending on the type of multiprocessing system, you can install two or more CPUs. You must determine the number of CPUs that you need, and then choose a system that supports that number of CPUs and has enough backplane slots available for the CPU boards.

• CPU processing speed

  CPUs have different processing speeds and other performance features. For example, some processors support EV56 technology.

  If your environment is CPU-intensive, you may want to choose a system that supports high-performance CPUs.

• CPU cache size

  CPUs have different sizes for on-chip caches, which can improve performance. Some systems have secondary caches that reside on the main processor board, and some have tertiary caches.

  Caches that reside on the CPU chip can vary in size up to a maximum of 64 KB (depending on the type of processor). These caches include the translation lookaside buffer, the high-speed internal virtual-to-physical translation cache, the high-speed internal instruction cache, and the high-speed internal data cache.

  The secondary direct-mapped physical data cache is external to the CPU, but usually resides on the main processor board. Block sizes for the secondary cache vary from 32 bytes to 256 bytes (depending on the type of processor). The size of the secondary cache ranges from 128 KB to 8 MB. The tertiary cache is not available on all Alpha CPUs; otherwise, it is identical to the secondary cache.

2.3.2    Memory and Swap Space Configuration

You must determine the total amount of memory and swap space that you need to handle your workload. Insufficient memory resources and swap space will cause performance problems. In addition, your memory bank configuration will affect performance.

To configure memory and swap space, perform the following tasks:

1. Determine how much physical memory your workload requires and choose a system that provides the necessary memory and has enough backplane slots for memory boards (Section 2.3.2.1).

2. Choose a swap space allocation mode (Section 2.3.2.2).

3. Determine how much swap space you need (Section 2.3.2.3).

4. Configure swap space in order to efficiently distribute the disk I/O (Section 6.2).

The following sections describe these tasks.

2.3.2.1    Determining Your Physical Memory Requirements

You must have enough system memory to provide an acceptable level of user and application performance. The amount of memory installed in your system must be at least as much as the sum of the following:

• The total amount of memory that will be wired, including operating system data and text, system tables, the metadata buffer cache, and dynamically allocated data structures

• The total amount of memory your processes need for anonymous memory, which holds data elements and structures that are modified during process execution (heap space, stack space, and space allocated by the malloc function)

• The total amount of memory that the UBC requires to cache data

In addition, each network connection to your server requires the following memory resources:

• Kernel socket structure

• Internet protocol control block (inpcb) structure

• TCP control block structure

• Any additional socket buffer space that is needed as packets arrive and are consumed

These memory resources total 1 KB for each connection endpoint (not including the socket buffer space), so you need 10 MB of memory to accommodate 10,000 connections. There is no limit on a system's ability to handle millions of TCP connections, if you have enough memory resources to service the connections. However, when memory is low, the server will reject new connection requests until enough existing connections are freed. Use the netstat -m command to display the memory that is currently being used by the network subsystem.

To ensure that your server has enough memory to handle high peak loads, you should have available 10 times the memory that is needed on a busy day. For optimal performance and for scalability, configure more than the minimum amount of memory needed.

2.3.2.2    Choosing a Swap Space Allocation Mode

There are two modes that you can use to allocate swap space. The modes differ in how the virtual memory subsystem reserves swap space for anonymous memory (modifiable virtual address space). Anonymous memory is memory that is not backed by a file, but is backed by swap space (for example, stack space, heap space, and memory allocated by the malloc function). There is no performance benefit attached to either mode.

The swap space allocation modes are as follows:

• Immediate mode--This mode reserves swap space when a process first allocates anonymous memory. Immediate mode is the default swap space allocation mode and is also called eager mode.

  Immediate mode may cause the system to reserve an unnecessarily large amount of swap space for processes. However, it ensures that swap space will be available to processes if it is needed. Immediate mode is recommended for systems that overcommit memory (that is, systems that page).

• Deferred mode--This mode reserves swap space only if the virtual memory subsystem needs to write a modified virtual page to swap space. It postpones the reservation of swap space for anonymous memory until it is actually needed. Deferred mode is also called lazy mode.

  Deferred mode requires less swap space than immediate mode and may cause the system to run faster because it requires less swap space bookkeeping. However, because deferred mode does not reserve swap space in advance, the swap space may not be available when a process needs it, and the process may be killed asynchronously. Deferred mode is recommended for large-memory systems or systems that do not overcommit memory (page).

You specify the swap space allocation mode by using the vm subsystem attribute vm_swap_eager. Specify 1 to enable immediate mode (the default); specify 0 to enable deferred mode.

See the System Administration manual for more information on swap space allocation methods.

2.3.2.3    Determining Swap Space Requirements

Swap space is used to hold the recently accessed modified pages from processes and from the UBC. In addition, if a crash dump occurs, the operating system writes all or part of physical memory to swap space.

It is important to configure a sufficient amount of swap space and to distribute swap space across multiple disks. An insufficient amount of swap space can severely degrade performance and prevent processes from running or completing. A minimum of 128 MB of swap space is recommended.

The optimal amount of swap space for your configuration depends on the following factors:

• Total amount of memory configured in the system--In general, systems with large amounts of memory require less swap space than low-memory systems.

• Your applications' anonymous memory requirements--Anonymous memory holds data elements and structures that are modified during process execution, such as heap space, stack space, and memory allocated with the malloc function.

• Crash dump configuration--Swap space should be large enough to hold a complete crash dump file. Tru64 UNIX supports compressed crash dumps, which require less swap space to hold the dump file than uncompressed crash dumps. See the System Administration manual for more information on compressed crash dumps.

• Swap space allocation mode--Less swap space is required if you are using deferred mode instead of immediate mode.

• Whether you use the AdvFS verify command--If you want to use the AdvFS verify command, allocate an additional 5 percent of total memory for swap space.

To calculate the amount of swap space required by your configuration, follow these steps:

1. Determine the total amount of anonymous memory (modifiable virtual address space) required by all of your processes. To do this, invoke the ps -o vsz command and add the values in the VSZ (virtual address size) column. For example:

   # ps -o vsz
    VSZ
    536K
   2.16M
   2.05M
   2.16M
   2.16M
   3.74M
   2.16M
   #
   

2. If you are using immediate mode, add 10 percent to the total amount of anonymous memory.

   If you are using deferred mode, divide the total amount of anonymous memory by 2.

3. If you are using AdvFS, add 5 percent of total memory to this value. The resulting value represents the optimal amount of swap space.

You can configure swap space when you first install the operating system, or you can add swap space at a later date. In addition, swap space must following the guidelines for distributing disk I/O. See Section 6.2 for information about adding swap space after installation and configuring swap space for high performance.

2.3.3    I/O Bus Slot Capacity

Systems provide support for different numbers of I/O bus slots to which you can connect external storage devices and network adapters. Some enterprise systems provide up to 132 PCI slots for external storage. These systems are often used in VLDB systems and cluster configurations.

You must ensure that the system you choose has sufficient I/O buses and slots available for your disk storage and network configuration. Some systems support I/O expansion units for additional I/O capacity.

2.3.4    Support for High-Performance Disk Storage

Systems support different local disk storage configurations, including multiple storage shelves, large disk capacity, and UltraSCSI devices. In addition, some systems support high-performance PCI buses, which are required for hardware RAID subsystems and clusters.

You must ensure that the system you choose supports the disk storage configuration that you need. See Section 2.4 and Section 2.5 for more information on disk storage configurations.

2.3.5    Support for High-Performance Network

Systems support various networks and network adapters that provide different performance features. For example, an Asynchronous Transfer Mode (ATM) high-performance network is ideal for applications that need the high speed and the low latency (switched, full-duplex network infrastructure) that ATM networks provide.

In addition, you can configure multiple network adapters or use NetRAIN to increase network access and provide high network availability.

2.4    Choosing Disk Storage Hardware

The disk storage subsystem is used for both data storage and for swap space. Therefore, an incorrectly configured or tuned disk subsystem can degrade both disk I/O and virtual memory performance. Using your resource model, as described in Section 2.1, choose the disk storage hardware that will meet your performance needs.

Table 2-3 describes some hardware options that can be used in a high-performance disk storage configuration and the performance benefit for each option.

Table 2-3:  High-Performance Disk Storage Hardware Options

For detailed information about hardware performance features, see the Compaq Systems & Options Catalog.

The following sections describe some of these high-performance disk storage hardware options in detail.

2.4.1    Fast Disks

Disks that spin with a high rate of revolutions per minute (RPM) have a low disk access time (latency). High-RPM disks are especially beneficial to the performance of sequential data transfers.

High-performance disks (7200 RPM) can improve performance for many transaction processing applications (TPAs). UltraSCSI disks (10,000 RPM) are ideal for demanding applications, including network file servers and Internet servers, that require high bandwidth and high throughput.

2.4.2    Solid-State Disks

Solid-state disks provide outstanding performance compared to magnetic disks, but at a higher cost. By eliminating the seek and rotational latencies that are inherent in magnetic disks, solid-state disks can provide very high disk I/O performance. Disk access time is under 100 microseconds, which allows you to access critical data more than 100 times faster than with magnetic disks.

Available in both wide (16-bit) and narrow (8-bit) versions, solid-state disks are ideal for response-time critical applications with high data transfer rates, such as online transaction processing (OLTP), and applications that require high bandwidth, such as video applications.

Solid-state disks complement hardware RAID configurations by eliminating bottlenecks caused by random workloads and small data sets. Solid-state disks also provide data reliability through a nonvolatile data-retention system.

For the best performance, use solid-state disks for your most frequently accessed data to reduce the I/O wait time and CPU idle time. In addition, connect the disks to a dedicated bus and use a high-performance host bus adapter.

2.4.3    Devices with Wide Data Paths

Disks, host bus adapters, SCSI controllers, and storage expansion units support wide data paths, which provide nearly twice the bandwidth of narrow data paths. Wide devices can greatly improve I/O performance for large data transfers.

Disks with wide (16-bit) data paths provide twice the bandwidth of disks with narrow (8-bit) data paths. To obtain the performance benefit of wide disks, all the disks on a SCSI bus must be wide. If you use both wide and narrow disks on the same SCSI bus, the bus performance will be constrained by the narrow disks.

2.4.4    High-Performance Host Bus Adapters

Host bus adapters and interconnects provide different performance features at various costs. For example, FWD (fast, wide, and differential) SCSI bus adapters provide high bandwidth and high throughput connections to disk devices. Other adapters support UltraSCSI.

In addition, some host bus adapters provide dual-port (dual-channel) support, which allows you to connect two buses to one I/O bus slot.

Bus speed (the rate of data transfers) depends on the host bus adapter. Different adapters support bus speeds ranging from 5 million bytes per second (5 MHz) for slow speed to 40 million bytes per second (20 MHz) for UltraSCSI.

You must use high-performance host bus adapters, such as the KZPSA adapter, to connect systems to high-performance RAID array controllers.

2.4.5    DMA Host Bus Adapters

Some host bus adapters support direct memory access (DMA), which enables an adapter to bypass the CPU and go directly to memory to access and transfer data. For example, the KZPAA is a DMA adapter that provides a low-cost connection to SCSI disk devices.

2.4.6    RAID Controllers

RAID controllers are used in hardware RAID subsystems, which greatly expand the number of disks connected to a single I/O bus, relieve the CPU of the disk I/O overhead, and provide RAID functionality and other high-performance and high-availability features.

There are various types of RAID controllers, which provide different features. High-performance RAID array controllers support dynamic parity RAID and battery-backed, write-back caches. Backplane RAID storage controllers provide a low-cost RAID solution.

See Section 8.5 for more information about hardware RAID subsystems.

2.4.7    Fibre Channel

Fibre Channel is a high-performance I/O bus that is an example of serial SCSI and provides network storage capabilities. Fibre Channel supports multiple protocols, including SCSI, Intellitan Protocol Interface (IPI), TCP/IP, and High-Performance Peripheral Interface (HPPI).

Fibre Channel is based on a network of intelligent switches. Link speeds are available up to 100 MB/sec full duplex. Although Fibre Channel is more expensive than parallel SCSI, Fibre Channel Arbitrated Loop (FC-AL) decreases costs by eliminating the Fibre Channel fabric and using connected nodes in a loop topology with simplex links. In addition, an FC-AL loop can connect to a Fibre Channel fabric.

2.4.8    Prestoserve

The Prestoserve product is a combination of NVRAM hardware and software. Prestoserve can speed up synchronous disk writes, including NFS server access, by reducing the amount of disk I/O.

Prestoserve uses nonvolatile, battery-backed memory to temporarily cache file system writes that otherwise would have to be written to disk. This capability improves performance for systems that perform large numbers of synchronous writes.

To optimize Prestoserve cache use, you may want to enable Prestoserve only on the most frequently used file systems, or configure Prestoserve to cache only UFS or AdvFS metadata. In addition, Prestoserve can greatly improve performance for NFS servers. See Section 9.2.9 for more information.

You cannot use Prestoserve in a cluster or for nonfile system I/O.

2.5    Choosing How to Manage Disks

There are various methods that you can use to manage your disk storage configuration. These methods provide different performance and availability features. You must understand your workload resource model, as described in Section 2.1, to determine the best way to manage disk storage.

In addition, if you require highly-available disk storage, see Section 2.6 for information about the available options.

After you choose a method for managing disk storage, see Chapter 8 for configuration guidelines.

Table 2-4 describes some options for managing disk storage and the performance benefit and availability impact for each option.

Table 2-4:  High-Performance Disk Storage Configuration Solutions

The following sections describe some of these high-performance storage configurations in detail.

2.5.1    Using a Shared Pool of Storage for Flexible Management

There are two methods that you can use to manage the physical disks in your environment. The traditional method of managing disks and files is to divide each disk into logical areas called disk partitions, and to then create a file system on a partition or use a partition for raw I/O.

Each disk type has a default partition scheme. The disktab database file lists the default disk partition sizes. The size of a partition determines the amount of data it can hold. It can be time-consuming to modify the size of a partition. You must back up any data in the partition, change the size by using the disklabel command, and then restore the data to the resized partition.

An alternative to managing disks with static disk partitions is to use the Logical Storage Manager (LSM) to set up a shared pool of storage that consists of multiple disks. You can create virtual disks (LSM volumes) from this pool of storage, according to your performance and capacity needs, and then place file systems on the volumes or use them for raw I/O.

LSM provides you with flexible and easy management for large storage configurations. Because there is no direct correlation between a virtual disk and a physical disk, file system or raw I/O can span disks as needed. In addition, you can easily add disks to and remove disks from the pool, balance the load, and perform other storage management tasks.

LSM also provides you with high-performance and high-availability RAID functionality, hot spare support, and load balancing.

See Section 8.4 for information about LSM configurations.

2.5.2    Striping Data or Disks to Distribute I/O

Data or disk striping (RAID 0) distributes disk I/O and can improve throughput. The striped data is divided into blocks (sometimes called chunks or stripes) and distributed across multiple disks in a array. Striping enables parallel I/O streams to operate concurrently on different devices, so that I/O operations can be handled simultaneously by multiple devices.

LSM enables you to stripe data across different buses and adapters. Hardware RAID subsystems provide only disk striping.

LSM data striping provides better performance than disk striping with hardware RAID and supports more flexible configurations. With LSM, you can create a striped 36-GB volume comprised of three 9-GB disks and two 4.5-GB disks, or you can create a striped 13.5-GB volume comprised of two 4.5-GB disks and half of a 9-GB disk. The other half of the 9-GB disk can be used in other LSM volumes.

The performance benefit of striping depends on the size of the stripe and how your users and applications perform disk I/O. For example, if an application performs multiple simultaneous I/O operations, you can specify a stripe size that will enable each disk in the array to handle a separate I/O operation. If an application performs large sequential data transfers, you can specify a stripe size that will distribute a large I/O evenly across the disks.

For volumes that receive only one I/O at a time, you may not want to use striping if access time is the most important factor. In addition, striping may degrade the performance of small data transfers, because of the latencies of the disks and the overhead associated with dividing a small amount of data.

Striping decreases data availability because one disk failure makes the entire disk array unavailable. To make striped data or disks highly available, you can combine RAID 0 with RAID 1 to mirror the striped data or disks.

See Chapter 8 for more information about LSM and hardware RAID subsystems.

2.5.3    Using Parity RAID to Improve Disk Performance

Parity RAID provides both high performance and high availability. Tru64 UNIX supports three types of parity RAID, each with different performance and availability benefits:

• RAID 3--Divides data blocks and distributes the data across a disk array, providing parallel access. RAID 3 provides a high data transfer rate and increases bandwidth, but it provides no improvement in throughput (the I/O transaction rate).

  Use RAID 3 to improve the I/O performance of applications that transfer large amounts of sequential data. RAID 3 provides no improvement for applications that perform multiple I/O operations involving small amounts of data.

  RAID 3 also provides high data availability by storing redundant parity information on a separate disk. The parity information is used to regenerate data if a disk in the array fails. However, performance degrades as multiple disks fail, and data reconstruction is slower than if you had used mirroring.

• RAID 5--Distributes data blocks across disks in an array. RAID 5 allows independent access to data and can handle simultaneous I/O operations.

  RAID 5 can be used for configurations that are mainly read-intensive. RAID 5 is not suitable for write-intensive applications.

  As a cost-efficient alternative to mirroring, you can use RAID 5 to improve the availability of rarely-accessed data. RAID 5 provides high data availability by distributing redundant parity information across disks. Each array member contains enough parity information to regenerate data if a disk fails. However, performance may degrade and data may be lost if multiple disks fail. In addition, data reconstruction is slower than if you had used mirroring. LSM supports only RAID 5 for parity RAID.

• Dynamic parity RAID--Dynamically adjusts, according to the workload, between data transfer-intensive algorithms and I/O operation-intensive algorithms, combining the performance benefits of RAID 3 and RAID 5. Also known as adaptive RAID 3/5, dynamic parity RAID improves disk I/O performance for a wide variety of applications. Only high-performance RAID controllers support dynamic parity RAID.

LSM can provide more flexible RAID configurations than hardware RAID, because LSM is able to utilize portions of disks, instead of requiring complete disks. See Chapter 8 for more information about LSM and hardware RAID subsystems.

2.6    Choosing a High-Availability Configuration

You can set up a configuration that provides the level of availability that you need. For example, you can make only disk data highly available or you can set up a cluster configuration with no single point of failure, as shown in Figure 1-4.

Table 2-5 lists each possible point of failure, the configuration solution that will provide high availability, and any performance benefits and tradeoffs.

Table 2-5:  High-Availability Configurations

The following sections describe some of the previous high-availability configurations in detail.

2.6.1    Using a Cluster for System Availability

If users and applications depend on the availability of a single system for CPU, memory, data, and network resources, they will experience down time if a system crashes or an application fails. To make systems and applications highly available, you must use the TruCluster Server product to set up a cluster.

A cluster is a loosely coupled group of servers configured as member systems and connected to highly available shared disk storage and common networks. Software applications are installed on every member system, but only one system runs an application at one time.

If a resource for a member system fails (for example, a network adapter or a bus fails), all cluster-configured applications running on that system will fail over to a viable member system. The new system then starts the applications and makes them available to users.

To protect against multiple system failures, use more than two member systems in a cluster.

In addition, TruCluster Server supports a high-performance cluster interconnect that enables fast and reliable communications between members. To protect against interconnect failure, use redundant cluster interconnects.

You can use only specific systems, host bus adapters, RAID controllers, and disks with the cluster products. In addition, member systems must have enough I/O bus slots for adapters, controllers, and interconnects.

You can use LSM or hardware RAID to improve the performance and availability of a cluster's shared storage. For example, LSM enables you to create a mirrored and striped volume that can be used to access an LSM volume simultaneously from different cluster member systems.

See the TruCluster Server Software Product Description for detailed information about the product.

2.6.2    Using RAID for Disk Data Availability

RAID technology provides you with high data availability, in addition to high performance. RAID 1 (data or disk mirroring) provides high data availability by maintaining identical copies of data on different disks in an array. If the original disk fails, the copy is still available to users and applications. To protect data against a host bus adapter or bus failure, mirror across disks located on different buses.

Mirroring can improve read performance because data can be read from two different locations. However, it decreases disk write performance, because data must be written to two different locations.

LSM provides more flexible RAID configurations than hardware RAID. LSM mirrors (and stripes) data. For example, LSM can mirror (and stripe) data across portions of disks or across different types of disks. Hardware RAID subsystems mirror entire disks.

Hardware RAID subsystems and LSM also use parity RAID, to provide high data availability and high performance. With parity RAID, data is spread across disks, and parity information is used to reconstruct data if a failure occurs. Tru64 UNIX supports three types of parity RAID (RAID 3, RAID 5, and dynamic parity RAID) and each provides different performance and availability benefits. LSM supports only RAID 5. Only high-performance RAID controllers support dynamic parity RAID. See Section 2.5.3 for more information.

See Chapter 8 for more information about LSM and hardware RAID subsystems.

2.6.3    Using Redundant Networks

Network connections may fail because of a failed network interface or a problem in the network itself. You can make the network connection highly available by using redundant network connections. If one connection becomes unavailable, you can still use the other connection for network access. Whether you can use multiple networks depends on the application, network configuration, and network protocol.

You can also use NetRAIN (redundant array of independent network adapters) to configure multiple interfaces on the same LAN segment into a single interface, and to provide failover support for network adapter and network connections. One interface is always active while the other interfaces remain idle. If the active interface fails, an idle interface is brought on line within less than 10 seconds.

NetRAIN supports only Ethernet and FDDI.

See nr(7) for more information about NetRAIN. See the Network Administration manual for information about network configuration. See Chapter 10 for information about improving network performance.

2.6.4    Using Redundant Power Supplies and Systems

To protect against a power supply failure for cabinets, systems, and storage shelves, use redundant power supplies. Alternately, for disk storage units, you can mirror across cabinets with different power supplies.

In addition, use an uninterruptible power supply (UPS) system to protect against a total power failure (for example, the power in a building fails). A UPS system depends on a viable battery source and monitoring software.