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Configuring the RAID Controller

Dell™ PowerEdge™ Expandable RAID Controller 4/SC, 4/DC, and 4e/DC User's Guide

  Configuring SCSI Physical Drives

  Physical Device Layout

  Device Configuration

  Setting Hardware Termination

  Configuring Arrays

  Assigning RAID Levels

  Optimizing Storage

  Planning the Array Configuration

This section describes how to configure for physical drives, arrays, and logical drives. It contains tables you can complete to list the configuration for the physical drives and logical drives.

Configuring SCSI Physical Drives

Your SCSI hard drives must be organized into logical drives in an array and must be able to support the RAID level that you select.

Observe the following guidelines when connecting and configuring SCSI devices in a RAID array:

• You can place up to 28 physical drives in an array.
  
  
• Use drives of the same size and speed to maximize the effectiveness of the controller.
  
  
• When replacing a failed drive in a redundant array, make sure that the replacement drive has the same or larger capacity than the smallest drive in the array (RAID 1, 5, 10, and 50).
  
  

NOTE: For RAID levels 10 and 50, the additional space in larger arrays can store data, so you can use arrays of different sizes.

When implementing RAID 1 or RAID 5, disk space is spanned to create the stripes and mirrors. The span size can vary to accommodate the different disk sizes. There is, however, the possibility that a portion of the largest disk in the array will be unusable, resulting in wasted disk space. For example, consider a RAID 1 array that has the following disks, as shown in Table 4-1.

Table 4-1. Storage Space in a RAID 1 Array

In this example, data is mirrored across the two disks until 20 GB on Disk A and B are completely full. This leaves 10 GB of disk space on Disk B. Data cannot be written to this remaining disk space, as there is no corresponding disk space available in the array to create redundant data.

Table 4-2 provides an example of a RAID 5 array.

Table 4-2. Storage Space in a RAID 5 Array

In this example, data is striped across the disks until 40 GB on Disks A, B, and C are completely full. This leaves 20 GB of disk space on Disk C. Data cannot be written to this remaining disk space, as there is no corresponding disk space available in the array to create redundant data.

Physical Device Layout

Use Table 4-3 to list the details for each physical device on the channels.

Table 4-3. Physical Device Layout 

Device Configuration

The following contain tables you can fill out to list the devices assigned to each channel. The PERC 4/SC controller has one channel; the PERC 4/DC and 4e/DC have two.

Use Table 4-4 to list the devices that you assign to each SCSI ID for SCSI Channel 0.

Table 4-4. Configuration for SCSI Channel 0 

Use Table 4-5 to list the devices that you assign to each SCSI ID for SCSI Channel 1.

Table 4-5. Configuration for SCSI Channel 1 

Setting Hardware Termination

NOTE: If you are using the PERC 4/DC RAID controller for clustering, then you must use hardware termination. Otherwise, software termination is OK.

The SCSI bus is an electrical transmission line and must be terminated properly to minimize reflections and losses. Termination should be set at each end of the SCSI cable(s). For PERC 4e/DC, the following headers specify control of the SCSI termination:

• J5 Termination Enable is a three-pin header that specifies control of the SCSI termination for channel 0.
  
  
• J6 Termination Enable is a three-pin header that specifies control of the SCSI termination for channel 1.
  
  

To enable hardware termination, leave the pins open. The default is hardware termination.

NOTE: See Step 7 Set SCSI Termination for additional information about setting SCSI termination.

Configuring Arrays

After you configure and initialize the hard drives, you are ready to configure arrays. The number of drives in an array determines the RAID levels that can be supported.

For information about the number of drives required for different RAID levels, see Table 4-7 in Assigning RAID Levels.

Spanned Drives

You can arrange arrays sequentially with an identical number of drives so that the drives in the different arrays are spanned. Spanned drives can be treated as one large drive. Data can be striped across multiple arrays as one logical drive.

You can create spanned drives using your array management software.

Hot Spares

Any hard drive that is present, formatted, and initialized, but is not included in an array or logical drive, can be designated as a hot spare. A hot spare should have the same or greater capacity than the smallest physical disk in the array it protects. You can designate hard drives as hot spares using your array management software.

Logical Drives

Logical drives, also known as virtual disks, are arrays or spanned arrays that are available to the operating system. The storage space in a logical drive is spread across all the physical drives in the array or spanned arrays.

You must create one or more logical drives for each array, and the logical drive capacity must include all of the drive space in an array. You can make the logical drive capacity larger by spanning arrays. In an array of drives with mixed sizes, the smallest common drive size is used and the space in larger drives is not used. The RAID controller supports up to 40 logical drives.

Configuration Strategies

The most important factors in RAID array configuration are:

• Drive capacity
  
  
• Drive availability (fault tolerance)
  
  
• Drive performance
  
  

You cannot configure a logical drive that optimizes all three factors, but it is easy to choose a logical drive configuration that maximizes one factor at the expense of the other two factors. For example, RAID 1(mirroring) provides excellent fault tolerance, but requires a redundant drive.

Configuring Logical Drives

After you have attached all physical drives, perform the following steps to prepare a logical drive. If the operating system is not yet installed, use the BIOS Configuration Utility to perform this procedure. If the operating system is installed, you can use the Dell Manager for Linux or OpenManage Array Manager (for Windows and Netware), depending on the operating system.

1. Start the system.
   
   
2. Run your array management software.
   
   
3. Select the option to customize the RAID array.
   
   

In the BIOS Configuration Utility and Dell Manager for Linux, use either Easy Configuration or New Configuration to customize the RAID array.

CAUTION: If you select New Configuration, all previous configuration information will be deleted.

4. Create and configure one or more system drives (logical drives).
   
   
5. Select the RAID level, cache policy, read policy, and write policy.
   
   

NOTE: Refer to the section Summary of RAID Levels for RAID level explanations.

6. Save the configuration.
   
   
7. Initialize the system drives.
   
   

After initialization, you can install the operating system.

See BIOS Configuration Utility and Dell Manager for detailed configuration instructions.

Logical Drive Configuration

Use Table 4-6 to list the details for each logical drive that you configure.

Table 4-6. Logical Drive Configuration 

Assigning RAID Levels

Only one RAID level can be assigned to each logical drive. Table 4-7 shows the minimum and maximum number of drives required.

Table 4-7. Physical Drives Required for Each RAID Level 

Summary of RAID Levels

RAID 0 uses striping to provide high data throughput, especially for large files in an environment that does not require fault tolerance.

RAID 1 uses mirroring and is good for small databases or other applications that require small capacity, but complete data redundancy.

RAID 5 provides high data throughput, especially for small random access. Use this level for any application that requires high read request rates, but low write request rates, such as transaction processing applications. Write performance is significantly lower for RAID 5 than for RAID 0 and RAID 1.

RAID 10 consists of striped data across mirrored spans. It provides high data throughput and complete data redundancy, but uses a larger number of spans.

RAID 50 uses parity and disk striping and works best with data that requires high reliability, high request rates, high data transfers, and medium-to-large capacity. Write performance is limited to the same as RAID 5.

Storage in an Array with Drives of Different Sizes

For RAID levels 0 and 5, data is striped across the disks. If the hard drives in an array are not the same size, data is striped across all the drives until one or more of the drives is full. After one or more drives are full, disk space left on the other disks cannot be used. Data cannot be written to that disk space because other drives do not have corresponding disk space available.

Figure 4-1 shows an example of storage allocation in a RAID 5 array. The data is striped, with parity, across the three drives until the smallest drive is full. The remaining storage space in the other hard drives cannot be used because not all of the drives have disk space for redundant data.

Figure 4-1. Storage in a RAID 5 Array

Storage in RAID 10 and RAID 50 Arrays

You can span RAID 1 and 5 arrays to create RAID 10 and RAID 50 arrays, respectively. For RAID levels 10 and 50, you can have some arrays with more storage space than others. After the storage space in the smaller arrays is full, you can use the additional space in larger arrays can store data.

Figure 4-2 shows the example of a RAID 50 span with three RAID 5 arrays of different sizes. (Each array can have from three to 14 hard disks.) Data is striped across the three RAID 5 arrays until the smallest array is full. The data is striped across the remaining two RAID 5 arrays until the smaller of the two arrays is full. Finally, data is stored in the additional space in the largest array.

Figure 4-2. Storage in a RAID 50 Array

Performance Considerations

The system performance improves as the number of spans increases. As the storage space in the spans is filled, the system stripes data over fewer and fewer spans and RAID performance degrades to that of a RAID 1 or RAID 5 array.

Optimizing Storage

Data Access Requirements

Each type of data stored in the disk subsystem has a different frequency of read and write activity. If you know the data access requirements, you can more successfully determine a strategy for optimizing the disk subsystem capacity, availability, and performance.

Servers that support video on demand typically read the data often, but write data infrequently. Both the read and write operations tend to be long. Data stored on a general-purpose file server involves relatively short read and write operations with relatively small files.

Array Functions

Define the major purpose of the disk array by answering questions such as the following, which are followed by suggested RAID levels for each situation:

• Will this disk array increase the system storage capacity for general-purpose file and print servers? Use RAID 5, 10, or 50.
  
  
• Does this disk array support any software system that must be available 24 hours per day? Use RAID 1, 5, 10, or 50.
  
  
• Will the information stored in this disk array contain large audio or video files that must be available on demand? Use RAID 0.
  
  
• Will this disk array contain data from an imaging system? Use RAID 0 or 10.
  
  

Planning the Array Configuration

Fill out Table 4-8 to help you plan the array configuration. Rank the requirements for your array, such as storage space and data redundancy, in order of importance, then review the suggested RAID levels. Refer to Table 4-7 for the minimum and maximum number of drives allowed per RAID level.

Table 4-8. Factors to Consider for Array Configuration 

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