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Introduction to RAID

RAID Benefits Host-Based RAID Solution RAID Overview Consistency Check Fault Tolerance Disk Striping Disk Mirroring Parity Hot Spares Disk Rebuild Logical Drive Hot Swap SCSI Drive States Logical Drive States Disk Array Types

RAID (Redundant Array of Inexpensive or Independent Disks) is an array of multiple independent hard disk drives that provide high performance and fault tolerance. A RAID disk subsystem improves I/O performance over a computer using only a single drive. The RAID array appears to the host computer as a single storage unit or as multiple logical units. I/O is expedited because several disks can be accessed simultaneously. RAID systems improve data storage reliability and fault tolerance compared to single-drive computers. Data loss because of a disk drive failure can be recovered by reconstructing missing data from the remaining data and parity drives.

RAID Benefits

RAID has gained popularity because it improves I/O performance and increases storage subsystem reliability. RAID provides data security through fault tolerance and redundant data storage.

Improved I/O

Although disk drive capabilities have improved drastically, actual performance has improved only three to four times in the last decade. Computing performance has improved over 50 times during the same time period. Implementing RAID improves the performance of the disk subsystem.

Increased Reliability

The electromechanical components of a disk subsystem operate more slowly, require more power, and generate more noise and vibration than electronic devices. These factors reduce the reliability of data stored on disks. Implementing RAID on disk subsystems improve the reliability of data.

Host-Based RAID Solution

RAID products are either:

  • software-based
  • host-based
  • external

A host-based RAID solution is a PCI adapter card that is installed in any available PCI expansion slot in a host system.


A host-based RAID product puts all of the RAID intelligence on an adapter card that is installed in a network server. A host-based RAID product provides much better performance than a software-based solution which runs on the host CPU. Host-based RAID also is a much better price/performance solution than external RAID.

The available sequential data transfer rate is determined by the following factors:

  • the sustained data transfer rate on the motherboard PCI bus
  • the sustained data transfer rate on the i960RN PCI to PCI bridge
  • the sustained data transfer rate of the SCSI controller
  • the sustained data transfer rate of the SCSI devices
  • the number of SCSI channels
  • the number of SCSI disk drives
  • Host-based solutions must provide operating system-specific drivers


An external RAID product puts the RAID intelligence inside the RAID chassis and uses a plain Host Bus Adapter installed in the network server. The data transfer rate is limited to the bandwidth of the HBA channel.


A SCSI-to-SCSI RAID product puts the RAID intelligence inside the RAID chassis and uses a plain SCSI Host Adapter installed in the network server. The data transfer rate is limited to the bandwidth of the SCSI channel. A SCSI-to-SCSI RAID product that has two wide SCSI channels that operate at speeds up to 80 MB/s must squeeze the data into a single wide SCSI (40 MB/s) channel back to the Host computer.

In SCSI-to-SCSI RAID products, the hard drive subsystem uses only a single SCSI ID, which allows you to connect multiple drive subsystems to a single SCSI controller.

RAID Overview

RAID (Redundant Array of Independent Disks) is a collection of specifications that describe a system for ensuring the reliability and stability of data stored on large disk subsystems. A RAID system can be implemented in a number of different versions (or RAID Levels) 0, 1, 5 and 10 (or 1+0).

Consistency Check

In RAID, check consistency verifies the correctness of redundant data in an array. For example, in a system with parity, checking consistency means computing the parity of the data drives and comparing the results to the contents of the parity drive.

Fault Tolerance

Fault tolerance is achieved through cooling fans, power supplies, and the ability to hot swap drives. Most RAID controllers provide hot swapping through the hot spare feature. A hot spare drive is an unused online available drive that instantly can use to rebuild a drive when an active drive fails.

After the hot spare is automatically moved into the RAID subsystem, the failed drive is automatically rebuilt on the spare drive. The RAID disk array continues to handle request while the rebuild occurs.

Disk Striping

Disk striping writes data across multiple disk drives instead of just one disk drive. Disk striping involves partitioning each drive storage space into stripes that can vary in size from 2 kilobytes (KB) to 128 KB. These stripes are interleaved in a repeated sequential manner. The combined storage space is composed of stripes from each drive. Most controllers support stripe sizes of 2 KB, 4 KB, 8 KB, 16 KB, 32 KB, 64 KB, or 128 KB.

For example, in a four-disk system using only disk striping (as in RAID level 0), segment 1 is written to disk 1, segment 2 is written to disk 2, and so on. Disk striping enhances performance because multiple drives are accessed simultaneously; but disk striping does not provide data redundancy.

Figure 1. Disk Striping

Stripe Width

Stripe width is the number of disks involved in an array where striping is implemented. For example, a four-disk array with disk striping has a stripe width of four.

Stripe Size

The stripe size is the length of the interleaved data segments that the RAID controller writes across multiple drives. Most RAID controllers support stripe sizes of 2 KB, 4 KB, 8 KB, 16 KB, 32 KB, 64 KB, or 128 KB.

Disk Mirroring

With mirroring (used in RAID 1), data written to one disk drive is simultaneously written to another disk drive. If one disk drive fails, the contents of the other disk drive can be used to run the system and reconstruct the failed drive. The primary advantage of disk mirroring is that it provides 100% data redundancy. Since the contents of the disk drive are completely written to a second drive, it does not matter if one of the drives fails. Both drives contain the same data at all times. Either drive can act as the operational drive.

Disk mirroring provides 100% redundancy, but is expensive because each drive in the system must be duplicated.

Figure 2. Disk Mirroring


Parity generates a set of redundancy data from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets. Parity data does not fully duplicate the parent data sets. In RAID, this method is applied to entire drives or stripes across all disk drives in an array. The types of parity are:

Table 1. Parity Type

Parity Type Description
Dedicated The parity of the data on two or more disk drives is stored on an additional disk.
Distributed The parity data is distributed across all drives in the system.

If a single disk drive fails, it can be rebuilt from the parity and the data on the remaining drives.

RAID level 5 combines distributed parity with disk striping. Parity provides redundancy for one drive failure without duplicating the contents of entire disk drives, but parity generation can slow the write process. A dedicated parity scheme during normal read/write operations is shown below:

Figure 3. Parity

Hot Spares

A hot spare is an extra, unused disk drive that is part of the disk subsystem. It is usually in standby mode, ready for service if a drive fails. Hot spares permit you to replace failed drives without system shutdown or user intervention.

Most RAID controllers implement automatic and transparent rebuilds using hot spare drives, providing a high degree of fault tolerance and zero downtime. The RAID management software allows you to specify physical drives as hot spares. When a hot spare is needed, the RAID controller assigns the hot spare that has a capacity closest to and at least as great as that of the failed drive to take the place of the failed drive.


Hot spares are employed only in arrays with redundancy, for example, RAID levels 1, 5, and 10.

A hot spare connected to a specific RAID controller can only be used to rebuild a drive that is connected to the same controller.

Disk Rebuild

You rebuild a disk drive by recreating the data that had been stored on the drive before the drive failed.

Rebuilding can be done only in arrays with data redundancy such as RAID level 1, 5, and 10.

A hot spare can be used to rebuild disk drives in RAID 1, 5 or 10 systems. If a hot spare is not available, the failed disk drive must be replaced with a new disk drive so that the data on the failed drive can be rebuilt. A rebuild will occur automatically when a drive is replaced (by hot-swapping it in the same drive bay.)

RAID controllers automatically and transparently rebuilds failed drives with user-definable rebuild rates. If a hot spare is available, the rebuild starts automatically when a drive fails. Most RAID controllers automatically restarts the system and the rebuild if the system goes down during a rebuild.

Rebuild Rate

The rebuild rate is the fraction of the compute cycles dedicated to rebuilding failed drives. A rebuild rate of 100 percent means the system is totally dedicated to rebuilding the failed drive.

The rebuild rate can be configured between 0% and 100%. At 0%, the rebuild is done only if the system is not doing anything else. At 100%, the rebuild has a higher priority than any other system activity. The default rebuild rate is 30%.

Physical Array

A RAID array is a collection of physical disk drives governed by the RAID management software. A RAID array appears to the host computer as one or more logical drives.

Logical Drive

A logical drive is a partition in a physical array of disks that is made up of contiguous data segments on the physical disks. A logical drive consists of an entire physical array.

Hot Swap

A hot swap is the manual replacement of a defective physical disk unit while the computer is still running. When a new drive has been installed, a rebuild will occur automatically if it is placed in the same drive bay as the failed drive it is replacing.

SCSI Drive States

Table 2. SCSI Drive States

State Description


The drive is functioning normally and is a part of a configured logical drive.


The drive is functioning normally but is not part of a configured logical drive and is not designated as a hot spare.
Hot Spare


The drive is powered up and ready for use as a spare in case an online drive fails.
Fail (FAIL) A fault has occurred in the drive, putting it out of service.
Rebuild (REB) The drive is being rebuilt with data from a failed drive.

Logical Drive States

Table 3. Logical Drive States

State Description
Optimal The drive operating condition is good. All configured drives are online.
Degraded The drive operating condition is not optimal. One of the configured drives has failed or is offline.
Failed The drive has failed.
Offline The drive is not available..

Disk Array Types

Table 4. Disk Array Types

Type Description
Software-Based The array is managed by software running in a host computer using the host CPU bandwidth. The disadvantages are the load on the host CPU and different software for each operating system.
External The array controller resides outside of the host computer and communicates with the host through a host bus adapter in the host. The array management software runs in the controller. It is transparent to the host and independent of the host operating system.

A subset of the external disk array type is SCSI to SCSI. In the SCSI to SCSI disk array type, the array controller resides outside of the host computer and communicates with the host through a SCSI adapter in the host. The array management software runs in the controller. It is transparent to the host and independent of the host operating system. The disadvantage is the limited data transfer between the SCSI adapter and the array controller.

Host-Based The array controller resides on the bus (for example, a PCI bus) in the host computer and has its own CPU to generate the parity and handle other RAID functions. It can transfer data at the speed of the host bus, but is limited to the bus it is designed for. The RAID controller resides on a PCI bus that handles data transfers at up to 132 MB/s. With the RAID controller, the channel can handle data transfer rates up to 80 MB/s per SCSI channel.


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Copyright 2002 Total Peripheral Repair - AAA Computer - RestEZ-PC
Last modified: 08/11/03