What Is a Parity Drive

What Is a Parity Drive

A parity drive is a storage device used as part of a computer system that contains parity data for redundancy and backup purposes. This is commonly part of a Redundant Array of Independent Disks (RAID), in which one or more disk drives are connected together to act as a single system. When data is stored on these devices, parity information can be created for use later in case one of the disks fails. A parity drive is not necessarily part of all RAID setups, but it allows for simple and effective data recovery.

The basic function of a parity drive is to provide additional storage of "parity bits," which are pieces of data used to backup the main drives in a disk array. An array is a computer setup in which multiple disks, such as two or more hard drives, are connected together and used as a single storage system. Although a number of different methods are used for this, a RAID is among the most common forms. There are various types of RAIDS, and more complex "levels" often include the use of a parity drive to provide effective backup and redundancy of information.

A parity drive functions through the use of parity bits that are stored on it. The simplest example of how parity bits function is in a RAID or other system that uses three drives in total. Two of the drives would be used as the actual data storage disks, while the third would function as a parity drive. Whenever data is saved to the RAID, each piece of information is split in half, with one part going onto one drive and the other part onto the second.

Computer data consists of bits, which are binary pieces of data represented by either a one or a zero. Whenever information is stored on a system with a parity drive, one bit from each storage drive is added to the other. If the result is an even number, then a parity bit with a value of zero is saved to the parity device, while an odd result creates a value of one. This can then be used if one of the storage drives fails, to recreate the data that is missing in order to restore what was lost.

For example, a "1" on one device, and a "0" on the other, would generate a "1" to be stored on the parity drive, since this is an odd value when added together. If the storage drive with the "0" data on it becomes corrupted, it can be replaced with a new, blank disk. The system can then look at the existing data, find the remaining "1" in data storage, compare that to the "1" in the parity device, and recognize that a "0" needs to be recreated to restore the lost data. This is redundancy and allows an array to effectively recover data even if part of the original system is lost.

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What are Disk Arrays

Disk arrays are storage systems that link multiple physical hard drives into one large drive for advanced data control and security. Disk arrays have several advantages over traditional single-disk systems.

A hard disk, while being the vital center of any computer system, is also its weakest link. It is the only critical device of a computer system that is not electronic, but relies on intricate moving mechanical parts that often fail. When this happens, data is irretrievable and unless a backup system has been employed, the user is out of luck. This is where disk arrays make a difference.

Disk arrays incorporate controls and a structure that pre-empts disaster. The most common disk array technology is RAID (Redundant Array of Independent Disks). RAID utilizes disk arrays in a number of optional configurations that benefit the user.

One advantage of RAID disk arrays is redundancy of data writes so that if a file is damaged or stored in a bad cluster or disk, it can be instantly and transparently replaced from another disk in the array. RAID also allows hot-swapping of bad disks and increased flexibility in scalable storage. Performance is also enhanced through a process called "striping."

There are many varieties of RAID, and though designed primarily for servers, disk arrays have become increasingly popular among individuals because of their many benefits. RAID is particularly suited for gamers and multimedia applications.

RAID controllers, built into motherboards, must set parameters for interacting with disk arrays. The controller sets the performance parameter to match the slowest disk. If it were to use the fastest disk as the benchmark, data would be lost when written to disks that cannot support that speed. For this reason, all disks in the array should be the same brand, speed, size and model for optimal performance. A mix of capacities, speeds and types of disks will negatively impact performance. The best drives for disk arrays are SATA (Serial ATA) RAID drives. These drives are optimized for RAID use and, being SATA, are hot-swappable.

Using disk arrays can provide peace of mind while improving data security and performance. Motherboards with built-in RAID controllers support certain types of RAID. For example, an older or inexpensive motherboard might only support RAID 0 and RAID 1, while a newer or more expensive board might support RAID 1 through RAID 5. Be sure to get a motherboard or third party RAID controller that supports the RAID configuration you require for your disk array.

What is RAID

RAID (redundant array of independent disks; originally redundant array of inexpensive disks) is a way of storing the same data in different places (thus, redundantly) on multiple hard disks. By placing data on multiple disks, I/O (input/output) operations can overlap in a balanced way, improving performance. Since multiple disks increases the mean time between failures (MTBF), storing data redundantly also increases fault tolerance.

Steps for configuring RAID volumes on various RAID cards

Creating & Managing the RAID volumes on Adaptec 2120S RAID card

1. Ensure that all the HDD’s are connected properly in their respective slots (In case of HS).

2. In the POST ensure that the Adaptec 2120S RAID card is getting sensed.

3. Enter the ARC utility by pressing Ctrl+A when the Adaptec message appears in POST.

4. Once its is inside the Adaptec BIOS you will see a blue-background screen appears with the

title Adaptec 2xx0S with different menu’s Use the “Disk Utilities” menu item to examine the drives showing as available to the system. If any seem to be missing, power the system down and inspect the hardware connections.

5.RAID Controller “ SCSI Select “ Utility. Press < ENTER > once the desired Array Configuration

Utility is highlighted by default.

Note: If more than one controller of the Adaptec SCSI RAID 2120S/2200S family is installed, the

first screen will show the controllers present. Highlight the controller you wish to use and press <

ENTER > to get to the screen in this step.

6. The “ Array Configuration “ Utility screen appears. Using the arrow keys, highlight “ Initialize

Drives “, then press < ENTER >.

Note: All drives being used in an array or volume must be initialized.

7. Using the arrow keys again, highlight the drives to be used for the RAID , press ‘Insert key’ when a desired drive is highlighted. Selected drives will be displayed in the box on the right side of the display. When the desired drives are on the right side, press < ENTER > to continue.

8. A red warning box appears. now Press Y, & then < ENTER > to continue(This is asking for a

confirmation to destroy all the data on the drives). The system will initialize the selected drives,

in a few seconds.

HowTo: Speed Up Linux Software Raid Building And Re-syncing

It is no secret that I am a pretty big fan of excellent Linux Software RAID. Creating, assembling and rebuilding small array is fine. But, things started to get nasty when you try to rebuild or resync large size array. You may get frustrated when you see it is going to take 22 hours to rebuild the array. You can always increase RAID resync performance using the following technique.

Recently, I build a small NAS server running Linux for one my client with 5 x 2TB disks in RAID 6 configuration for all in one backup server for Mac OS X and Windows XP/Vista clients computers. Then whey I cat /proc/mdstat it reported that md0 is created and resync is in progress. The resync speed was around 4000K/sec and resync will complete in approximately in 22 hours. I wanted to finish this early.

/proc/sys/dev/raid/{speed_limit_max,speed_limit_min}

The /proc/sys/dev/raid/speed_limit_min is config file that reflects the current "goal" rebuild speed for times when non-rebuild activity is current on an array. The speed is in Kibibytes per second, and is a per-device rate, not a per-array rate . The default is 1000.