RAID 1

From ProPHOTO WIKI

Introduction

One of a number of different RAID levels that combines two or more hard drives to protect the user against drive failures. In these arrays, an identical copy of all data is mirrored onto all of the drives in the array. As such, all but one of the drives in the array can fail without any data loss.

While RAID 1 is the most secure RAID level, it is also relatively inefficient. A RAID 1 array composed of two 160GB hard drives will result in only 160GB of usable space. This means that to use this mode users must purchase twice as much capacity as is needed.

Method of Operation

RAID 1 arrays mirror all of the data stored on the array onto each component drive. As long as one drive survives a failure all of the data on the array will remain intact.

One of the simplest RAID levels, when opperating in RAID 1 mode a copy of all data fed to the controller will be stored on each component drive. As such, each drive in the array is an identical copy of all of the others. This provides a high level of redundancy and makes it easy to recover from any failures.

As everything written to the array must be stored on all drives, a RAID 1 array is no faster than a conventional hard drive. In some cases controller overhead and unit variations can mean that these arrays may even reduce write performance.

Read performance is dependant on the implementation of the controller in question. Most RAID 1 controllers simply read data on all drives so read performance will be no different than with conventional discs. Higher-end controllers can instruct each drive to retreive different blocks of the file, in which case read performance can be significantly improved.

Reliability

RAID 1 is one of the most reliable RAID levels as the entire content of the array is stored redundantly. As long as one drive is still working, all data will remain intact and the array will continue to work unimpeded. Naturally, the more drives that are used in a RAID 1 array the safer the data stored on them is - a four drive RAID 1 array, for instance, can survive three drive failures without any adverse effects.

Overhead

The primary weakness of RAID 1 arrays is that they are relatively inefficient. A N-drive RAID 1 array only provides 1/N of the capacity of the component disks. That is, four 160GB hard drives in a RAID 1 configuration will only provide 160GB of usable space dispite the 640GB of space on the drives. This makes these arrays relatively expensive to set up, so they are generally used for situations where continuous access to the data is critical.

Requirements

A RAID 1 array requires a minimum of two hard drives of equal size as well as a hard drive controller than supports it. As RAID 1 arrays are generally only as fast as their slowest disc, it is generally a good idea for the two drives to be of the same make and model.

See Also

• RAID - General overview of RAID and all of its different levels.
• RAID 0 - Stripes the data at the block level to maximize performance, however that increases the risk of lost data versus individual disks.
• RAID 2 - Similar to RAID 0 but stripes at the bit level rather than the block level. This level is very uncommon and not supported by any modern RAID controllers.
• RAID 3 - Uses byte-level striping with a dedicated parity disc. This can provide similar performance to RAID 0 but can survive the failure of one drive.
• RAID 4 - Same as RAID 3, but uses block-level striping.
• RAID 5 - Uses block-level striping with a distributed parity disc. This can provide similar performance to RAID 0 but can survive the failure of one drive.
• RAID 6 - Uses block-level striping with two distributed parity discs. This can provide similar performance to RAID 0 but can survive the failure of up to two drives.
• RAID 10 - A combonation of RAID 0 and RAID 1. Data is striped across two RAID 1 arrays. This provides the performance advantages of RAID 0 and can survive at least one drive failure.
• RAID 0+1 - A combonation of RAID 0 and RAID 1. In this mode, two RAID 0 arrays are mirrored. This provides the performance advantages of RAID 0 and can survive at least one drive failure.
• Matrix RAID - A propreitary technology used by some Intel chipsets that allows drives to be partitioned and different RAID levels used on each block.