RAID 1

RAID 1 uses one disk to store data and a second disk to store a copy for for redundancy, the R in RAID. When a single disk breaks, you replace the broken disk and rebuild the broken disk from the data on the other disk. RAID 1 is also called mirroring because the second disk is a mirror image of the first disk.

You need a minimum of two disks for RAID 1. If you have more disks, look at RAID 5.

You can also use multiple spare disks. If you have three disks and want extreme reliability, you could use RAID 1 with two spare disks. When you have more than three disks, look at RAID 5 with multiple spare disks.

During writes to RAID 1, the data is written to both disks in parallel and the is little overhead from the duplicate write. Reads are spread across the different disks and produce faster results. Two disks could read twice as fast as one disk. Three disks could read three times as fast as one disk.

In practice the maximum speed occurs only when reading small chunks of data and the software directs each read to the disk with the read head closest to the data. A continuous read of two long files in parallel could be slower if the RAID 1 software continually sends the reads to the wrong disks and the disk heads have to fly all over the place to access the data. Moderns disks use command queuing to reduce the disk head movement and some will read ahead to improve access speed. Both features improve the average access speed and occasionally make specific speed tests show a slower speed.

In a typical workstation you have a small number of disks and RAID 1 is a good fit for workstations with two disks. A workstation with four disks could use two arrays of RAID 1 for speed and simple recovery. Read about an example in Linux workstation disk RAID 1. A server with more disks could use two disks in a RAID 1 array for the system files then a separate RAID 5 array for the data files.

Recovery of RAID 1 arrays is faster than recovery of RAID 5 because RAID 1 is a simple copy from the working disk to the other disk.

Good RAID 1 makes both disks identical and independent. You can remove one disk from the RAID array and use that disk in another computer, a vital part of recovering from a computer failure when you use RAID 1. Include a test where you pretend your current computer is dead, remove one of the disks, then access it in another computer. All your data should be accessible.

RAID 1 can be used as a way to clone computer disks. You con configure a master computer using multiple spare disks in a RAID 1 array then remove spare disks for use as the first disk in a new computer.

There is some bad RAID 1 out there that makes stupid weird changes to the disk configuration and you cannot use the disks outside of the RAID array. When you are testing RAID 1 software and hardware, make sure you can remove a working disk and use that disk in another computer without requiring the RAID software or hardware. Also try starting the original computer with RAID 1 switched off. You should be able to switch RAID 1 off at any time and operate from either disk.