disk mirroring (RAID 1)

Contributor(s): Paul Crocetti

Disk mirroring, also known as RAID 1, is the replication of data to two or more disks. Disk mirroring is a good choice for applications that require high performance and high availability, such as transactional applications, email and operating systems.

Because both disks are operational, data can be read from them simultaneously, which makes read operations quite fast. The RAID array will operate if one disk is operational. Write operations, however, are slower because every write operation is done twice.

Disk mirroring provides instantaneous failover for data required by mission-critical applications. If primary arrays are damaged, traffic is switched to secondary or mirrored backup arrays.

RAID and RAID levels

RAID, or redundant array of independent disks, is a method of grouping individual physical drives together to form one bigger drive called a RAID set. Because the server has more spindles to read from or write to when data is accessed from a drive, performance is improved.

The various ways in which data is grouped across drives is called the RAID level. Each RAID level is denoted by a number following the word RAID. The most common levels are RAID 0, RAID 1 and RAID 5. The RAID level depends on the application running on the server. RAID 0 is the fastest, RAID 1 is the most reliable and RAID 5 is considered a good combination of both

 RAID levels explained

Combinations of RAID levels may be used together for additional data protection. For example, a user can create two RAID 0 sets, and then combine the RAID 0 sets into a RAID 1 set. This essentially provides the performance benefits of RAID 0 with the availability benefits of RAID 1.

It is important to note that not all storage array vendors support all the various RAID levels.

How RAID 1 compares to other RAID levels

RAID 1 requires a minimum of two physical disks, as data is written simultaneously to two places. The disks are essentially mirror images of each other, so if one disk fails, the other one can retrieve the data. Disk mirroring is good for very fast read operations, but it's slower when writing to the disks since the data needs to be written in two locations.

RAID 1 and RAID 10 are both mirroring technologies that use half of their available drives for data, but the main difference between them is the number of available drives. While RAID 1 uses two drives, RAID 10 involves at least four drives and combines the mirroring of RAID 1 with the striping of RAID 0.


RAID 1 vs. RAID 5 comes down to what's more important in terms of performance and cost. A RAID 1 drive pair can handle twice the amount of reads of a standalone disk drive. The main drawback of RAID 1 is that for the required disk space, the amount of capacity will double. For example, to store 300 GB of data requires 600 GB of capacity. With RAID 5, if a user buys five disks, four-fifths of the capacity is usable, but RAID 5 volumes take a while to rebuild.

This was last updated in May 2016

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In your opinion, how well does RAID 1 work versus other RAID levels?
The cool thing about RAID 1 is that you can recover from a production disk failure almost immediately, without having to wait for a sometimes lengthy rebuild process to complete. The not-so-cool things about RAID 1 are its capacity requirements because it simply mirrors production data without using any hashing or other reduction techniques and its potential to drag down performance as each write needs to occur twice. That said, RAID 1 might be a good fit for certain applications that might not use too much disk capacity--and that have to be recovered quickly to minimize impact on business processes.
We were thinking to move our RAID 5 array to RAID 1 for better performance in written but with this article I see that I was wrong. I see that for better written performance we need to use RAID 0 combined with RAID 1. Thanks for this information.
You get a performance benefit with striping as writing across multiple disks speeds up the operation. But RAID 0 just does striping--so no data protection. Your plan to use RAID 0+1 should give you both--a performance boost and data protection--and that data protection comes without having to do any disk rebuilds if the original gets trashed.
I am a hobbiest photographer and would hate to loose my collection (~650GB) for last 12 years due to the failure of external hard disk. 
I have tried multiple backup options and recently a photographer suggested me to use RAID 1 config.
I was wondering if I use two 2TB HDDs as RAID 1 and designate one disk as main disk?

What i wish to do is use one disk to copy my photos from laptop on a regular basis and once a week or so when I plug in the other drive of RAID 1 config, the data is copied on that drive as well and then keep it separately until I am ready to copy again.

Is this possible or both HDDs for RAID 1 config have to be plugged in while copying the data to them?

Thank you all,


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