Definition

disk mirroring (RAID 1)

Contributor(s): Rich Castagna and 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. Disk mirroring also works with solid state drives so “drive monitoring” may be a better term for contemporary storage systems.

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

Disk (drive) mirroring is particularly advantageous for disaster recovery scenarios as it provides instantaneous failover for data required by mission-critical applications. If primary drives in the array are damaged or unable to operate, traffic is switched to secondary or mirrored backup drives. The mirror copy is able to become operational on failover because the operating system and application software are replicated to the mirror along with the data used by the applications.

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 can simultaneously access more spindles (drives) for reads and writes, performance is improved when data is accessed from a “RAID-ed” drive.

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, RAID 5 and RAID 6. The RAID level is generally determined by the requirements of  the applications 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-1

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 or RAID controller card vendors support all the various RAID levels. Before purchasing a storage system or RAID controller you should determine your organization’s specific needs relative to data protection and recovery, and application performance.

How RAID 1 compares to other RAID levels

RAID 1 requires a minimum of two physical drives, as data is written simultaneously to two places. The drives are essentially mirror images of each other, so if one drive fails, the other one can take over and provide access to the data that’s stored on that drive. Mirroring drives is good for very fast read operations, but it's slower when writing to the drives since the data needs to be written to two locations.

While RAID 1 drive mirroring provides comprehensive data protection that can be a lifesaver in the event of disaster that disables the primary drive, that level of data protection comes at a cost of requiring 100% drive capacity overhead. That means if you use drive mirroring RAID 1 on an array you have to double the amount of installed drive capacity—half to accommodate active storage and half for the mirrored copy. Because that amount of overhead capacity can be very expensive, very few enterprise-class storage systems use RAID 1, opting for more space-conserving RAID 5 or RAID 6 levels.

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 (e.g., to store 100 TB of data requires 200 TB of capacity). With RAID 5, if a user buys five disks, four-fifths of the capacity is usable, so much less additional capacity is needed. However, RAID 5 and RAID 6 cannot recover as quickly as a RAID 1 implementation. While RAID 1 may be able to offer a near instantaneous recovery time objective (RTO), RAID 5 and RAID 6 need time—considerable time for large capacities—to rebuild the data across their arrays.

This was last updated in May 2019

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In your opinion, how well does RAID 1 work versus other RAID levels?
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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.
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Hi Rich. So to understand what I’ve found out so far, I am loading a Synology 218+ with two 4 TB hard drives. In Raid 1 I will only benefit from 2 TB of disk space? I am for max space and recovery security out of my NAS.
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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.
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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.
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Hello,
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,
RJ2307
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Thank you for the comment and question! I would recommend contacting the HDD vendor in this situation to make sure you can do what you're hoping to do. 
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