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.
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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.
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
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.
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