Data are stored twice by writing them to both the data disk (or set of data disks) and a mirror disk (or set of disks) . If a disk fails, the controller uses either the data drive or the mirror drive for data recovery and continues operation. You need at least 2 disks for a RAID 1 array.
RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.
Advantages
RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single disk.
In case a disk fails, data do not have to be rebuild, they just have to be copied to the replacement disk.
RAID 1 is a very simple technology.
Disadvantages
The main disadvantage is that the effective storage capacity is only half of the total disk capacity because all data get written twice.
Software RAID 1 solutions do not always allow a hot swap of a failed disk (meaning it cannot be replaced while the server keeps running). Ideally a hardware controller is used.
Ideal use
RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two disks will be used.
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