RAID 1 - Mirroring

Writes two copies of the data simultaneously on two separate drives. This is called fault tolerant because if one of the mirrored drives suffers a mechanical failure (e.g. spindle failure) or does not respond, the remaining drive will continue to function. The RAID 1 configuration is performed either by a hardware RAID controller… or performed in software. It is suited to applications requiring high fault tolerance at a low cost and where a duplicated set of data is more secure than using parity. RAID 1 is popular for accounting and other financial data. It is also commonly used for small database systems, enterprise servers, and home PCs where a fairly inexpensive fault tolerance is required.

A RAID device maintains a mirror of all the data in a partition... on another partition. This second partition is usually on another drive (note: in Linux systems the second partition can be on the same drive). There is a small performance hit to be expected when configuring your hard disks in a RAID 1 partition as the data has to be written to every disk in the RAID array.

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More information on RAID

How to install RAID 1 on a Linux system

RAID 1 can be used together with RAID 0 (RAID 0 + 1) providing the advantages of both striping and mirroring. An interesting concept is that RAID 0+1 is not the same as RAID 1+0 and this technical article explains the difference and makes a case for why RAID 1+0 is better than RAID 0+1.