Managing RAID on Linux

RAID-1 (Mirroring)

RAID-1 provides the most complete form of redundancy because it can survive multiple disk failures without the need for special data recovery algorithms. Data is mirrored block-by-block onto each member disk (see Figure 2-7). So for every N disks in a RAID-1, the array can withstand a failure of N-1 disks without data loss. In a four-disk RAID-1, up to three disks could be lost without loss of data.

Figure 2-7. Fully redundant RAID-1.

As the number of member disks in a mirror increases, the write performance of the array decreases. Each write incurs a performance hit because each block must be written to each participating disk. However, a substantial advantage in read performance is achieved through parallel access. Duplicate copies of data on different hard drives allow the system to make concurrent read requests.

For example, let’s examine the read and write operations of a two-disk RAID-1. Let’s say that I’m going to perform a database query to display a list of all the customers that have ordered from my company this year. Fifty such customers exist, and each of their customer data records is 1 KB. My RAID-1 array receives a request to retrieve these fifty customer records and output them to my company’s sales engineer. The drives in my array store data in 1 KB chunks and support a data throughput of 1 KB at a time. However, my controller card and system bus support ...