Linksys NSS4100 Cisco NSS4000 and NSS6000 Series Network Storage System Admini - Page 51

Configuring your Storage Choosing a RAID Array Level 4 increases the reliability (for example, a two-disk RAID1 is half as likely to fail, a three-disk RAID1 is one-third as likely to fail, and so on). Increased read performance. • Disadvantages: Decreased aggregate storage capacity (each mirror disk does not contribute to the total capacity of the array). Decreased write performance. I/O intensive when resyncing mirrors. RAID5: RAID5 stripes data written to the array like RAID0, except that, unlike RAID0, RAID5 generates parity information that also gets striped across the array. This parity information is used during the reconstruction of the lost data when a drive fails in the array. When a failed disk is then replaced, the array must regenerate and rewrite the parity information of the array. This process is referred to as "rebuilding". During a rebuild, the array continues to be available for reads and writes. If the parity information in the array gets out of sync with the data in the array, usually as the result of an unexpected loss of power to the system, the array must be rebuilt. Like RAID1, if a disk is missing from the array, the array is said to be operating in "degraded" mode. • Minimum Number of Disks: 3 • Total capacity: m x (n-1) • Advantages: Increased reliability. The RAID array can sustain the loss of any single disk without any data loss. Increased read and write performance. • Disadvantages: Decreased aggregate storage capacity (you lose the equivalent capacity of a single disk). CPU intensive calculation of parity, especially during rebuild operation. I/O intensive during rebuild operation. RAID10: RAID10 is a combination of RAID1 and RAID0. The NSS implements RAID10 as two, 2-disk RAID1 mirrors, which are then used to build a RAID0 array. RAID10 can be thought of as "mirror then stripe". • Minimum Number of Disks: 4 • Total capacity: 2 x m • Advantages: Increased read and write performance; increased reliability. The RAID array can sustain the loss of any single disk without data loss. Depending on which disks fail, the array can lose up to two disks without data loss. • Disadvantages: Decreased aggregate storage capacity (you lose half of the storage space). Linksys Small Business NSS4000 and NSS6000 Series Network Storage System Administration Guide 43