HP StorageWorks Modular Smart Array 1000 HP StorageWorks 1000/1500 Modular Sma - Page 46

For data in the logical drive to be readable, the data block sequence must be the same in every stripe. This sequencing process is performed by the array controller, which sends the data blocks to the drive write heads in the correct order. A natural consequence of the striping process is that each physical drive in a given logical drive will contain the same amount of data. If one physical drive has a larger capacity than other physical drives in the same logical drive, the extra capacity is wasted, because it cannot be used by the logical drive. The group of physical drives containing the logical drive is called a drive array, or array. Because all physical drives in an array are commonly configured into just one logical drive, the term array is often used as a synonym for logical drive. However, an array can contain several logical drives, each of a different size (Figure 4). A1 A2 L1 L2 D1 D2 D3 L3 L4 L5 D4 D5 15313 Figure 4 Two arrays (A1, A2) containing five logical drives (L1 through L5) spread across five physical drives (D1 through D5) Each logical drive in an array is distributed across all of the physical drives within the array. A logical drive can also extend across more than one storage enclosure attached to the array system. Drive failure, although rare, is potentially catastrophic. For example, in Figure 4, failure of any one physical drive in an array causes every logical drive in the array to suffer irretrievable data loss. To protect against data loss due to physical drive failure, logical drives are usually configured with fault tolerance. Fault-tolerance methods To protect against data loss due to physical drive failure, logical drives are usually configured with fault tolerance. The following configuration types are available: • RAID 0-Data striping only (no fault tolerance) • RAID 1+0-Drive mirroring • RAID 5-Distributed data guarding • RAID 6 (ADG)-Advanced data guarding For any configuration except RAID 0, further protection against data loss can be achieved by assigning a drive as an online spare. This drive contains no data and is connected to the same controller as the array. When any other physical drive in the array fails, the controller automatically rebuilds information data protection. (In the unlikely event that another drive in the array fails while data is being rewritten to the spare, the logical drive will still fail.) A spare is assigned to an array and is automatically assigned to all logical drives in the same array. You do not need to assign a separate spare to each array; you can configure one hard drive to be the spare for several arrays. 46 Storage overview