HP StorageWorks 1000 Modular Smart Array - Manual (Page 46)

46 Storage overview
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 ).
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.
L4
L5
L3
A1
L1
L2
D1 D2 D3 D4 D5
A2
15313