HP StorageWorks 1510i Modular Smart Array - Manual (Page 86)

S1
S2
S3
S4
B1
B4
B7
B2
B5
B8
B11 B10 B12
D2 D1 D3
B6
B3
B9
15312
Figure 17 Data striping (S1-S4) and data blocks (B1-B12) on multiple physical drives
(D1, D2, D3)
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 usable space. 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 just array). Because all
physical drives in an array are commonly configured into just one logical drive, the term array is also
often used as a synonym for logical drive. However, an array can contain several logical drives, each of
a different size ( Figure 18 ).
L4
L5
L3
A1
L1
L2
D1 D2 D3 D4 D5
A2
15313
Figure 18 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 18 , 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 levels
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—no fault tolerance
• RAID 1+0—drive mirroring
• RAID 5—distributed data guarding
• RAID 6—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
86 Storage overview