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E-14 Understanding Drive Arrays
Writer: Rickard Project: Compaq Smart Array 3200 Controller Reference Guide Comments: 340862-002
File Name: L-APPE.DOC Last Saved On: 12/7/98 9:30 AM
COMPAQ CONFIDENTIAL - NEED TO KNOW REQUIRED
The number of sectors on the physical drives can not be increased. This can
occur as a result of a request to increase stripe-size; the newly selected stripe-
size would not be allowed. For example, if the end user desires to change the
stripe-size from 32 to 256 sectors, there may not be enough sectors at the end of
the disks for a complete stripe of the larger size. Since a partial stripe cannot be
supported, the request would not be allowed. In a new configuration, the area at
the end of the disks is not used if there are not enough sectors left for that
particular stripe-size. To select the new stripe-size, the current configuration
would have to be deleted and the array configured with the new stripe-size.
Since the data would be destroyed, you would need to back up the data and
restore it after the array was reconfigured.
Array Accelerator
By serving as a posted write cache and read-ahead cache, the Smart Array 3200
Controller Array Accelerator dramatically improves the performance of Read
and Write commands. The Array Accelerator is particularly useful for
increasing performance in database and fault-tolerant configurations.
Performance
The Array Accelerator increases performance by having the Smart Array 3200
Controller write data to the cache memory on the Array Accelerator rather than
directly to the drives. The system can access this cache memory more than 100
times faster than accessing disk storage. The Smart Array 3200 Controller
writes the data in the Array Accelerator to the drive array at a later time, when
the controller is otherwise idle.
The controller also uses the Array Accelerator to increase performance by
anticipating requests. The Array Accelerator uses a multi-threaded algorithm to
predict the next likely Read operation for the array. That data is read into the
Array Accelerator and, therefore, is ready before you access it. When the Smart
Array 3200 Controller receives a Read request for the cached data, it can be
burst immediately into system memory at PCI bus speeds.