Compaq 3200 manual Array Accelerator, Performance, 14Understanding Drive Arrays

E-14Understanding Drive Arrays

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.