HP Surestore Disk Array FC60 Storage Manager 60-NT Introduction Guide - Page 46

Tuning Storage Arrays Option Description Use this option to set three tuning parameters: - Write caching - Cache read-ahead - Volume Modification Priority Write Caching - To maximize I/O request rates, enable write- caching on a volume. Write caching frees an application to continue after writing to the controller cache memory, a faster operation than a disk write. The cache memory is flushed to disk when it contains a percentage of unwritten data you specify using the option. Cache Read-ahead - If an individual volume is experiencing a low cache hit percentage, consider enabling cache read-ahead (or prefetch) for that volume. Cache read-ahead can increase the cache hit percentage for a sequential I/O workload. If cache read-ahead is enabled, the cache reads the data from the disk. But in addition to the requested data, the cache also fetches more data, usually from adjacent data blocks on the drive. This feature increases the chance that a future request for data could be fulfilled from the cache rather than requiring a disk access. The cache read-ahead multiplier values specify the multiplier to use for determining how many additional data blocks are read into cache. Choosing a higher cache read-ahead multiplier can increase the cache hit percentage for volumes with sequential I/ O access patterns. (In the case of a random I/O pattern, the optimal multiplier is zero.) Volume Modification Priority - The volume Modification Priority defines how much processing time is allocated for volume modification operations versus system performance. The higher the priority, the faster volume modification operations complete but the slower system I/O is serviced. Volume modification operations include reconstruction, copyback, initialization, media scan, defragmentation, change of RAID level, and change of segment size. The modification priority is set for each volume using a slider bar on the dialog. There are five relative settings on the reconstruction rate slider bar ranging from Low to Highest. The actual speed of each setting is determined by the controller. Choose the Low setting to maximize the I/O request rate. If the controller is idle (not servicing any I/O) it will ignore the individual volume rate settings and process volume modification operations as fast as possible. 46 TUNING STORAGE ARRAYS