HP Surestore Disk Array FC60 HP SureStore E Disk Array FC60 Service Manual (A5 - Page 63

Product Description Disk Array Caching Disk caching is the technique of storing data temporarily in RAM while performing I/Os to the disk array. Using RAM as a temporary storage medium can significantly improve the response time for many types of I/O operations. From the host's perspective the data transfer is complete, even if the disk media was not involved in the transaction. Both write caching and read caching are always enabled. Caching enhances disk array I/O performance in two ways: Read I/O If a read I/O requests data that is already in read cache, the disk array services the request from cache RAM, thus avoiding the much slower process of accessing a disk for the data. A pre-fetch capability enables the disk array to anticipate needed data (for example, on a file transfer) and read it from disk into the read cache, which helps significantly with sequential read I/Os. Write I/O During a write I/O, the disk array writes the requested data into write cache. Rather than writing the modified data back to the disk immediately, the disk array keeps it in cache and informs the host that the write is complete. If another I/O affects the same data, the disk array can update it directly in cache, avoiding another disk write. Data is flushed to disk at regular intervals (10 seconds) ro when the cache flush threshold is reached. Write cache is mirrored between the two disk array controllers. Each controller maintains an exact image of the write cache on the other controller. If a controller fails, its write cache content is flushed to the disk by the remaining controller. Because write cache is mirrored, the operational controller automatically disables write caching until the failed controller is replaced. After it is replaced, the operational controller automatically reenables write caching. Mirroring effectively reduces the size of available cache by half. A controller with 256 Mbytes of cache will use half of the memory to mirror the other controller, leaving only 128 Mbytes for its own cache. The write cache contents cannot be flushed when both controllers are removed from the disk array simultaneously. In this case the write cache image is lost and data integrity on the disk array is compromised. To avoid this problem, never remove both controllers from the disk array simultaneously. Capacity Management Features 63