Flash-backed write cache

HPE Smart Array controllers use a write-back caching scheme that lets host applications continue without waiting for write operations to complete to the disk. A controller without a write-back cache returns completion status to the OS after it writes the data to the drives. A controller with write-back caching can “post” write data to high-speed cache memory, and then immediately return completion status to the OS. The write operation completes in microseconds rather than milliseconds. The controller writes data from the controller’s write cache to disk later, at an optimal time for the controller.

Once the controller locates write data in the cache, subsequent reads to the same disk location come from the cache. Subsequent writes to the same disk location will replace the data held in cache. This is a “read cache hit.” It improves bandwidth and latency for applications that frequently write and read the same area of the disk.

The write cache will typically fill up and remain full usually in high-workload environments. The controller uses this opportunity to analyze the pending write commands to improve their efficiency. The controller can

• Use write coalescing that combines small writes to adjacent logical blocks into a single larger write for quicker execution

• Perform command reordering, rearranging the execution order of the writes in the cache to reduce the overall disk latency

• Store and analyze a larger number of pending write commands, increasing the opportunities for write coalescing and command reordering while delivering better overall performance

When the HPE Smart Array controller has a large cache memory size, it can coalesce and reorder commands efficiently, which improves overall array performance.

You can use HPE Smart Storage Administrator to configure the percentage of the cache to use for write caching. The default configuration on Smart Array controllers assigns 90% of the available cache space for write cache.

Flash-backed write cache (FBWC) uses flash devices to retain cache data and the energy pack to provide power during a power loss. The FBWC offers significant advantages over earlier BBWC systems. While a battery-backed write cache (BBWC) requires backup power during the entire power loss, an FBWC only needs power during the time it takes to backup from DRAM to flash. Since the FBWC writes the contents of memory to flash devices, there is no longer a 48-hour energy pack life limitation, and the data posts to the disk drive on the next power-up.