HP Integrity BLc3000 ISS Technology Update, Volume 9, Number 3 - Page 2

Enabling physical drive write cache on HP Smart Array controllers HP Smart Array RAID controllers use controller-level write cache to improve the local storage subsystem write performance of ProLiant servers. Controller write cache acts as an output buffer, allowing server applications to post write commands to the controller without having to wait for the write operation to complete to disk. The Smart Array controller then executes the lowlevel write operations. Smart Array controller-level write cache is protected by a back-up system that preserves the pending write commands in the cache even if the server's power is completely lost. These pending writes to disk are then executed once power is restored. Smart Array controllers use either battery-backed or flash-backed cache modules to implement this critical protection which ensures data integrity in mission-critical environments. Drive write cache, on the other hand, is a separate cache that exists on each physical drive in an array. It can be enabled for all drives attached to a controller using HP array configuration tools, including the Array Configuration Utility (ACU). Enabling drive write cache can sometimes further improve array performance; however, you must first understand its potential impact on data integrity. Understanding physical drive write cache In simplest terms, drive write cache does at the drive level what Smart Array write cache does at the controller level. Drive write cache lets the Smart Array controller post a low level disk write to a physical drive's low-level controller and continue to its next operation without having to wait for the write to be completed to the disk drive. In this sense, drive write cache can be part of a two-tiered cache of the overall execution stack of write commands. This starts at the OS file system level and extends down to the actual drive level commands (Figure 1-1). Figure 1-1. Architecture of the Smart Array write caching stack with physical drive write cache enabled. Smart Array Write Cache Stack Operating System / Application Logical Drive Writes Drive write cache Smart Array Controller 4 Controller Write Cache 4 - Deconstruction to physical drive writes - Placement in write cache - Analysis of write commands - Coalescing/reordering for efficiency Drive write cache Physical Drive Writes Drive write cache Drive write cache 2