HP ProLiant DL280 Memory technology evolution: an overview of system memory te - Page 11

correctable memory errors, the channel goes off line and the system copies data to the spare channel. This prevents data corruption, a server crash, or both, and you can replace the defective DIMM at your convenience during a scheduled shutdown. This also maintains server availability and memory reliability without service intervention or server interruption. Online Spare Memory technology reduces the chance of an uncorrectable error bringing down the system; however, it does not fully protect your system against uncorrectable memory errors. NOTE Online Spare Memory mode can run on some systems with only one memory channel populated. But a single-channel memory configuration requires dual-rank DIMMs. For more information, refer to the server's user guide. In a system with three channels per memory controller, two channels operate normally and the third channel is the spare. Online Spare Memory mode does not require operating system support or special software beyond the system BIOS. However, to support messaging and logging at the console along with messages in HP Systems Insight Manager, the operating system must have system management and agent support for Advanced Memory Protection. Implementing Online Spare Memory mode over Advanced ECC requires extra DIMMs for the spare memory channel and reduces the system's memory capacity. Mirrored Memory mode Mirrored Memory mode is a fault-tolerant memory option that gives you a higher level of security than Online Spare mode. Mirrored Memory mode provides full protection against single-bit and multi-bit errors. With Mirrored Memory mode, the memory subsystem writes identical data to two channels simultaneously. If a memory read from one of the channel returns incorrect data due to an uncorrectable memory error, the system automatically retrieves the data from the other channel. A transient or soft error in one channel does not affect mirroring, and operation continues unless there is a simultaneous error in exactly the same location on a DIMM and its mirrored DIMM. Mirrored Memory mode reduces the amount of memory available to the operating system by 50% since only one of the two populated channels provides data. Lockstep Memory mode Lockstep Memory mode uses two memory channels at a time and offers you an even higher level of protection. In lockstep mode, two channels operate as a single channel-each write and read operation moves a data word two channels wide. Both channels split the cache line to provide 2x 8-bit error detection and 8-bit error correction within a single DRAM. In three-channel memory systems, the third channel is unused and left unpopulated. The Lockstep Memory mode is the most reliable, but it reduces the total system memory capacity by one-third in most systems. Memory protection mode summary The Online Spare Memory, Mirrored Memory, and Lockstep Memory modes enhance reliability and availability. A faulty DIMM does not have to result in unscheduled downtime. The server can remain up and running until it is convenient to take the equipment off line. Unfortunately, memory protection modes sacrifice performance. The reduction in available channels limits bandwidth and reduces a system's capacity. Nevertheless, server availability is the highest priority for enterprise environments. You can configure the system for the desired memory protection mode through the RBSU. Online Spare, Mirrored, and Lockstep memory protection modes are mutually exclusive, meaning that only one mode operates at a time. You must use DIMMs that are identical in type, size, and rank. 11