IBM 88554RU Installation Guide - Page 29
When memory mirroring is enabled the data that is written to memory is
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Currently, other industry-standard servers use 8 bits of the 72-bit data packets for ECC functions and the remaining 64 bits for data. However, the x455 (and several other xSeries servers) use an advanced ECC algorithm that is based not on bits but on memory symbols. Symbols are groups of multiple bits, and in the case of the x455, each symbol is 4 bits wide. With two-way interleaved memory, the algorithm needs only three symbols to perform the same ECC functions, thus leaving one symbol free (2 bits on each DIMM). In the event that a chip failure on the DIMM is detected by memory scrubbing, the memory controller can re-route data around that failed chip through the spare symbol (similar to the hot-spare drive of a RAID array). It can do this automatically without issuing a Predictive Failure Analysis® (PFA) or light path diagnostics alert to the administrator. After the second DIMM failure, PFA and light path diagnostics alerts would occur on that DIMM as normal. Memory scrubbing Memory scrubbing is an automatic daily test of all the system memory that corrects soft errors and reports recoverable errors. An excessive rate of recoverable errors reported triggers Memory ProteXion to replace the failing locations. Memory mirroring Memory mirroring is equivalent to RAID-1 in disk arrays, in that memory is divided in two ports and one port is mirrored to the other (see Figure 1-8 on page 14). If 8 GB is installed, then the operating system sees 4 GB once memory mirroring is enabled (it is disabled by default). All mirroring activities are handled by the hardware without any additional support required from the operating system. When memory mirroring is enabled the data that is written to memory is stored in two locations. One copy is kept in the port 1 DIMMs, while a second copy is kept in the port 2 DIMMs. During the execution of the read command, the data is read simultaneously from both ports, and error-free data from either port is forwarded. This provides an extra level of error recovery capability. When an unrecoverable memory error from one of the memory ports is encountered, good data from the non-failing memory port is forwarded to the system. The failing DIMM is reported and indicated with light path. The failing memory port is then disabled. Certain restrictions exist with respect to placement and size of memory DIMMs when memory mirroring is enabled. These are discussed in "Memory mirroring" on page 58. Chapter 1. Technical description 15