28 IBM eX5 Implementation Guide2.3.6 Reliability, availability, and serviceability (RAS) featuresIn addition to Hemisphere Mode, DIMM balance and memory size, memory performance isalso affected by the various memory reliability, availability, and serviceability (RAS) featuresthat can be enabled from the Unified Extensible Firmware Interface (UEFI) shell. Thesesettings can increase the reliability of the system; however, there are performance trade-offswhen these features are enabled.The available memory RAS settings are normal, mirroring, and sparing. On the X5 platforms,you can access these settings under the Memory option menu in System Settings.This section is not meant to provide a comprehensive overview of the memory RAS featuresthat are available in the Xeon 7500 processor, but rather it provides a brief introduction toeach mode and its corresponding performance effects. For more information about memoryRAS features and platform-specific requirements, see the following sections: System x3850 X5: 6.9, “UEFI settings” on page 259 System x3690 X5: 7.8, “UEFI settings” on page 337 BladeCenter HX5: 8.5, “UEFI settings” on page 396The following sections provide a brief description of each memory RAS setting.Memory mirroringTo further improve memory reliability and availability beyond error checking and correcting(ECC) and Chipkill, the chip set can mirror memory data to two memory ports. To successfullyenable mirroring, you must have both memory cards per processor installed, and populate thesame amount of memory in both memory cards. Partial mirroring (mirroring of part but not allof the installed memory) is not supported.Memory mirroring, or full array memory mirroring (FAMM) redundancy, provides the userwith a redundant copy of all code and data addressable in the configured memory map.Memory mirroring works within the chip set by writing data to two memory ports on everymemory-write cycle. Two copies of the data are kept, similar to the way RAID-1 writes to disk.Reads are interleaved between memory ports. The system automatically uses the mostreliable memory port as determined by error logging and monitoring.If errors occur, only the alternate memory port is used, until bad memory is replaced.Because a redundant copy is kept, mirroring results in having only half the installed memoryavailable to the operating system. FAMM does not support asymmetrical memoryconfigurations and requires that each port is populated in identical fashion. For example, youmust install 2 GB of identical memory equally and symmetrically across the two memory portsto achieve 1 GB of mirrored memory.FAMM enables other enhanced memory features, such as Unrecoverable Error (UE)recovery, and is required for support of memory hot replace.Memory mirroring is independent of the operating system.For more information about system-specific memory mirroring installation options, see thefollowing sections: x3850 X5: 3.8.4, “Memory mirroring” on page 87 x3690 X5: 4.8.6, “Memory mirroring” on page 141 BladeCenter HX5: 5.10.4, “Memory mirroring” on page 200
