IBM 86884RX Installation Guide - Page 33

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logs. Errors from scrubbing are not reported unless bit steering has already been invoked. If the error is not recoverable, then memory scrubbing sends an error message to the light path diagnostics, which then turns on the proper lights and LEDs to guide you to the defective DIMM. If memory mirroring is enabled, then the mirrored copy of the data in the damaged DIMM is used until the system is powered down and the DIMM replaced. Memory mirroring Memory mirroring is roughly equivalent to RAID-1 in disk arrays, in that memory is divided in two ports and one port is mirrored to the other half (see Figure 1-11). If 8 GB is installed, then the operating system sees 4 GB once memory mirroring is enabled (it is disabled in BIOS by default). All mirroring activities are handled by the hardware without any additional support required from the operating system. Port 1 Port 2 Front of server Figure 1-11 Memory DIMMs are divided into two ports When memory mirroring is enabled (see "Enabling memory mirroring" on page 83), 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 Chapter 1. Technical description 19

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Chapter 1. Technical description
19
logs. Errors from scrubbing are not reported unless bit steering has already
been invoked.
If the error is not recoverable, then memory scrubbing sends an error
message to the light path diagnostics, which then turns on the proper lights
and LEDs to guide you to the defective DIMM. If memory mirroring is enabled,
then the mirrored copy of the data in the damaged DIMM is used until the
system is powered down and the DIMM replaced.
±
Memory mirroring
Memory mirroring is roughly equivalent to RAID-1 in disk arrays, in that
memory is divided in two ports and one port is mirrored to the other half (see
Figure 1-11). If 8 GB is installed, then the operating system sees 4 GB once
memory mirroring is enabled (it is disabled in BIOS by default). All mirroring
activities are handled by the hardware without any additional support required
from the operating system.
Figure 1-11
Memory DIMMs are divided into two ports
When memory mirroring is enabled (see “Enabling memory mirroring” on
page 83), 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
Port 1
Port 2
Front of server