IBM 440 manual Port

￿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 1-12). 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

Figure 1-12 Memory DIMMs are divided into two ports

When memory mirroring is enabled (see 4.1.2, “Enabling memory mirroring” on page 108), 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 from the DIMM with the least amount of reported memory errors through memory scrubbing.

If memory scrubbing determines the DIMM is damaged beyond use, read and write operations are redirected to the partner DIMM in the other port. Memory scrubbing then reports the damaged DIMM and the Light Path Diagnostics display the error. If memory mirroring is enabled, then the mirrored copy of the

Chapter 1. Technical description 21