Hitachi Compute Rack CR 210H User Manual page 46

In memory mirroring, a memory read and write are performed as follows:
A read is performed from either the primary slot or mirror slot.

A write is performed to both primary and mirror slots in appearance order.

If an uncorrectable memory error occurs on one of the memory boards in the
primary slot, all read and write operations are performed only on the memory
boards in the mirror slot. If an error occurs on one of the memory boards in the
mirror slot, contrarily, all read and write operations are performed only on the
memory boards in the primary slot.
The precautions on use of memory mirroring are as follows:
To validate the memory mirroring function, set Chipset > North Bridge >

Memory Mode to Mirroring on the system BIOS setup menu.
When the memory mirroring function is set valid, the half of the memory

boards installed are used as a mirror and thus the actual memory capacity
used becomes half. On the system BIOS setup menu, confirm Chipset >
North Bridge > Total Memory.
The mirror memory slot is displayed as "0 MB" even if memory boards are
installed.
If memory degradation occurs, the memory mirroring function becomes

invalid.
In this case, the MAINTENANCE LED 2 in front of the system unit is turned on.
See MAINTENANCE LEDs
You cannot use the memory mirroring function simultaneously with online

spare memory, lock step, and device tagging.
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on page 2-3
System Unit Functions
Hitachi Compute Rack 210H User's Guide