Specifications

Chapter 4. Continuous availability and manageability 163
򐂰 Hypervisor data that is not mirrored
– Advanced Memory Sharing (AMS) pool
– Memory used to hold contents of platform dump while waiting for offload to
management console
򐂰 Partition data that is not mirrored
– Desired memory configured for individual partitions is not mirrored.
To enable mirroring, the requirement is to have eight equally sized functional memory DIMMs
behind at least one POWER7 chip in each CEC enclosure. The DIMMs will be managed by
the same memory controller. The sizes of DIMMs might be different from one Power 7 chip
to another.
A write operation in the memory begins on the first DIMM of a mirrored DIMM pair. When
this write is complete, the POWER7 chip writes the same data to a second DIMM of the
DIMM pair.
The read operations alternate between both DIMMs.
Figure 4-4 shows the hardware implementation of Memory Mirroring for Hypervisor.
Figure 4-4 Hardware implementation of Memory Mirroring for Hypervisor
The impact on performance is very low. Whereas writes operations are slightly slower
because two writes are actually done, reads are faster because two sources for the data are
used. Measured commercial workloads show no gain or loss in performance due to mirroring.
HPC workload performing huge amounts of string manipulation might see a slight
performance effect.
DIMMs
Memory controller 0
Power7 processor
chip
0
1
2
3
4
5
6
7
Mirrored Data
Data
Memory controller 1