IBM 71413SU - System x3950 M2 Reference page 24

There are a number of advanced features implemented in the x3850 M2 and x3950 M2
memory subsystem, collectively known as
Memory ProteXion
The Memory ProteXion feature (also known as
equivalent of a hot-spare drive in a RAID array. It is based in the memory controller, and it
enables the server to sense when a chip on a DIMM has failed and to route the data
around the failed chip.
Normally, 128 bits out of every 144 are used for data and the remaining 16 bits are used
for ECC functions. However, the x3850 M2 and x3950 M2 needs only 12 bits to perform
the same ECC functions, thus leaving four bits free. In the event that a chip failure on the
DIMM is detected by memory scrubbing, the memory controller can reroute data around
that failed chip through these spare bits.
It can do this automatically without issuing a Predictive Failure Analysis® (PFA) or light
path diagnostics alert to the administrator, although an event is logged to the service
processor log. After the second DIMM failure, PFA and light path diagnostics alerts would
occur on that DIMM as normal.
Memory scrubbing
Memory scrubbing
reports memory errors that might be developing before they cause a server outage.
Memory scrubbing and Memory ProteXion work in conjunction with each other and do not
require memory mirroring to be enabled in order to work properly.
When a bit error is detected, memory scrubbing determines if the error is recoverable or
not.
– If the error is recoverable, Memory ProteXion is enabled and the data that was stored
in the damaged locations is rewritten to a new location. The error is then reported so
that preventative maintenance can be performed. As long as there are enough good
locations to allow the proper operation of the server, no further action is taken other
than recording the error in the error logs.
– 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 damaged DIMM. If memory mirroring is enabled, then the mirrored copy of the data
from the damaged DIMM is used until the system is powered down and the DIMM is
replaced.
Because x3850 M2 and x3950 M2 are now capable of supporting large amounts of
memory, IBM has added the Initialization Scrub Control setting to the BIOS, to let
customers choose when this scrubbing is done and therefore potentially speed up the
boot process. Refer to Table 10 on page 26 for further detail about these settings.
Memory mirroring
Memory mirroring
halved and a second copy of data is written to the other half. If 8 GB is installed, then the
operating system sees 4 GB after memory mirroring is enabled. It is disabled in the BIOS
by default. Because all mirroring activities are handled by the hardware, memory mirroring
is operating system-independent.
When memory mirroring is enabled, certain restrictions exist with respect to placement
and size of memory DIMMs and the placement and removal of memory cards. Refer to
System x3950M2 and x3850 M2 Installation Guide for details.
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IBM System x3950 M2 and x3850 M2 Technical Introduction
Active Memory
is an automatic daily test of all system memory, which detects and
is roughly equivalent to RAID-1 in disk arrays, in that usable memory is
:
redundant bit steering
) provides the