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See 3.2, "Memory subsystem" on page 111 for further discussion of how
memory is implemented in the x3850 M2 and x3950 M2 and what you should
consider before installation.
A number of advanced features are implemented in the x3850 M2 and x3950 M2
memory subsystem, collectively known as
Memory ProteXion
The Memory ProteXion feature (also known as
provides the 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 of every 144 are used for data and the remaining 16 bits
are used for error checking and correcting (ECC) functions. However, the
x3850 M2 and x3950 M2 require only 12 bits to perform the same ECC
functions, thus leaving 4 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 reroutes the data automatically without issuing a Predictive Failure
Analysis® (PFA) or light path diagnostics alerts to the administrator, although
an event is recorded 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 is an automatic daily test of all the system memory that
detects and reports memory errors that might be developing before they
cause a server outage.
Memory scrubbing and Memory ProteXion work in conjunction and do not
require memory mirroring to be enabled to work properly.
When a bit error is detected, memory scrubbing determines whether the error
is recoverable:
– 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.
If the number of good locations is sufficient 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, 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,
40
Planning, Installing, and Managing the IBM System x3950 M2
Active Memory
:
redundant bit steering
)
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