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IBM
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xSeries 450 Planning and Installation Guide
Memory ProteXion
Memory ProteXion, also known as “redundant bit steering”, is the technology
behind using redundant bits in a data packet to provide backup in the event of
a DIMM failure.
Currently, other industry-standard servers use 8 bits of the 72-bit data packets
for ECC functions and the remaining 64 bits for data. However, the x450 uses
an advanced ECC algorithm that is based not on bits but on memory symbols.
Symbols are groups of multiple bits, and in the case of the x450, each symbol
is 4 bits wide. With two-way interleaved memory, the algorithm needs only
three symbols to perform the same ECC functions, thus leaving one symbol
free (2 bits on each DIMM). See Figure 1-10.
Figure 1-10 Memory ProteXion
In the event that a chip failure on the DIMM is detected by memory scrubbing,
the memory controller can re-route data around that failed chip through the
spare symbol (similar to the hot-spare drive of RAID array). It can do this
automatically without issuing a Predictive Failure Analysis® (PFA) or light
path diagnostics alert to the administrator. 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 with each
other, but they do not require memory mirroring (as described below) to be
enabled to work properly.
When a bit error is detected, memory scrubbing determines if the error is
recoverable or not. If it 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
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Summary of Contents for 450 xSeries
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