IBM xSeries 450 Planning And Installation Manual page 32

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.
S0 S0 S0 S1 S1 S1
S16 S17 S16 S17 S16 S17
S32 S32 S32 S33 S33 S33
S48 S49 S48 S49 S48 S49
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
IBM ^ xSeries 450 Planning and Installation Guide
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S2 S3 S2 S3 S2 S3 S4 S5 S4 S5 S4 S5 S6 S7 S6 S7 S6 S7 S8 S9 S8 S9 S8 S9
S18 S19 S18 S19 S18 S19 S20 S21 S20 S21 S20 S21 S22 S23 S22 S23 S22 S23 S24 S25 S24 S25 S24 S25
S34 S35 S34 S35 S34 S35 S36 S37 S36 S37 S36 S37 S38 S39 S38 S39 S38 S39 S40 S41 S40 S41 S40 S41
S50 S51 S50 S51 S50 S51 S52 S53 S52 S53 S52 S53 S54 S55 S54 S55 S54 S55 S56 S57 S56 S57 S56 S57
S10 S11 S10 S11 S10 S11 S12 S13 S12 S13 S12 S13 S14 S15 S14 S15 S14 S15
S26 S27 S26 S27 S26 S27 S28 S29 S28 S29 S28 S29 S30 S31 S30 S31 S30 S31
S42 S43 S42 S43 S42 S43 S44 S45 S44 S45 S44 S45 S46 S47 S46 S47 S46 S47
S58 S59 S58 S59 S58 S59 S60 S61 S60 S61 S60 S61 S62 S63 S62 S63 S62 S63
C0 C0 C0 C1 C1 C1
C2 C2 C2 K1 K1 K1
C3 C3 C3 C4 C4 C4
C5 C5 C5
K2 K2 K2