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the mirrored copy of the data from the damaged DIMM is used until the
system is powered down and the DIMM replaced.
As x3850 M2 and x3950 M2 is 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. See 3.2.3, "Memory mirroring" on page 118for
more details on these settings.
Memory mirroring
Memory mirroring is roughly equivalent to RAID-1 in disk arrays, in that
usable memory is halved and a second copy of data is written to the other
half. If 8 GB is installed, the operating system sees 4 GB once 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. See 3.2, "Memory subsystem" on page 111 and 3.2.3,
"Memory mirroring" on page 118for details.
Chipkill memory
Chipkill is integrated into the XA-64e chipset, so it does not require special
Chipkill DIMMs and is transparent to the operating system. When combining
Chipkill with Memory ProteXion and Active Memory, the x3850 M2 and x3950
M2 provides very high reliability in the memory subsystem.
When a memory chip failure occurs, Memory ProteXion transparently handles
the rerouting of data around the failed component as previously described.
However, if a further failure occurs, the Chipkill component in the memory
controller reroutes data. The memory controller provides memory protection
similar in concept to disk array striping with parity, writing the memory bits
across multiple memory chips on the DIMM. The controller is able to
reconstruct the missing bit from the failed chip and continue working as usual.
One of these additional failures can be handled for each memory port for a
total of eight Chipkill recoveries.
Hot-add and hot-swap memory
The x3850 M2 and x3950 M2 support the replacing of failed DIMMs while the
server is still running. This hot-swap support works in conjunction with
memory mirroring. The server also supports adding additional memory while
the server is running. Adding memory requires operating system support.
These two features are mutually exclusive. Hot-add requires that memory
mirroring be disabled, and hot-swap requires that memory mirroring be
enabled. For more information, see 3.2, "Memory subsystem" on page 111.
Chapter 1. Technical overview
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