Advanced memory protection for HP ProLiant 300 series G4 servers

Symmetric memory mode
Symmetric Memory mode is a performance enhancement supported with certain DIMM configurations
on the ProLiant 300-series G4 servers. With either four identical dual-rank DIMMs or eight identical
single-rank DIMMs installed in a G4 system configured for Advanced ECC Mode, Symmetric Memory
mode will automatically be enabled. When enabled, the system takes advantage of the particular
DIMM configuration to improve performance. This mode cannot be entered when configured for
Online Spare mode.
Memory Scrubbing
What is memory scrubbing?
There are two types of memory scrubbing, demand and background. ProLiant 300-series G4 servers
support both types of memory scrubbing. Previous generations of 300-series platforms typically
supported only demand scrubbing.
Demand scrubbing is a feature that allows the system to differentiate between soft and hard
correctable memory errors. It allows the system to ignore soft errors while still notifying the customer of
a true DIMM failure. When a correctable memory error occurs due to a soft error, correct data is
written back to the memory device. This prevents the same soft error from occurring again in the
future. This prevents a soft error from resulting in a DIMM exceeding the correctable error threshold.
Background scrubbing reduces the chances that multiple soft errors will result in an uncorrectable
memory error. With background scrubbing, the system is constantly correcting any potential soft
errors “in the background.” Without affecting normal system operation, the memory controller will
continuously perform read/write operations on memory correcting any soft errors that may exist in
memory. By doing this, a soft error that would result in a single-bit failure will likely be corrected
before another soft error potentially occurs which might result in multiple-bits of corrupted data. If
multiple bits of data are corrupted, an uncorrectable error could result causing the system to crash
(see description of Advanced ECC protection).
See the section below for additional detail on how demand and background scrubbing work.
Detailed description of memory scrubbing
As mentioned previously, Advanced ECC and Standard ECC utilize data and ECC bits to perform
memory error detection and correction. Through the data and ECC bits, the system can correct certain
memory errors. For Standard ECC, the system can correct single-bit errors. For Advanced ECC, the
system can correct single or multi-bit errors as long as all failed bits are on the same DRAM device on
the DIMM. If the ECC bits are correct for the corresponding data, then no error occurs when a
memory read occurs. However, if the data does not match the ECC bits, then an error occurs when a
memory read occurs. In many cases, the proper data can be reconstructed through using the ECC
check bits, resulting in a correctable error. The data and ECC bits are checked on memory reads to
detect and potentially correct errors. The system writes correct data and ECC bits on memory writes.
Also mentioned previously, soft errors are errors that occur in memory but which do not indicate a
hardware problem. A cosmic ray hitting the DRAM device on a DIMM can in rare cases cause one or
more bits to change states. This can result in a soft correctable or soft uncorrectable error. It is
extremely rare to have multiple soft errors occur to the same memory location that would result in an
uncorrectable error due to a soft error. Since soft memory errors don’t indicate a problem with the
hardware, and are simply the result of a bit in memory changing state due to cosmic rays, the
memory error will only occur until the data and ECC bits are properly written back to the DIMM.
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