Dimm Error Detection/Correction Technologies; The Increasing Possibility Of Memory Errors - Compaq BL10e - HP ProLiant - 512 MB RAM Overview

To prevent this and other memory-related problems, HP urges customers to use only HP-certified
DIMMs, which are available in the memory option kits for each ProLiant server (see the "Importance
of using HP-certified memory modules in ProLiant servers" section).
Another important difference between single-rank and dual-rank DIMMs is cost. Typically, memory
costs increase with DRAM density. For example, the cost of an advanced, high-density DRAM chip is
typically more than twice that of a conventional DRAM chip. Because large capacity single-rank
DIMMs are manufactured with higher-density DRAM chips, they typically cost more than dual-rank
DIMMs of comparable capacity.

DIMM error detection/correction technologies

Memory modules used in servers are inherently susceptible to memory errors. As described earlier,
each DRAM chip stores data in columns and rows of capacitors (memory cells) that must be
continuously recharged (refreshed) to preserve the data. The operating voltage of the memory device
determines the level of the electrical charge. However, if a capacitor's charge is affected by some
external event, the data may become incorrect. Such memory errors can cause applications and
operating systems to crash and can result in the permanent loss of business data.
Memory errors are classified by the number of bits that are affected—single-bit or multi-bit—and the
cause of error. A 64-bit wide data bus transports 64 bits at a time. These 64 bits constitute an ECC
data word. An error in one bit of a data word is a single-bit error. An error in more than one bit of a
data word is a multi-bit error.
Depending on the cause, a memory error is referred to as either a hard or soft error. A hard error is
caused by a broken or defective piece of hardware, so the device consistently returns incorrect results.
For example, a memory cell may be stuck so that it always returns "0" bit, even when a "1" bit is
written to it. Hard errors can be caused by DRAM defects, bad solder joints, connector issues, and
other physical issues. Soft errors are more prevalent. They occur randomly when an electrical
disturbance near a memory cell alters the charge on the capacitor. A soft error does not indicate a
problem with a memory device because once the stored data is corrected (for example, by a write to
a memory cell), the same error does not recur.

The increasing possibility of memory errors

Two trends increase the likelihood of memory errors in servers: expanding memory capacity and
increasing storage density.
Software vendors are developing increasingly complex and memory-intensive applications. This drives
operating systems to address more memory, which causes manufacturers to expand the memory
capacity of the servers. For example, while the HP ProLiant DL585 G2 of 2007 could support a
maximum of 128 GB, some of the latest servers now support up to 256 GB of memory. As
manufacturers continue to expand the memory capacity of servers, the possibility of memory errors
likewise increases.
Two parameters of DRAM are inextricably tied together—the storage density of the DRAM chips and
the operating voltage of the memory system. As the size of memory cells decreases, both DRAM
storage density and the memory-cell voltage sensitivity increase. Initially, industry-standard DIMMs
operated at 5 volts. However, due to improvements in DRAM storage density, the operating voltage
decreased first to 3.3 V, then 2.5 V, and then 1.8 V to allow memory to run faster and consume less
power. Because memory storage density is increasing and operating voltage is shrinking, there is a
higher probability that an error may occur. Whenever a data bit is misinterpreted and not corrected,
the error can cause an application to crash. The only true protection from memory errors is to use
some sort of memory detection or correction protocol. Some protocols can only detect errors, while
others can both detect and correct memory problems, seamlessly.
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