ECC Memory (Error-Correcting Code RAM)

ECC memory (Error-Correcting Code memory) is a type of RAM that stores extra check bits with each word of data. The memory controller uses them to detect and automatically correct single-bit errors, and to detect some larger ones.

Why it matters

Bits in memory can flip because of electrical noise, manufacturing weakness or, rarely, cosmic radiation. In a desktop, one flipped bit might crash a program. In a database server, it could silently corrupt stored data or cause a hard-to-reproduce outage.

ECC is standard in servers and many workstations. It costs somewhat more and requires support from both the CPU and the motherboard, so ECC modules do not always work in consumer boards.

Example

A typical server module stores 72 bits for every 64 bits of data:

64 data bits + 8 check bits = 72 bits per word
$ journalctl -k | grep -i edac
EDAC MC0: 1 CE on DIMM_A1 (corrected error)

The log line shows a corrected error (CE) on one module. A rising count of corrected errors is an early warning to replace that DIMM before uncorrectable errors appear.

Common misconceptions

• ECC makes memory faster. It is slightly slower, because of the extra checking.
• ECC protects against every error. It corrects single-bit errors and detects many multi-bit ones, but cannot fix them all.
• Any ECC module works in any board. The CPU and motherboard must support it.

Related

• RAM
• Motherboard
• MTBF
• Uptime

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