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» Articles » Monitoring Memo... Login Error Detection and Correction Jeff Layton Data protection and checking takes place various places throughout a system. Some of it is in hardware and some of it is in uncorrectable memory error (module unknown) software. The goal is to ensure that data is not corrupted (changed), either coming correctable memory error rate exceeded for dimm from or going to the hardware or in the software stack. One key technology is ECC memory (error-correcting code memory).The standard ECC memory used in systems today can detect and correct what are called single-bit errors, and although it can detect double-bit errors, it cannot correct them. A simple flip of one bit in a byte can make http://h20564.www2.hp.com/hpsc/doc/public/display?docId=mmr_kc-0107529 a drastic difference in the value of the byte. For example a byte (8 bits)with a value of 156 (10011100)that is read from a file on disk suddenly acquires a value of 220 if the second bit from the left is flipped from a 0 to a 1 (11011100) for some reason.ECC memory can detect the problem and correct it so with the user unaware. Notice, however, that only one bit in the http://www.admin-magazine.com/Articles/Monitoring-Memory-Errors byte has been changed and then corrected. If two bits change – perhaps by both the second and seventh from the left – the byte is now 11011110 (i.e., 222); typical ECC memory can detect that the “double-bit” error occurred, but it cannot correct it. In fact, when a double-bit error happens, memory should cause what is called a “machine check exception” (mce), which should cause the system to crash. After all, you are using ECC memory, so ensuring the data is correct is important; if an uncorrectable memory error occurs, you would probably want the system to stop.The source of bit-flipping usually originates in some sort of electrical or magnetic interference inside the system. This interference can cause a bit to flip at seemingly random times, depending on the circumstances. According to the Wikipedia article and a paper on single-event upsets in RAM, most single-bit flips are the result of background radiation – primarily neutrons from cosmic rays.The same Wikipedia article reports that the error rates reported from 2007 to 2009 varied all over the map, ranging from 10–10 (errors/bit-hr) to 10–17 (seven orders of magnitude difference). The lower number is just about one error per gigabit of memory per hour. The upper number indicates roughly one error every 1,000 years per gigabit of m
following sections: DIMM Replacement Guidelines How DIMM Errors Are Handled by the System Isolating and Correcting DIMM ECC Errors Note - Refer to the service manual or service label for the system that you are servicing for information on DIMM population rules. https://docs.oracle.com/cd/E19150-01/820-4213-11/dimms.html DIMM Replacement Guidelines Replace a DIMM when one of the following events takes place: The DIMM fails memory testing under BIOS due to Uncorrectable Memory Errors (UCEs). UCEs occur and investigation shows that the errors originated from memory. More than 24 Correctable Errors (CEs) originate in 24 hours from a single DIMM and no other DIMM is showing further CEs. Note - If more than one DIMM has experienced multiple CEs, other possible causes of CEs must be ruled out memory error by a qualified Sun Support specialist before replacing any DIMMs. Retain copies of the logs showing the memory errors to send to Sun for verification prior to calling Sun. How DIMM Errors Are Handled by the System This section describes the following topics: Uncorrectable DIMM Errors Correctable DIMM Errors DIMM Fault LEDs Uncorrectable DIMM Errors For all operating systems, the behavior is the same for uncorrectable errors (UCEs): 1. When a UCE occurs, the memory controller causes an immediate reboot of correctable memory error the system. 2. During reboot, the BIOS checks the Machine Check registers and determines that the previous reboot was due to a UCE. The uncorrectable ECC error is displayed in the service processor’s system event log (SEL) as shown here: Memory | Uncorrectable ECC | Asserted | DIMM A0 Correctable DIMM Errors If a DIMM has 24 or more correctable errors (CE)s in 24 hours, it is considered defective and should be replaced. CEs will be captured in the SEL and light the fault LED after 24 single bit errors are detected in 24 hours. They are reported or handled in the supported operating systems as follows: Windows server: a. A Machine Check error-message bubble appears on the task bar. b. Open the Event Viewer to view errors. Access the Event Viewer through this menu path: Start-->Administration Tools-->Event Viewer c. View individual errors (by time) to see the details of the error. Solaris: Solaris FMA reports and sometimes retires memory with correctable Error Correction Code (ECC) errors. See your Solaris documentation for details. To view ECC errors, use the following command: fmdump -eV DIMM Fault LEDs When you press the Remind button on the motherboard (or memory tray for x4450), the LEDs next to the DIMMs flash to indicate that the system has detected 24 or more CEs in a 24-hour period on that DIMM. DIMM fault LED is off: The DIMM is operating properly. DIMM fault LED is flashin
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