Eye on Blades Blog: Trends in Infrastructure
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Information on DDR3 memory lock-step technology

Lots of questions about advaced memory protection and capabilities with the latest HP ProLiant G6 server announcement.


HP BladeSystem Announcing today page: www.hp.com/go/bladesystem/news


How does DDR3 memory work and one of the new features is memory lock-step capabilities?


Lock-step mode is an advanced memory protection feature supported in many of the G6 servers announced yesterday (3/30/09), including the BL460c G6 and BL490c G6.  It takes two of the Xeon 5500 processor's three memory channels and runs them together, which enables 8-bit error correction instead of the 4-bit correction you get in normal Advanced ECC (non-lockstep) mode. Positives:smileysad:1) Achieves the same level of protection as ChipKill*, so there are some additional scenarios in which the system can correct memory errors.   Negatives: (1) You have to leave one of the three memory channels on each processor un-populated, so you cut your available number of DIMM slots by 1/3.  (2) Performance is measurably slower than normal Advanced ECC mode.(3) You can only isolate uncorrectable memory errors to a pair of DIMMs (instead of down to a single DIMM). Lock-Step mode is not default operation; it must be enabled in RBSU. We don't know how many customers will want to use it.   *Normal" ECC can correct single-bit errors and detect double-bit errors.  HP's term "Advanced ECC" means that the server corrects single-bit errors, detects multi-bit errors, and corrects some multi-bit errors that occur on the same DRAM.   Advanced ECC is not the exact same thing as ChipKill, which is an IBM term.   In some but not all scenarios, Advanced ECC offers the same protection as ChipKill.
 
We are updating the "HP Advanced Memory Protection technologies - Technology Brief" to include info about new features like this.http://h18004.www1.hp.com/products/servers/technology/whitepapers/index.html?jumpid=servers/technology

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