4 ms·

What if twitter flipped the high bit in your advertising bill? I bet you'd notice that. Anyway, that's not really how memory errors work. In aggregate, if yo

What if twitter flipped the high bit in your advertising bill? I bet you'd notice that. Anyway, that's not really how memory errors work. In aggregate, if you consider all of the memory on the planet as a whole, the bit error rate is extremely low. However, any given chip that's prone to bit errors will have them very frequently. Without ECC you have no way to detect this, and with OEM-provided, soldered-to-motherboard memory chips you also have no way to qualify your memory, either. You'd have to qualify and reject the entire box using something like a periodic memtest86+.

JohnBooty 12y ago
However, any given chip that's prone to bit errors will have them very frequently. Yes, but ECC is not going to help you much with this kind of memory issue. That's not the purpose of ECC. It's not "RAID for memory"; it's not designed to keep you up and running when your memory is physically failing. ECC "can detect and correct errors of a single bit per 64-bit 'word' (the unit of bus transfer), and detect (but not correct) errors of two bits per 64-bit word." ref: http://en.wikipedia.org/wiki/ECC_memory#Pros_and_cons_of_ECC http://en.wikipedia.org/wiki/ECC_memory#Pros_and_cons_of_ECC (yeah, I know... Wikipedia) A failing ECC memory chip's errors are going to overwhelm the ability of ECC to correct it - they're almost always going to be more than 2 bits per 64 bit word. Where ECC shines is, as I said, correcting the occasional flipped bit. Which happens way more than any of us would really like to think about - cosmic rays, electrical interference... it happens. Soft errors. with OEM-provided, soldered-to-motherboard memory chips you also have no way to qualify your memory, either. You'd have to qualify and reject the entire box using something like a periodic memtest86+. Yes. Soldered-on memory sucks, aside from the fact that it can offer form factor benefits. (Which is a really dubious benefit when it comes to the Mac Mini. The 2012 Minis used replaceable DIMMs in the same form factor as the 2014 models)
- thrownaway2424 12y ago
  Your understanding of this topic seems to stem from something other than operating servers at large scale. Single bit errors are extremely common in the absence of multiple bit errors. One of the reasons that memtest86+ is so thorough and weird is because these marginal bits are access-pattern-dependent. With the right access pattern certain defective drams will always yield SBEs without ever causing an MBE.
  - JohnBooty 12y ago
    That's interesting to know; thanks for posting that. I've never seen failing RAM give SBEs but not MBEs but, like you said, I have not operated servers at large scale. It's good to know that can happen!

JohnBooty 12y ago

However, any given chip that's prone to bit errors will have them very frequently. Yes, but ECC is not going to help you much with this kind of memory issue. That's not the purpose of ECC. It's not "RAID for memory"; it's not designed to keep you up and running when your memory is physically failing. ECC "can detect and correct errors of a single bit per 64-bit 'word' (the unit of bus transfer), and detect (but not correct) errors of two bits per 64-bit word." ref: http://en.wikipedia.org/wiki/ECC_memory#Pros_and_cons_of_ECC http://en.wikipedia.org/wiki/ECC_memory#Pros_and_cons_of_ECC (yeah, I know... Wikipedia) A failing ECC memory chip's errors are going to overwhelm the ability of ECC to correct it - they're almost always going to be more than 2 bits per 64 bit word. Where ECC shines is, as I said, correcting the occasional flipped bit. Which happens way more than any of us would really like to think about - cosmic rays, electrical interference... it happens. Soft errors. with OEM-provided, soldered-to-motherboard memory chips you also have no way to qualify your memory, either. You'd have to qualify and reject the entire box using something like a periodic memtest86+. Yes. Soldered-on memory sucks, aside from the fact that it can offer form factor benefits. (Which is a really dubious benefit when it comes to the Mac Mini. The 2012 Minis used replaceable DIMMs in the same form factor as the 2014 models)

thrownaway2424 12y ago
Your understanding of this topic seems to stem from something other than operating servers at large scale. Single bit errors are extremely common in the absence of multiple bit errors. One of the reasons that memtest86+ is so thorough and weird is because these marginal bits are access-pattern-dependent. With the right access pattern certain defective drams will always yield SBEs without ever causing an MBE.
- JohnBooty 12y ago
  That's interesting to know; thanks for posting that. I've never seen failing RAM give SBEs but not MBEs but, like you said, I have not operated servers at large scale. It's good to know that can happen!