4 ms·

Other comments are assuming this is flash wearing out, but is this actually the flash wearing out or is it something else (buggy firmware, cheap memory cells, w

by darren_ 15y ago

Other comments are assuming this is flash wearing out, but is this actually the flash wearing out or is it something else (buggy firmware, cheap memory cells, what have you)? The time-to-failure seems way too low for it to be excessive writes (15 days for an 80GB IBM!). Actually, come to think of it, what is the failure mode for a drive when it's all worn out? Is it catastrophic data loss or is it just an unwritable drive?

wazoox 15y ago
I've read a test done on a basic SD card, and apparently it just turned to read-only mode when worn out. We could reasonably hope that higher end devices would fail the same gentle way :)
Symmetry 15y ago
Theoretically it should just be an unwritable drive since flash tends to only fail on erase. If the firmware is badly programmed or something other than the flash fails who knows what could happen though.
- kabdib 15y ago
  There is a "read disturbance effect" on MLC flash that causes wear on reads. MLC flash gets multiple bits per cell using several voltage levels. MLC flash is also around 5K erase cycles (and about the same number of reads until you need to do an erase-and-rewrite). Multiplied out and properly wear-leveled this is not a big deal. We might be seeing crappy wear leveling, or badly written firmware, or the need for more ECC bits (yes, MLC has these, and they are /not/ optional) than the EE types think they can get away with. I've been writing flash file systems since 1991. They're fun as hell to work with.
  - adbge 15y ago
    > I've been writing flash file systems since 1991. They're fun as hell to work with. Could you recommend any good resources for someone new to file system technology that is interested in their inner-workings? Thanks!
    - kabdib 15y ago
      Practical File System Design: http://www.letterp.com/~dbg/practical-file-system-design.pdf http://www.letterp.com/~dbg/practical-file-system-design.pdf NT File System Design (Rajeev Nagar) -- you can find this used. I believe his web site had a PDF copy for a while, sans pictures. My favorite book on transactions is Bernstein's Principles of Transaction Processing (may also be available as a PDF somewhere, from the author). Read the v6 Unix file system code. That will date me, but it's /simple/ and it works. You can move up from there. btrfs is neat. I haven't looked at the code.

by darren_ 15y ago

wazoox 15y ago

I've read a test done on a basic SD card, and apparently it just turned to read-only mode when worn out. We could reasonably hope that higher end devices would fail the same gentle way :)

Symmetry 15y ago

Theoretically it should just be an unwritable drive since flash tends to only fail on erase. If the firmware is badly programmed or something other than the flash fails who knows what could happen though.

kabdib 15y ago
There is a "read disturbance effect" on MLC flash that causes wear on reads. MLC flash gets multiple bits per cell using several voltage levels. MLC flash is also around 5K erase cycles (and about the same number of reads until you need to do an erase-and-rewrite). Multiplied out and properly wear-leveled this is not a big deal. We might be seeing crappy wear leveling, or badly written firmware, or the need for more ECC bits (yes, MLC has these, and they are /not/ optional) than the EE types think they can get away with. I've been writing flash file systems since 1991. They're fun as hell to work with.
- adbge 15y ago
  > I've been writing flash file systems since 1991. They're fun as hell to work with. Could you recommend any good resources for someone new to file system technology that is interested in their inner-workings? Thanks!
  - kabdib 15y ago
    Practical File System Design: http://www.letterp.com/~dbg/practical-file-system-design.pdf http://www.letterp.com/~dbg/practical-file-system-design.pdf NT File System Design (Rajeev Nagar) -- you can find this used. I believe his web site had a PDF copy for a while, sans pictures. My favorite book on transactions is Bernstein's Principles of Transaction Processing (may also be available as a PDF somewhere, from the author). Read the v6 Unix file system code. That will date me, but it's /simple/ and it works. You can move up from there. btrfs is neat. I haven't looked at the code.