During the last couple of weeks, numerous reports of Samsung SSD 840 and 840 EVO having low read performance have surfaced around the Internet. The most extensive one is probably a forum thread over at Overclock.net, which was started about month ago and currently has over 600 replies. For those who are not aware of the issue, there is a bug in the 840 EVO that causes the read performance of old blocks of data to drop dramatically like the HD Tach graph below illustrates. The odd part is that the bug only seems to affect LBAs that have old data (>1 month) associated with them because freshly written data will read at full speed, which also explains why the issue was not discovered until now. 

Source: @p_combe

I just got off the phone with Samsung and the good news is that they are aware of the problem and have presumably found the source of it. The engineers are now working on an updated firmware to fix the bug and as soon as the fix has been validated, the new firmware will be distributed to end-users. Unfortunately there is no ETA for the fix, but obviously it is in Samsung's best interest to provide it as soon as possible.

Update 9/27: Samsung just shed some light on the timeline and the fixed firmware is scheduled to be released to the public on October 15th.

I do not have any further details about the nature of the bug at this point, but we will be getting more details early next week, so stay tuned. It is a good sign that Samsung acknowledges the bug and that a fix is in the works, but for now I would advise against buying the 840 EVO until there is a resolution for the issue.

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  • hojnikb - Friday, September 19, 2014 - link

    And while issue may not be critical (although we really need for info on that) its certanly a VERY big annoyance to have a few month old files reading much slower than with an old HDD. Afterall, you do buy SSDs for speed.
  • hojnikb - Friday, September 19, 2014 - link

  • stickmansam - Friday, September 19, 2014 - link

    Well the issue took a while to surface due to the nature of the problem

    While the EVO offers good value, until a fix is released and proven working in the wild, why not just get other similar drives that offer very much the same performance/value and are not beset by potential issues.
  • mrdude - Friday, September 19, 2014 - link

    "Well the issue took a while to surface due to the nature of the problem"

    So Samsung doesn't thoroughly test their drives? They never bothered to read data that was written and not accessed a month ago?

    I'm not sure I like that answer... at all, frankly.
  • stickmansam - Friday, September 19, 2014 - link

    I wonder how much long term retention testing is done

    and if the tests are done, do they only look if the data is accessible or do they consider if there is any speed degradation

    I'm thinking this is an issue with TLC long term retention
  • wintermute000 - Friday, September 19, 2014 - link

    Its doesn't affect all users/all drives or all data > a month.
    I have two of these in two different systems and have not noticed a single hiccup.
    If it affected all users, there would be a LOT more noise than an overclock.net thread, there are so many of these drives in the wild!
  • Gigaplex - Saturday, September 20, 2014 - link

    They'd need to test for 6 months or so at least for this issue to show up. That's 6 months of holding off from releasing a product in a fast moving industry. I'm all for well tested products but you can't test for everything.
  • Kristian Vättö - Friday, September 19, 2014 - link

    You are correct that the EVO is still a solid drive and it has in fact been one of our recommendations since it was launched, but I don't feel comfortable recommending it until we have peace of mind that the issue has been solved. I'm confident that Samsung is able to deliver a fix on a timely matter, but there is always the possibility of "what if it's not fixable by firmware", in which case we would have an egg on our face for recommending a drive with a known problem.
  • hrrmph - Saturday, September 20, 2014 - link

    Even a firmware fix is an irritating disruption.

    Samsung apparently moved too quickly when it pulled so far ahead of Intel. Now Intel reliability is looking more attractive again.
  • fallaha56 - Sunday, September 21, 2014 - link

    buddy this is absolutely critical! my whole system (like many others) has been slowing down

    i generally love my Samsung gear but until this is fixed they're on probation -you sound a little too much like a marketing department employee for my liking!

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