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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  • xkiller213 - Saturday, September 20, 2014 - link

    definitely affects the 840 too... see my screenshot: http://i.imgur.com/NLAMGmp.png
  • chizow - Friday, September 19, 2014 - link

    Good to know, glad to see the community staying on top of this one until a fix came about.

    Another odd thing I've noticed about these Samsung SSDs is RAID0 performance with an asynchronous number of SSDs. For example, 2 of these drives result in near-linear performance gains. Adding a 3rd drops results to less than 2, 4 returns it to near linear performance but limited to the same speed as 2 drives, adding a 5th drops it again. Very odd behavior. Other SSDs I have tested do not exhibit this problem, like Kingston HyperX and Sandisk Extreme II.

    Hopefully this is addressed too in the firmware fix.
  • Gigaplex - Saturday, September 20, 2014 - link

    That sounds like an issue with the controller, the drives themselves don't know they're in a RAID. Possibly odd number of drives causes misaligned block sizes?
  • Indio22 - Friday, September 19, 2014 - link

    This article by Kristian begins by mentioning: "numerous reports of Samsung SSD 840 and 840 EVO having low read performance". So does this mean the 840 Pro and the 840 EVO versions both have this issue? And what about the previous generation 830 SSDs, do they have this issue? I wonder if anyone has tested the 830 models yet to confirm.
  • Gigaplex - Saturday, September 20, 2014 - link

    Pro is likely unaffected since it doesn't use TLC NAND.
  • xkiller213 - Friday, September 19, 2014 - link

    I do hope they would release a firmware update for the discontinued 840 non-Evo drives too... I have a 500GB one installed in a macbook and the it is getting between 10-20MB/s read speeds...
  • ThisWasATriumph - Friday, September 19, 2014 - link

    Wow I just tested mine and its happening to me too. [IMG]http://i.imgur.com/lGVzZeS.jpg[/IMG] First 50GB are reading very slowly. Trying to refresh the drive with a utility I found in the meantime.
  • TheWrongChristian - Monday, September 22, 2014 - link

    And the fact you never previously noticed tells you what?
  • Samus - Saturday, September 20, 2014 - link

    This seems to only apply to TLC NAND drives?
  • Friendly0Fire - Saturday, September 20, 2014 - link

    Interesting, I seem not to be affected, or at least not to a significant level. My sequential reads never drop below ~160mb/s (and that's a few specific dips, otherwise it's 200+). This is my OS drive so there are definitely areas that haven't seen a write for over a month.

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