As we previously reported back in mid-October, some Samsung 840 EVO SSD owners were experiencing issues with low read speeds of data that had not been touched or moved for an extended period of time after being written. In most cases, the data had been present in the flash memory for a period of weeks or months, and as time went on attempting to read this data would result in ever-slower read speeds — even as low as 50-100 MB/s. The previous article announced a firmware update and performance restoration tool from Samsung to address the problem.
We now have a number of 840 EVO users reporting that they implemented the restoration tool and the firmware update, and either the update was ineffective, or merely a one-time reset with the same problem occurring once the same conditions were subsequently met again. For the latter group, the only improvement was that the degradation was spread out over a longer time period, and the performance “floor” was now 200-250MB/s rather than previous “floor” of 50-100MB/s.
Allyn Malventano over at PC Perspective has been doing some excellent research and analysis of the whole situation, and has been instrumental in working with Samsung to attempt to address and fix the performance degradation problem. His article from 10/14 is a must-read for anyone affected by (or curious about) the problem. Allyn’s theory was that the slow down was due to cell voltage drift over time, as depicted in his graph below.
Allyn has now posted a more detailed follow-up article concerning the only partially effective firmware update. As Allyn notes in his latest report, the slow speeds turned out to be caused by the SSD controller working extra hard to apply error correction to the data being received from flash memory that had been (reportedly) miscalibrated at the factory. This miscalibration was causing the 840 EVO to erroneously adapt to cell voltage drifts over time. This effect occurs to some degree in all flash-based storage, but TLC NAND (as is used in the 840 EVO) is the most susceptible to this cell voltage drift. Ambient temperature can also impact the slower read speeds, as the controller is forced to work outside of its normal load levels. This can result in the controller thermally throttling itself when asked to perform too many error correction operations.
image source: pcperspective.com
The PC Perspective article(s) are partially based on a discussion forum thread started at overclock.net about the performance degradation. Users on that forum thread have now been posting the results of their experiences with the effectiveness (or lack thereof) of the firmware update. Allyn has summarized the results of their experiences, and has found there be three basic categories of results:
- Users reporting drives that still slow down over time, except with the downward trend spread over a longer time period, and bottom read speeds of 200-250MB/s. In other words, some mitigation of the issue, but the core issue is still present.
- Users whose posts are inconsistent, but may be due to other issues than the focus of the article(s). Some seem to not grasp the issue being discussed and/or are running the wrong tests. Others presented results compromised by heavy random writes, such as bittorrent or Steam users, that were dragging down read speeds.
- The third group is those who have reported that the firmware update was a success, and are no longer experiencing read speeds degradation.
image source: pcperspective.com
Allyn has reached out to Samsung once again for a statement, and thus far the reply has been a rather generic “we are looking into it.” Samsung reacted relatively quickly the first time in pushing out the performance restoration tool and firmware update, so a more comprehensive fix may take a little time. In the mean time, a petition has been started on the overclock.net forums, asking Samsung to replace their affected 840 EVO SSDs.
The ball is now squarely back in Samsung’s court, which in this case is also the court of public opinion. Stay tuned for further developments.