Some 840 EVOs are still reading slower over time.

Well here we are again with this Samsung 840 EVO slow down issue cropping up here, there, and everywhere. The story for this one is so long and convoluted that I’m just going to kick this piece off with a walk through of what was happening with this particular SSD, and what was attempted so far to fix it:

The Samsung 840 EVO is a consumer-focused TLC SSD. Normally TLC SSDs suffer from reduced write speeds when compared to their MLC counterparts, as writing operations take longer for TLC than for MLC (SLC is even faster). Samsung introduced a novel way of speeding things up with their TurboWrite caching method, which adds a fast SLC buffer alongside the slower flash. This buffer is several GB in size, and helps the 840 EVO maintain fast write speeds in most typical usage scenarios, but the issue with the 840 EVO is not its write speed – the problem is read speed. Initial reviews did not catch this issue as it only impacted data that had been stagnant for a period of roughly 6-8 weeks. As files aged their read speeds were reduced, starting from the speedy (and expected) 500 MB/sec and ultimately reaching a worst case speed of 50-100 MB/sec:

There were other variables that impacted the end result, which further complicated the flurry of reports coming in from seemingly everywhere. The slow speeds turned out to be the result of the SSD controller working extra hard to apply error correction to the data coming in from flash that was (reportedly) miscalibrated at the factory. This miscalibration caused the EVO to incorrectly adapt to cell voltage drifts over time (an effect that occurs in all flash-based storage – TLC being the most sensitive). Ambient temperature could even impact the slower read speeds as the controller was working outside of its expected load envelope and thermally throttled itself when faced with bulk amounts of error correction.

An example of file read speed slowing relative to age, thanks to a tool developed by Techie007.

Once the community reached sufficient critical mass to get Samsung’s attention, they issued a few statements and ultimately pushed out a combination firmware and tool to fix EVO’s that were seeing this issue. The 840 EVO Performance Restoration Tool was released just under two months after the original thread on the forums was started. Despite a quick update a few weeks later, that was not a bad turnaround considering Intel took three months to correct a firmware issue of one of their own early SSDs. While the Intel patch restored full performance to their X25-M, the Samsung update does not appear to be faring so well now that users have logged a few additional months after applying their fix.

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