Last month, Samsung confirmed that its 840 EVO SSD can suffer from slower read speeds with older data. The company promised to address the issue with updated firmware, and we've been experimenting with the fix.
Before we get into the results of our testing, we should spend a moment on the problem itself. Here's how Samsung describes the issue:
SSDs usually calibrate changes in the statuses of cells over time via the flash management software algorithm. Due to the error in the software algorithm, the 840 EVO performed read-retry processes aggressively, resulting in a drop in overall read performance. This only occurs if the data was kept in its initial cell without changing, and there are no symptoms of reduced read performance if the data was subsequently migrated from those cells or overwritten.
Read-retry is used to compensate for changes in cell voltages, suggesting that voltage drift over time was a key factor here. The EVO's flash management routines evidently haven't been responding to voltage changes correctly.
Fixing the problem is a two-part process managed by Samsung's appropriately named Performance Restoration tool. First, the utility flashes the SSD's firmware with the new EXT0CB6Q revision. Then, it reboots the system and completes the restoration process, which we're told involves "conditioning" the NAND. The entire process is non-destructive, so it shouldn't compromise any data on the drive. It also only needs to be run once. That said, the target drive must have at least 10% free capacity for the tool to work its magic.
The data on the 840 EVOs we have in the lab isn't old enough to be affected by the flaw. Fortunately, our resident developer, Bruno "morphine" Ferreira, has been running an 840 EVO 500GB in his personal system for more than 10 months. Having already noticed slower performance with older files, he bravely volunteered to test Samsung's fix.
The restoration process went off without a hitch, though it took over an hour to condition Bruno's drive. Read speeds seem to be much improved. Below is a screenshot from HD Tach, which tests read performance across the extent of the drive. Click the buttons below the image to switch between results from before and after the fix was applied.