Ever since it came out a year ago, the Samsung 840 Evo has been one of the best consumer SSDs on the market. While it relies on the same TLC (triple-level cell) NAND as the Samsung 840, the 840 Evo combines this slower, cheaper NAND flash with high speed SLC (single-level cell) NAND. Unfortunately, it seems there’s a problem with the SSD that has undercut its peanut butter-and-chocolate blend of excellent performance and cost-per-GB.
According to a thread at Overclock.net, the problem only occurs in drives with old data on them, where old is defined as more than 30 days. The drive graphs shown below are from two 840 Evos run on the same motherboard using the same AHCI solution. The top drive already has a significant amount of data on it and has been used for some time, while the bottom drive is a just-wiped model.
The reason reviews haven’t caught this particular errata is that it doesn’t crop up on filled drives or empty drives, but apparently only when data within the drive is marked as being of a certain age. Reviews don’t typically include a “Copy everything to the SSD, wait two months, then benchmark” method, if for no other reason than because this tends to muck up deadlines and anger my editors.
Samsung has said that it’s aware of the problem and is prepping a firmware fix for the issue. What’s happening, at this point, is unclear — users in the Overclock forum have reported that certain disk management utilities can fix the issue, but all such fixes are temporary at the moment until Samsung repairs the problem. Some readers are reporting even more drastic drops than those shown here, with performance falling to as low as 20MBps or less than a tenth of expectations.
SSDs remain far more complicated than HDDs
One thing this illustrates is that SSDs are far more complex than your average hard drive. A hard drive controller is still a complex bit of silicon, but much of the jobs that were once handled by the HDD controller were absorbed into the southbridge.
More than that, however, there are challenges to maximizing SSD performance or minimizing power cost that hard drives simply don’t encounter. SSDs typically have large DDR3-backed caches, perform sophisticated write-leveling, and must track the health of the drive through sophisticated ECC methods that hard drives generally don’t have to utilize. Drives like Samsung’s 840 Evo, which utilize both TLC and SLC NAND, perform an additional level of caching on top of that — the SSD has to know which data should be written back to TLC and which should be kept in the SLC NAND buffer.
All that sophisticated work going on beneath the hood means more opportunity for things to go wrong — and it looks like that’s what happened here. We’ll report back when Samsung’s fix is ready — hopefully it’ll solve the problem for all concerned. We’re still of the opinion that the 840 Evo is a solid design — especially compared to some of the hijinks we’ve seen other low-end manufacturers pull.