Samsung has released its 840 Evo SSD repair tool, as well as additional information on what causes the problem. It turns out that the issue is a bug in how the drive calculates what the voltage level within a cell should be in order to perform a proper data read. When data is written to triple-level (TLC) NAND, it’s stored at one of eight distinct voltage states. As time passes, the state shifts slightly and the drive has to compensate for that shift in order to read the older data. Auto-refreshing the data with a periodic re-write isn’t an option — TLC NAND already has lower durability than other form factors, and a periodic background rewrite would quickly exhaust the number of program/erase (P/E) cycles.
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The 840 Evo’s problem was that the calibration algorithm that’s supposed to detect the voltage levels in the cells apparently wasn’t calibrated correctly. As the data in the cells got older (the 30-day window was something of a moving target), the drive had more trouble reading the information — what it expected to see and what it was actually seeing were two different things. This led to a quick succession of read cycles as the controller attempted to compensate, which substantially degraded performance.
Smaller gaps between voltage levels make precise software calibration essential.
The good news is that this aggressive retry cycle won’t have harmed the flash in any way — writes are destructive to NAND, but reads are not. The further good news is that Samsung expects the new repair tool to fully resolve the drive’s problems. While we don’t have any SSDs configured appropriately to test it, we can confirm that the flash process went off without a hitch.
The caveats
When it comes to flashing a product, its always a good idea to read the instructions; firmware updates remain one of the few ways to truly trash piece of equipment. Samsung has a list of 17 — here are some of the most important:
- Only MBR and GPT partitions are supported.
- Performance restoration will not work if a drive is locked with a user password or if TCG/Opal or Encrypted Drive standards are in place.
- The application will not run unless at least 10% of the drive is free. This is non-negotiable.
- Firmware updates may fail if you are using an AMD drive controller. Samsung notes that the “latest” AMD driver prevents this, but doesn’t actually give the driver version number. AMD’s latest chipset drivers can be downloaded here; users also have the option to simply revert back to Microsoft’s AHCI driver before applying this patch. Any custom storage driver can cause the procedure to fail, so fall back to Intel, AMD’s latest, or Microsoft standard — or hook the drive to one of these ports if you’re currently using a different class of controller (Asmedia, Marvell, etc).
- NTFS is the only file system supported. Mac OS X and Linux patches are coming at an unspecified later date.
- RAID configurations are not supported. Drives configured in RAID cannot be patched at this time.
- Dynamic disks are not supported.
- Your PC will reboot 20 seconds after the procedure finishes. (Really? This is still a thing?)
And finally: If you need to update an 840 Evo with an OS installation on it, make sure you boot from that OS when you do the update. Dropping the 840 Evo in a secondary system and performing the update that way will kill the OS installation (Samsung only says this is due to a “Windows policy.”
The update can be downloaded from Samsung’s website; it doesn’t seem to be searchable from the standard 840 Evo product page.
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