The news of Samsung's SSD 840 EVO read performance degradation started circulating around the Internet about a month ago. Shortly after this, Samsung announced that they have found the fix and a firmware update is expected to be released on October 15th. Samsung kept its promise and delivered the update yesterday through its website (download here). 

The fix is actually a bit more than just a firmware update. Because the bug specifically affects the read speed of old data, simply flashing the firmware isn't enough. The data in the drive has to be rewritten for the changes in the new firmware to take place. Thus the fix comes in the form of a separate tool, which Samsung calls Performance Restoration Software. 

For now the tool is limited to the 840 EVO (both 2.5" and mSATA) and will only work under Windows. An OS-independent tool will be available later this month for Mac and Linux users, but currently there is no word on whether the 'vanilla' 840 and the OEM versions will get the update. Samsung told me that they've only seen the issue in the 840 EVO, although user reports suggested that the 'vanilla' 840 is affected as well. I'll provide an update as soon as I hear more from Samsung.

The performance restoration process itself is simple and doesn't require any input from the user once started. Basically, the tool will first update the firmware and ask for a shut down after the update has been completed. Upon the next startup the tool will run the actual three-step restoration process, although unfortunately I don't have any further information about what these steps actually do. What I do know is that all data in the drive will be rewritten and thus the process can take a while depending on how much data you have stored in your drive. Note that the process isn't destructive if completed successfully, but since there is always a risk of data loss when updating the firmware, I strongly recommend that you make sure that you have an up-to-date backup of your data before starting the process.

The restoration tool has a few limitations, though. First, it will require at least 10% of free space or the tool won't run at all, and there is no way around the 10% limitation other than deleting or moving files to another drive before running the tool. Secondly, only NTFS file system is supported at this stage, so Mac and Linux users will have to wait for the DOS version of the tool that is scheduled to be available by the end of this month. Thirdly, the tool doesn't support RAID arrays, meaning that if you are running two or more 840 EVOs in a RAID array, you'll need to delete the array and switch back to AHCI mode before the tool can be run. Any hardware encryption (TCG Opal 2.0 & eDrive) must be disabled too.

In regards to driver and platform support, the tool supports both Intel and AMD chipsets and storage drivers as well as the native Microsoft AHCI drivers. The only limitation is with AMD storage drivers where the driver must be the latest version, or alternatively you can temporarily switch to the Microsoft driver by uninstalling the AMD driver. Samsung has a detailed installation guide that goes through the driver switch process along with the rest of the performance restoration process. 

Explaining the Bug

Given the wide spread of the issue, there has been quite a bit of speculation about what is causing the read performance to degrade over time. I didn't officially post my theory here, although I did Tweet about it and also mentioned it in the comments of the original news post. It turns out that my theory ended up being pretty much spot on as Samsung finally disclosed some details of the source of the bug.

As most of you likely know already, the way NAND works is by storing a charge in the floating gate. The amount of charge determines the voltage state of the cell, which in turn translates to the bit output. Reading a cell basically works by sensing the cell voltage, which works by increasing the threshold voltage until the cell responds.


However, the cell charge is subject to multiple variables over time. Electron leakage through the tunnel oxide reduces the cell charge over time and may result in a change in the voltage state. The neighboring cells also have an impact through cell-to-cell interference in the form of floating gate coupling, which is at its strongest when programming a neighbor (or just a nearby) cell. That will affect the charge in the cell and the effect becomes stronger over time if the cell isn't erased and reprogrammed for a long time (i.e. more neighbor cell programs = more interference = bigger shift in cell charge). 

Because cell voltage change is a characteristic of NAND, all SSDs and other NAND-based devices utilize NAND management algorithm that takes the changes into account. The algorithm is designed to adjust the voltage states based on the variables (in reality there are far more than the two I mentioned above) so that the cell can be read and programmed efficiently.

In case of the 840 EVO, there was an error in the algorithm that resulted in an aggressive read-retry process when reading old data. With TLC NAND more sophisticated NAND management is needed due to the closer distribution of the voltage states. At the same time the wear-leveling algorithms need to be as efficient as possible (i.e. write as little as possible to save P/E cycles), so that's why the bug only exists on the 840 and 840 EVO. I suspect that the algorithm didn't take the change in cell voltage properly into account, which translated into corrupted read points and thus the read process had to be repeated multiple times before the cell would return the correct value. Obviously it takes more time if the read process has to be performed multiple times, so the user performance suffered as a result.

Unfortunately I don't have an 840 EVO that fits the criterion of the bug (i.e. a drive with several months old data), so I couldn't test more than the restoration process itself (which was smooth, by the way). However, PC Perspective's and The Tech Report's tests confirm that the tool restores the performance back to the original speeds. It's too early to say whether the update fixes long-term performance, but Samsung assured that the update does actually fix the NAND management algorithm and should thus be a permanent fix. 

The EVO has been the most popular retail SSD so far, so it's great to see Samsung providing a fix in such a short time. None of the big SSD manufacturers have been able to avoid widespread bugs (remember the 8MB bug in the Intel SSD 320 and the 5,000-hour bug in the Crucial m4?) and I have to give Samsung credit for handling this well. In the end, this bug never resulted in data loss, so it was more of an annoyance than a real threat.

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  • risingstars - Thursday, October 16, 2014 - link

    Do you and your wife really perform massive amounts of read/write that waiting a month or so is that detrimental?

    I game pretty heavily every day and I honestly do not even notice the problem.
  • djdes - Sunday, October 19, 2014 - link

    > I game pretty heavily every day and I honestly do not even notice the problem.

    You obviously didn't read the article. It's old data, not data that is constantly being written/read. Of course you didn't see an issue. Completely opposite. Waiting a month IS what is detrimental.
  • BobDaMann12 - Tuesday, October 21, 2014 - link

    "You may have missed the part of the article that states the problem only occurs when reading old data" - doesn't that sound much nicer than "You obviously didn't read the article"

    He was just asking a question
  • omgyeti - Friday, December 26, 2014 - link

    Your questions seems to imply that going ahead and wiping her drive so I can update it, and then time machining the data back, is some herculean task. Sure I could've just waited a month for the DOS fix, but it took very little effort to just do it that way and not have to worry about it.
  • skilimavro - Friday, October 17, 2014 - link

    shouldn't complain, at least it will be a few weeks, I'm running Linux... I may never get the update
  • djdes - Sunday, October 19, 2014 - link

    What I did: which refreshed everything (which I will have to repeat):

    1. Shrink ext4 partition to less than half of drive size (smaller the better)
    2. Move partition to opposite side of the drive.
    3. FSTRIM
    4. Expand partition to full size again.

    Doesn't fix the firmware though.
  • Crushnaut - Sunday, October 19, 2014 - link

    If you have the extra space to make that partition to move the data around, just keep the blank partition, install windows into it, get the update tool, run it until the firmware is updated, then blow away the windows partition. That should be a done deal.

    I had to do something similar as the tool wouldn't run on my Windows Server install. So, I installed windows 7 on my other drive, booted into it, installed the firm ware, switched back to Windows Server and blew away the Win7 install.
  • sicid - Monday, October 20, 2014 - link

    This wouldn't work. The tool states that right now it only works with NTFS filesystems
  • Solandri - Friday, October 17, 2014 - link

    From the description of the bug and fix, it sounds like all you have to do on non-Windows OSes is to copy the data off the drive (or make a drive image if it's got system/boot files on it), update the firmware, reformat, and restore the data. It sounds like the firmware fixes the problem of data becoming "stale" on the drive, while the Windows-only tool just moves data around to insure all data has been freshly-written.
  • rewen - Monday, October 20, 2014 - link

    Just pull the drive from the non-Windows machine, put it as a secondary drive in the Windows machine, run the s/w choosing the secondary drive, and then put it back into the Mac.

    There's a drop-down list of connected drives in the s/w, so I have to imagine you could do what I suggested above.

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