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Last reply by 12-19-2020 Unsolved
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3 Silver

PM851 SSD Slowdown (Like SSung's 840EVO)??

I noticed the 9530 XPS 15 laptop with the PM851 512GB SSD getting slower. Knowing it is a Samsung supplied msata drive, I was suspecting it had the same slowdown problem as the 840 EVO.

I ran HD tune and found several areas of the drive that had about 30MB/s read speed - less than about 10% of the rest of the drive.

Note that benchmarks such as ATTO only read/write very small parts of the drive and will NOT find this problem. HD tune (and HD tach) actually measure across the entire drive.

So I re-installed the factory OS from a USB image made from Dell Backup and recovery - which completely reformats and re-writes to the drive.

I found those really slow spots were gone, and the average access time of the reads dropped by a factor of about 3, and the maximum access time observed dropped by a factor of 10.

Anyone else seen this, or know of a firmware update to fix this?

(The documented problem with the Samsung 840evo is that when data is 'old' (about a month or so), the read speed of that old data drops like a lead balloon.)

So unless a fix is found, the drive will again become just as slow in a month or so.

Replies (11)
2 Bronze
2 Bronze

Having the same problem as you. The PM851 is getting slower and I have random lag spikes from time to time. I ran several HD tune scans and the slow parts will EXACTLY overlap so its not something related to random SSD usage by a process or virus scanner or anything of the sorts.

See the image below, scan #1 is a dark blue line and I overlayd scan #2 on top as a light blue line you can see during the slowdown both lines overlap perfectly. My slowdown is now down to 25mb/s.

I used to have the same problem with 840 EVO on my Desktop computer but that got fixed by samsung's restoration tool (link) . However this tool is not compatible with PM851 drives.

So is Dell talking to samsung to make them release a restoration tool for the PM851 drives?

2 Bronze
2 Bronze

I'd like to know this as well! :)

My PM851 is a new drive/fresh install but I'd still like to head this off at the pass before it actually becomes a problem.

Speeds for me are currently showing 540/260-ish, if they can be better than this due to the restoration tool etc I'd certainly like to get it fixed ASAP.

I wish the Samsung Magician tool was compatible too.. are there any other really good tools that are similar to this that I could look at installing?

2 Bronze
2 Bronze

I added a 500GB mSATA 840 EVO to my Precision M4800. After using it for some weeks, the slow read speed problem showed up.

After doing some research, I decided to perform a monthly "refresh" of the partitioned area in the SSD  (I`ve left about 40% unallocated space for over provisioning).

I do it with DiskFresh, which can be downloaded from

2 Bronze
2 Bronze

This thread is about the PM851 model.

You can permamently fix a 840 evo with the patch from samsung.

Just google Samsung SSD 840 EVO Performance Restoration Software.

2 Bronze
2 Bronze

Well, not really. The patch Samsung issued some months ago works when applied (it can be run only once), but read speed goes down again after some weeks. That's why I'm "refreshing" mine with diskfresh, with very good results so far. If the cause of the problems with PM851 is the same as with 840 EVO, this workaround should be of help.

3 Silver

Thanks for the responses. This confirms my suspicions. The HDtune pic posted looks very much like my experience. Erratic, bouncy  overall read speeds and very very slow in spots.

Note the access time scale on that posted chart....will mention again a bit later...  

As far as 840EVO's, it is a very good assumption that they ALL have the slowdown problem and the Dell drive is a derivative of that one. Again, benchmarks such as ATTO will not find the problem.

So I picked up an 850EVOmsata 500G and replaced the Dell drive. With the 850,HD tune essentially flat lines around 500-520 MB/s, and essentially ALL of the read access times are in the .03ms range - not a typo. The HDtune chart for access time for the 850EVO only goes up to 0.55ms on the scale. Some 10x or better faster access times than the 851!

Samsung led Dell down the rosy path with this one. Can't blame Dell - they had no idea at the time.

But Samsung may take their own sweet time issuing a firmware for the Dell OEM'd drive, and keep in mind, the 840EVO's may have this problem permanently built in, and only a monthly drive re-write will be a fix for it.

3 Silver

Here are some screen shots of HDtune -  Dell 851 drive after 4 months (bad, but not as bad as many examples)


dell 851 drive after full factory image refresh using Dell image written to a USB. Note how access times improved overall and the access time scale changed due to the lower variability in this 'freshened' state

850 EVO after full system re-image from that same USB with the dell factory image. Note how low and consistent the access time is compared to the 851 drive - even with the 851 in a 'freshened' state.. And how comparatively steady the read throughput is.

2 Bronze
2 Bronze

So is Dell ever going to update the firmware of the 851 to allow us to use the Magician/Restoration tool etc?

Thank you!

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