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Samsung SSD 840 250GB review: affordable and fast?

New generation SSD with triple level cell memory

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Calculating TLC effects

Update [20-6-2013]: it turns out that our initial calculations and conclusions were off the mark, when discussing the lifespan of TLC memory chips. After thoroughly testing the lifespan of Samsung 840 SSDs over the course of 2.5 months, our new and more accurate conclusion is that you can expect these SSDS to last you about 75 years. Click here to read the full story.

Samsung appears to have taken preventative measures with the 840 SSDs, by reserving a relatively large amount of space for overprovisioning. The storage capacities are 120, 250 en 500 GB, while the flash chips have available 128, 256 and 512 GB (or GiB). Even when the SSDs are (almost) completely full, this leaves enough room for wear-leveling.

Let's examine this, for the example the 120 GB model. In our calculations we make a distinction between so-called gibibytes (1 GiB = 1024x1024x1024 bytes) and gigabytes (1 GB = 1000x1000x1000 bytes). We usually don't do this in reviews in the interest of readability, but we'll make an exception here.

In a Samsung 840 120 GB SSD there are 128 GiB worth of flash chips. The available storage capacity is 120 GB, but Windows uses GiBs which is why you in Windows will see 111.8 GiB actually available. That means the SSD offers 12.5 percent less storage than actually exists, and that is the overprovisioning.

In our example we will examine a normal scenario and an almost worst-case scenario. In the first case the available storage is 50 percent full, leaving 72.1 GiB including overprovisioning. In the second scenario the SSD is filled 90 percent, leaving 27.4 GiB.

Then we have to make an assumption. The experts haven't quite agreed on yet what the average amount of data is that is written to a disk in Windows. It's more than you think. Hibernation, for example, writes all the contents of your RAM to a file. It's generally thought that between 5 GB and 10 GB per day is typical for consumer PCs. Using those numbers, it takes 7.2 days and 2.7 days, respectively, before each available memory cell has been written to once. This also assumes that the wear leveling algorithm of the SSD works optimally and all memory cells are equally used.

If the memory cells can be overwritten 750 times, that is equal to 5,409 days or 14.8 years (normal scenario) and 2,056 days or 5.6 years (almost worst-case).

The chart below shows our calculations for the 120 GB model and the 250 GB model.

SSD / Percentage full250 GB / 50%250 GB / 90%120 GB / 50%120 GB / 90%
Capacity flash chips 256 GiB 256 GiB 128 GiB 128 GiB
Available capacity OS 250 GB 250 GB 120 GB 120 GB
Available capacity OS 232.8 GiB 232.8 GiB 111.8 GiB 111.8 GiB
Percentage used 50% 90% 50% 90%
Available flash memory 139.6 GiB 46.5 GiB 72.1 GiB 27.4 GiB
Writes per day 10 GiB/day 10 GiB/day 10 GiB/day 10 GiB/day
# of days for filling all chips 14.0 4.6 7.2 2.7
# of days for filling 750x 10468.9 3483.9 5409.0 2056.3
# of years for filling 750x 28.7 9.5 14.8 5.6

With more intensive use the picture changes. We know that Intel validates its SSDs at 20 GiB written per day, and bases the warranty period on that. 20 GiB/day is probably a good estimate for someone that uses their PC intensely or for professional ends. In that case the 50 percent full SSD will last 3.6 days, and the 90 percent one 1.4 days before each memory cell has been written to once. The lifespan then becomes 7.4 years and 2.8 years, respectively.

SSD / Percentage full250 GB / 50%250 GB / 90%120 GB / 50%120 GB / 90%
Capacity flash chips 256 GiB 256 GiB 128 GiB 128 GiB
Available capacity OS 250 GB 250 GB 120 GB 120 GB
Available capacity OS 232.8 GiB 232.8 GiB 111.8 GiB 111.8 GiB
Percentage used 50% 90% 50% 90%
Available flash memory 139.6 GiB 46.5 GiB 72.1 GiB 27.4 GiB
Writes per day 20 GiB/day 20 GiB/day 20 GiB/day 20 GiB/day
# of days for filling all chips 7.0 2.3 3.6 1.4
# of days for filling 750x 5234.4 1742.0 2704.5 1028.1
# of years for filling 750x 14.3 4.8 7.4 2.8

This still doesn't mean that you need to worry. A lifespan of 5.6 years and 14.8 years for normal daily use is still quite acceptable, in our opinion. However, people that use their PC intensively or for professional purposes should probably instead get an 840 Pro or different MLC SSD.


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