Samsung SSD 845DC PRO

Samsung SSD 845DC PRO
Capacity 400GB 800GB
Controller Samsung MDX
NAND Samsung 128Gbit 24-layer 40nm MLC V-NAND
Sequential Read 530MB/s 530MB/s
Sequential Write 460MB/s 460MB/s
4KB Random Read 92K IOPS 92K IOPS
4KB Random Write 50K IOPS 51K IOPS
Idle Power 1.0W 1.0W
Load Power (Read/Write) 1.7W / 3.1W 1.7W / 3.3W
Endurance (TBW) 7,300TB 14,600TB
Endurance (DWPD) 10 DWPD
Warranty Five years

Surprisingly the 845DC PRO goes for the older MDX controller that was used in the SSD 840 and 840 Pro. Architecturally the MDX and MEX are the same since both are based on the 3-core ARM Cortex R4 base, but the MEX just runs at a higher clock speed (400MHz vs 300MHz). I suspect the MEX controller does not really offer a major benefit for MLC NAND based SSDs because there is much less NAND management to do, but with TLC the extra processing power is certainly useful given the amount of ECC and management TLC needs.

The 845DC PRO is only available in two capacities: 400GB and 800GB. I heard Samsung has plans to add a higher capacity version (1,600GB?) later on but for the time being the 845DC PRO is limited to just 800GB. I suspect that going above 1TiB of raw NAND requires a controller update, which would explain why higher capacities are not available yet. In the end, the 845DC PRO is using silicon that is now two years old, which adds some design limitations.

Similar to the 845DC EVO, the PRO has capacitors that offer data protection in case of a power loss.

The 845DC PRO uses Samsung's first generation V-NAND, which is a 24-layer design with a die capacity of 128Gbit. The part numbers of the first and second generation are almost equal and the only way to distinquish the two is to look at the third, fourth and fifth characters since they reveal the number of die per package as well as the total capacity of the package. Our 400GB sample has four and our 800GB has eight 8-die packages on the PCB, so the raw NAND capacities work out to be 512GiB and 1024GiB respectively with over-provisioning being 28%.

I am not going to cover V-NAND in detail here as I did that in the 850 Pro review and architecturally the first generation V-NAND is no different – it is just 24 layers instead of 32. The first generation is an older, more mature process and thus more suitable for enterprise SSDs. I measured the endurance of the first generation V-NAND to be 40,000 P/E cycles, whereas the second generation V-NAND in the 850 Pro is only rated at 6,000 P/E cycles. For the record, you would either need eMLC or SLC to get 40,000 P/E cycles with 2D NAND, but V-NAND does that while being normal MLC. The benefit over eMLC is performance as eMLC sacrifices program and erase latencies for higher endurance, and the eMLC manufacturing process is also more complicated than regular MLC (although I am pretty sure that V-NAND is still more complicated and hence more expensive).

Samsung SSD 845DC EVO Samsung PM853T
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  • hojnikb - Wednesday, September 03, 2014 - link

    Looks like, they ain't doing Turbowrite on TLC models :) Reply
  • Kristian Vättö - Wednesday, September 03, 2014 - link

    TurboWrite wouldn't make any sense on an enterprise drive because all that matters is sustained performance. Client drives are a different case because IO activity tends to happen in bursts, so having a fast buffer is beneficial (and it makes the drive look better in benchmarks). Reply
  • hojnikb - Wednesday, September 03, 2014 - link

    Yeah i figured as much. Aside from being more suited for short bursts (as you'd said) its really great for marketing department, since they can slap magical 500MB/s+ speeds, that every uninformed buyer is after. 500MB/s sure sells a lot better than 250MB/s (or even less, if we're dealing with lower capacities) :) Reply
  • Spirall - Wednesday, September 03, 2014 - link

    Thanks for the article. Would suggest to make a $/manufacturer declared write endurance comparison (PB writen until manufacturer write warranty end). If there's a chance (due to the difficult on testing time) also make a $/tested write life (PB writen until tested dead). Reply
  • Kristian Vättö - Wednesday, September 03, 2014 - link

    $/endurance is certainly something I will be looking at in the future. This was more of a preview than a full review and in the full review I will have more graphs and comparisons to support the conclusions :)

    As for actual endurance testing, that is something I will not do. It took me two weeks of continuos writing to get the rated endurance to drop by 1% in the 845DC PRO, so it would take way too long to wait for the drive to die.
    Reply
  • hojnikb - Wednesday, September 03, 2014 - link

    And there is a good chance, that thing would still go strong after rated endurance would drop to 0% (unless its hardcapped to die after that). Reply
  • Essence_of_War - Wednesday, September 03, 2014 - link

    Another excellent article, Kristian.

    A question/comment, though, do you think it would make more sense to report the stdev measurements as an "error bar" attached to each data point rather than an additional graph with a 2nd y-axis? I think that might be more compact without having to worry about having multiple y-scales to read. Then it might even be possible to plot an average + error bars data set for multiple different SSDs on the same axis w/o having to worry about which curve is the avg, which curve is the stdev, etc.
    Reply
  • hulu - Wednesday, September 03, 2014 - link

    Another way to present the standard deviation data would be to draw it in two average +/- stdev graphs above and below the average graph. This would better allow visualizing the actual values the average minus stdev has. Reply
  • Kristian Vättö - Thursday, September 04, 2014 - link

    Thanks for the suggestion. I will certainly look into alternative ways to present the standard deviation. I need to redo the graphs anyway for our new suite, so while I'm at it I can see if there is a better way to present the data. Reply
  • Essence_of_War - Thursday, September 04, 2014 - link

    Best luck then, I'm sure you'll figure out a good way to do it. Reply

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