Samsung SSD 840 (250GB) Review
by Kristian Vättö on October 8, 2012 12:14 PM EST- Posted in
- Storage
- SSDs
- Samsung
- TLC
- Samsung SSD 840
Lower Endurance: Hardly an Issue
With perfect wear-leveling and write amplification of 1x, you would get 256,000GiB (that's ~275TB) of writes out of a 250GB Samsung 840 with TLC NAND and 1,000 P/E cycles. That is still a lot but wear-leveling and write amplification are never perfect. Giving any specific numbers for endurance is hard because every drive behaves differently and users have different workloads, but it's unlikely for a light consumer workload to see more than 10GiB of writes per day. That's 3,650GiB per year, which is only 1.4% out of 256,000GiB. In the real world NAND writes will be bigger than host writes but even with a write amplification factor of 10x, you will only end up writing 36,500GiB each year and exhausting ~143 P/E cycles out of the available 1,000. In other words, it would take roughly seven years for you to wear out the NAND.
| SSD Lifetime Estimation | ||||
| NAND | MLC—3K P/E Cycles | TLC—1K P/E Cycles | ||
| NAND Capacity | 128GiB | 256GiB | 128GiB | 256GiB |
| Writes per Day | 10GiB | 10GiB | 10GiB | 10GiB |
| Write Amplification | 10x | 10x | 10x | 10x |
| Total Estimated Lifespan | 10.5 years | 21.0 years | 3.5 years | 7.0 years |
For the 120GB Samsung 840, the lifespan is half of the 250GB model but we are still talking about years. Samsung doesn't offer a 60/64GB Samsung 840, although that makes sense as it wouldn't be hard to wear that out in less than three years, which is the warranty Samsung gives to the 840 SSD.
DSP to the Rescue
However, there is actually more to SSD endurance than just P/E cycles and write amplification. There has been a lot of talk lately about digital signal processing (DSP) in the industry, which is supposedly the solution for lower endurance NAND.
The basic idea behind DSP is very simple: you read changes in voltages and adapt to the changes. As I mentioned in the previous page, the voltages change as the NAND wears out and if your controller can't adapt to the changes, you'll be stressing the NAND even more. Each time you're trying to program or erase the cell, you are wearing it out, so you don't even have to succesfully program or erase the cell to cause damage. That's why the guess and test process for writing to NAND is so harmful; it may take multiple tries and each try will wear out the NAND even more.
Graphical presentation of a change in voltage state
However, if your controller can read the changes in program and erase voltages, you will know what voltages to use to program/erase the cell. Even though DSP doesn't make NAND immortal, it causes a lot less stress on the NAND, allowing it to last for more P/E cycles than what you would get without DSP.
Again, it's hard to give out any specific numbers of DSP usefulness in real world, but for example STEC is claiming that their CellCare technology can extend the endurance of regular 3K P/E cycle MLC up to as much as 60K. I've heard unofficial figures as high as 100K for some companies' DSPs, but I would take all figures with a grain of salt until they are tested by a third party. Either way, even if a good DSP is only able to double the endurance of NAND, it's a huge deal as we move to even smaller process nodes and possibly even more bits per cell.
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JarredWalton - Monday, October 8, 2012 - link
What price are you looking at? The table shows $199.99 for the 250GB Samsung 840 (MSRP) and $249.99 for the 256GB Samsung 840 Pro.Kevin G - Monday, October 8, 2012 - link
On the fourth page:"With perfect wear-leveling and write amplification of 1x, you would get 256TiB of writes out of a 250GB Samsung 840 with TLC NAND and 1,000 P/E cycles. "
Shouldn't it be 256 GiB?
JarredWalton - Monday, October 8, 2012 - link
No; he's accounting for the visible storage capacity (and spare area). So 250GB is 256GiB of NAND but only 250GB end-user storage. You can still write 256TiB of data (256GiB * 1000 P/E), which means on a 250GB SSD you end up with the ability to write 281TB of data (for an effective P/E cycles of 1125).schizoide - Monday, October 8, 2012 - link
I'm not comfortable with TLC, particularly since it doesn't come at a huge absolute cost savings. I've been burned too often with SSDs, and now only buy the most bulletproof devices available. Maybe next generation. Maybe.Death666Angel - Tuesday, October 9, 2012 - link
I wouldn't draw any conclusion for pricing just yet. Everyone is comparing street prices of current gen SSDs to MSRP of next gen SSDs. Give it a few months to get the 840 into stores and selling and then we'll se how prices fair.I'm personally fairly confident that there will be a distinct cost advantage.
name99 - Tuesday, October 9, 2012 - link
Have ANY SSDs given you a failure based on the flash cells themselves, rather than on crappy firmware?It's stupid to demonize a technology because some companies sold you a bad product --- especially when your response is to refuse to buy a product from a company that is known (in this respect at least) NOT to have shipped crappy firmware.
harijan - Monday, October 8, 2012 - link
Pretty sure 3650GiB != 3.65TiB.3650GB == 3.65TB.
It doesn't do anything to your percentages, but this is Anandtech, we hold you to higher standards ;)
Kristian Vättö - Tuesday, October 9, 2012 - link
You are correct, I forgot that binary units don't scale up linearly (1000GiB is not 1TiB); metrics are just so much simpler. Thanks for the heads up, I've fixed the math now.iaco - Monday, October 8, 2012 - link
I wonder if Samsung will ever make a SSD made of SLC NAND. The performance would be amazing.Conficio - Monday, October 8, 2012 - link
First thanks to the Kristan and the Anand Team for another comprehensive review and the inclusion of in depth knowledge about the technology behind it.I'd like to see included in the test (or in another article) a review of the available tools for each SSD. I think it is important to know if the manufacturer supports its tools to low level format, secure erase, TRIM, ROM update etc. beyond the obligatory Windows (7). Are those available for Linux (command line and/or GUI)? Are they available for Mac OSX? Do they work if the drive is connected over USB? Are these tools user friendly to use?
It is useless if I buy an SSD and have to find out that Mac OSX does not support the tools. I have a couple of Vertex drives which I can't find a way to secure erase in order to restore full performance. They are pre TRIM drives and Mac OS X does not support trim on them anyway.
As with motherboards, the the BIOS or UEFI is important, so is with SSDs the ability to actually perform some of the low level functions. I hope you can add that to your workload! A comparison of the different tools for each manufacturer for current drives would be a great start. Thanks again!