Samsung SSD 840 EVO Review: 120GB, 250GB, 500GB, 750GB & 1TB Models Tested
by Anand Lal Shimpi on July 25, 2013 1:53 PM EST- Posted in
- Storage
- SSDs
- Samsung
- TLC
- Samsung SSD 840
TurboWrite: MLC Performance on a TLC Drive
All NAND trends towards lower performance as we move down to smaller process geometries. Clever architectural tricks are what keep overall SSD performance increasing each generation, but if you look at Crucial's M500 you'll see that it's not always possible to do. Historically, whenever a level of the memory hierarchy got too slow, the industry would more or less agree to insert another level above it to help hide latency. The problem is exascerbated once you start talking about TLC NAND. Samsung's mitigation to the problem is to dedicate a small portion of each TLC NAND die as an SLC write buffer. The feature is called TurboWrite. Initial writes hit the TurboWrite buffer at very low latency and are quickly written back to the rest of the TLC NAND array.
Since the amount of spare area available on the EVO varies depending on capacity, TurboWrite buffer size varies with capacity. The smallest size is around 3GB while the largest is 12GB on the 1TB EVO:
| Samsung SSD 840 EVO TurboWrite Buffer Size vs. Capacity | |||||||
| 120GB | 250GB | 500GB | 750GB | 1TB | |||
| TurboWrite Buffer Size | 3GB | 3GB | 6GB | 9GB | 12GB | ||
I spent some time poking at the TurboWrite buffer and it pretty much works the way you'd expect it to. Initial writes hit the buffer first, and as long as they don't exceed the size of the buffer the performance you get is quite good. If your writes stop before exceeding the buffer size, the buffer will write itself out to the TLC NAND array. You need a little bit of idle time for this copy to happen, but it tends to go pretty quickly as it's just a sequential move of data internally (we're talking about a matter of 15 - 30 seconds). Even before the TurboWrite buffer is completely emptied, you can stream new writes into the buffer. It all works surprisingly well. For most light use cases I can see TurboWrite being a great way to deliver more of an MLC experience but on a TLC drive.
TurboWrite's impact is best felt on the lower capacity drives that don't have as many NAND die to stripe requests across (thus further hiding long program latencies). The chart below shows sequential write performance vs. time for all of the EVO capacities. The sharp drop in performance on each curve is when the TurboWrite buffer is exceeded and sequential writes start streaming to the TLC NAND array instead:
On the 120GB drive the delta between TurboWrite and standard performance is huge. On the larger drives the drop isn't as big and the TurboWrite buffer is also larger, the combination of the two is why the impact isn't felt as muchon those drives. It's this TurboWrite buffer that gives the EVO its improvement in max sequential write speed over last year's vanilla SSD 840.
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Timur Born - Sunday, July 28, 2013 - link
Just did a quick test: On my 8 gb RAM system Windows 8 uses quite exactly 1 gb for write caching and all available RAM for read caching. It doesn't matter whether the 1 gb consist of one or several files and whether they fit into the cache as a whole or not (first 1 gb is cached if not).1Angelreloaded - Friday, July 26, 2013 - link
Hold on a second correct me if I'm wrong on this paradox. Did Samsung not scale back on NAND production in order to drive the price up for greater bloated profits, now as stated in Korea press conference they want "SSDs for everyone". WTF is going on here, and why are SSDs not at more reasonable pricing by now about .33cents per gig.?They had a complete shot at burying HDDs after the flood and the price hike.FunBunny2 - Friday, July 26, 2013 - link
Don't confuse capitalists with intelligence. They look at unit margin and ignore gross profit. IOW, they'd rather sell 100 at $2 margin than 1,000 at $1 margin. They're stupid.MrSpadge - Saturday, July 27, 2013 - link
There's also the factor of marget saturation to take into account. You can't sell an infinite number of drives.Notmyusualid - Friday, July 26, 2013 - link
Exciting technology indeed! Impressive numbers, nice identification of spare computer resources, and put to good use too. I'd imagine this would be the go-to drive for most users...But I'd like my clocks available for my applications thanks.
In addition, I'm not willing to put my data on any non-enterprise disk now, cost be damned. Burned too many times now.
Interesting product though....
z28dreams - Friday, July 26, 2013 - link
I recently saw the Plextor M5P (pro) for $190 on sale.If the 840 evo comes out in the same price range, which would be a better buy?
It looks like the write seems of the M5P are better, but I'm not about overall performance.
K_Space - Friday, July 26, 2013 - link
Help a noob here: How is Rapid any different to a custom nonvolatile RAM disk with your selected cached files stored on it & these being written to the SSD at an interval? Is it mainly because Rapid can writes in blocks and it's more intelligent in its choice of cached files?wpapolis - Saturday, July 27, 2013 - link
Hey there all,I have a MacBook 13" from late 2008, the first gen of the unibody construction (Model MB467*/A).
My bus speed is SATA 3Gbit/s.
What's the best SSD for me?
Trim doesn't work automatically for me, though I have found the commands to use in terminal to enable it.
This Samsung drives looks really good, but it seems like I won't be able to use RAPID, or perhaps even TRIM. Plus I am limited by my bus speed. Should I still go for this Samsung just because the price might be the same as lower featured alternatives?
What do you guys suggest? I want one in the 250GB range.
Thanks,
Bill
TheinsanegamerN - Saturday, July 27, 2013 - link
if you have a mac, the samsung is your best bet. TRIM can be enabled quite easily on a mac if it is not done automatically, so you can use trim. as for RAPID, it replaces window's terrible i/o caching process. osx does not have this problem, so you dont have to worry about that. now, the sata2 interface will be a bottleneck, but it will still be much faster than a hard drive. id go for either this evo drive or the 840 250gbwpapolis - Saturday, July 27, 2013 - link
Yes, you reaffirmed what I was already thinking.Plus, when I upgrade this MacBook, I have the option to move the drive. Though, I have to say, performance is still pretty good, but each OS upgrade seems to make things a bit more sluggish.
With 8GB RAM, and a current SSD, things should be good for a bit longer.
Thanks for the feedback,
Bill