Latency Percentile and TurboWrite Cache Testing
Latency Percentile
For this article, I'm going to put a bit of a twist on our Latency Distribution and Latency Percentile testing. In order to better approximate actual consumer usage, I will be intentionally pacing the test at a 50% duty cycle. This should give us random write results closer to what typical users will actually see, and the majority of the latency results should result from writes to the SLC area of these caching SLC/TLC hybrid solutions.
The four results include a first pair, in which the 120GB and 250GB 750 EVO saw a 100 second continuous 4k random run at QD=4. The second two results are of the same pair of SSDs seeing the exact same workload, but at a 50% duty cycle. This means the workload ceased every two seconds, giving the SSD two seconds of idle time between pulses.
It's interesting to note that in both cases, all capacities of the 750 EVO perform in an extremely tight latency band at QD=4. I had originally expected the 50% duty cycle to yield more of a different result, but as it turns out, QD=4 was not high enough to saturate the TurboWrite buffer over the 100 second continuous test run, making the results for both test cases nearly identical. We did note an additional cluster of ~100 total IOs falling at the ~100 microsecond mark in the intermittent run, but that can easily be explained as the first IO of each two second burst, which saw the 750 coming out of what was previously an idle state (where it was likely emptying its SLC buffer into TLC). To demonstrate just how tight the latencies are here, take a look at the percentiles:
Such a tight band across two different conditions and two different capacities is mighty impressive. To give this some additional perspective, lets add some preliminary results from a pair of other competing low cost SSDs into the mix:
What we see here are a few things (after noting that I had to increase the range on the chart by another 10x to include the slower results). First, the OCZ Trion SSDs both run out of cache at different percentages of the 100 second run (~22% for the Trion 100 and ~73% for the Trion 150). We also see that the SLC latency of the Trion SSDs is much greater (~47 us) than the 750 EVO (~27 us, or 1.7x faster response per IO). These latencies taper off to far greater figures as the Trion Series products fill their cache and transition to TLC writing. Once this happens, the performance gap increases to a range of 5x – 13x for that fraction of writes going to TLC on the Trions. Note from the legend that IOPS are similar across all of the models / situations tested, but the latency of those IOs is what really dictates how SSDs 'feel' in actual use.
As you can see, with proper context, the previously boring and tightly packed 750 EVO results can show a bit more perspective on just how good they actually are.
Cache Testing
Now lets wrap this up with a quick check on sequential copies to the 750 EVO from a fast source:
120GB:
250GB:
Apologies for the overlapping window inadvertently captured in that last one. The test file here was ~4GB, and we can see the obvious drop from SLC speed down to TLC speed, which takes place at the 3GB mark at both capacities. That slower sustained write speed comes out to 133 MB/s and 211 MB/s when tested within Windows (note: the Windows write cache is disabled for this test, so sustained speeds are a tad higher in real use). These are a bit shy of the original sustained ratings of the 850 EVO, but we suspect some additional care is being taken during writes as a means to correct the slow down issue originally present in the 840 EVO.
it’ll soon be 10 cents per
it’ll soon be 10 cents per gb! ;-p
I thought Samsung had decided
I thought Samsung had decided to kill of the 120/128GB models as 250/256GB SSDs cost less than 100$.
I don’t see the point of
I don’t see the point of 120gb HDD’s anymore, at least not at only $20 less than a 240gb. Sure, they are just going to be used for boot drives only, but the 240’s always seem to perform better so you’d want to go that route for a boot drive anyway and reap the benefit of being able to put more of your most used apps on it.
They look ridiculous when
They look ridiculous when opened up. I guess we need a different form factor for SSDs.
hehe, I totally agree, 2.5″
hehe, I totally agree, 2.5″ doesn’t make any sense for these drives. Luckily we have m.2
Yeah this is what mSATA and
Yeah this is what mSATA and M.2 are for 🙂
I am almost surprised they
I am almost surprised they didn’t just put an m.2 SATA device in there with a little adaptor.
I don’t know if I can trust
I don’t know if I can trust Samsung after that massive Evo debacle, at least not without waiting a year or two while everyone beta-tests this drive. The three year warranty doesn’t really inspire much confidence either. Kind of puts me off SSDs entirely.
I agree with you, and we will
I agree with you, and we will leave data on these for safe keeping / future retests, but Samsung specifically pointed out to me that the 750 EVO will not see a repeat of the slow down issue seen on the 840 EVO. I believe them for four reasons:
I have two 840 EVO 250 GB
I have two 840 EVO 250 GB running as extra drives on this system (with an 850 EVO 500GB as system drive) and I still see the slowdown after the firmware fix. Every 3-4 months or so I have to run the Advanced Performance Optimization in Magician, the firmware itself is not enough for older files.
Now I have hammered these drives a bit using them as temp/caching for torrents etc. but still…
Random writes from torrent
Random writes from torrent writing will slow down drives for reasons other than the fixed issue – you're fragmenting the flash itself. Performance optimization forces a defrag of the flash (in addition to rewriting everything), which explains the recovery you are seeing.
But who on earth would buy a
But who on earth would buy a 120GB SSD at today’s prices? I wouldn’t even touch a 250GB drive now, the cost per GB has fallen so much, but at least that should be the new entry level, just drop the 120GB unit and do the cheapskates a long term favour.
120GB actually still works
120GB actually still works when you consider typical desktop folks using mostly productivity apps, especially if they have the bulk storage handled by their home NAS. Can't disagree on that $20 difference to double the capacity though. It's almost a no brainer decision.
“Lets compare those prices to
“Lets compare those prices to what we saw for the 840 EVO launch back in July of 2014”
You mean 2013.
Fixed. Thanks!
Fixed. Thanks!
Even more interesting pricing
Even more interesting pricing wise is launch 840 Evo 512 GB vs 950 Pro. 950 Pro is less for like 5x performance in 2.5 years. Can we get another 5x in next few years?
How do you feel about raid 0
How do you feel about raid 0 with 2 (or 4 because of price) 750’s
These should do just fine in
These should do just fine in RAID-0, and a 4-SSD array of these would be 1TB for $300. Also, it might actually outperform a single 950 Pro when it comes to low latencies seen in SSD RAID.
I wish companies would stop
I wish companies would stop cranking-out these cookie-cutter SATA-limited drives and start focusing on PCIE nvme.
I’d love to replace the slow PM951 m.2 PCIE drive that shipped in my new XPS15 with an 850 Pro, but the price per GB is still too high.
Some competition would be nice to drive prices down, and really, every SSD these days pegs SATA, so why keep ‘innovating’ in that space?
It’s all in the name of
It’s all in the name of driving down $/GB. I agree that we would all like to see m.2 PCIE nvme $/GB come down as well, but I think a lot of the more casual users and OEMs are just looking for the lowest $/GB, and with SATA maxed out that’s more than enough performance for that class of user.
They cant drop SATA as then
They cant drop SATA as then the market shrinks by a massive amount, probably 90%+, blame intel for choosing to not provide nvme bios updates for older chipsets.
Maybe I missed it, but will
Maybe I missed it, but will there be an M.2 form factor?
“I don’t see the point of
“I don’t see the point of 120gb HDD’s anymore,”
“But who on earth would buy a 120GB SSD at today’s prices? I wouldn’t even touch a 250GB drive now,”
“I wish companies would stop cranking-out these cookie-cutter SATA-limited drives and start focusing on PCIE nvme.”
“Maybe I missed it, but will there be an M.2 form factor?”
Guys-go back to page 1-
“Samsung will drop the 120GB capacity of that line and replace it with a new OEM / system builder destined 750 EVO:”
These drives are mainly for OEM’s-And they pay much less than we do.I’d much prefer to see a 120 GB SSD in a $300 lappy than
5400 RPM Spinning rust….
It’s not certain yet what availability of these drives will be in retail……………………………………….
The 120GB are really for
The 120GB are really for legacy computers. Some older hardware is limited in the amount of storage it can recognise. I just installed a 120GB 750EVO in my old Netbook. I use Lubuntu and a limited number of applications, I still have 112GB free as it stands, largely because I don’t put movies or music on my netbook.