Samsung 850 EVO 250GB SATA 2 instead of 3

The laptops (and motherboards) from that era typically had 2 SATA 3 ports from the Intel chipset, and 4-6 SATA 2 ports added on from a third party chipset (usually Marvell). If a budget laptop developer was having problems getting the Intel SATA 3 drivers to work with their hardware, but their Marvell SATA 2 drivers were working fine, they could've taken the easy way out and simply connected the laptop's devices to the SATA 2 ports, instead of fixing the SATA 3 ports.

I wouldn't worry about it though.


1. SATA 2 is limited to 300 MB/s while SATA 3 can reach 600 MB/s. That sounds like a scary huge difference, but it isn't. The 300+ MB/s speeds are only for sequential read/writes. Most users only hit these speeds when they're copying movies. If you're not spending all day copying movies from one SSD to another SSD, you can pretty much ignore the sequential read/write speeds.

The 512k and 4k read/write speeds are much more important to typical computer use. 512k roughly measures speeds for files a half MB to a few MB in size. 4k measures speeds for small files like Word documents. The vast majority of most people's computer use falls into these sizes. Most modern SSDs top out at about 300 MB/s at 512k read/writes, and 30-70 MB/s for 4k read/writes.

Both of these are within SATA 2's limits. So in practical use, unless you're in the habit of making copies of large movie files on your SSD, you won't be able to tell the difference between SATA 2 and SATA 3. If you don't believe me, just read this article. You'll notice SATA 3 is impressively faster in benchmarks. But there's practically no difference in real-world usage tests. Real-time video editing is about the only practical use which takes advantage of SSD sequential speeds.

http://www.tomshardware.com/reviews/sata-6gbps-performance-sata-3gbps,3110-7.html


2. t's the 4k speeds which make SSDs feel so much faster than HDDs.

• 
  ■A HDD can hit about 125 MB/s sequential speeds, while a SSD can hit 500 MB/s, so the SSD is just 4x faster.
  ■A HDD can hit 1 MB/s 4k speeds, while a SSD can hit 30-70 MB/s, so the SSD is about 50x faster.

So when you're doing sequential disk transfers, a SSD is "oh nice, that's faster" than a HDD. But when you're doing 4k disk transfers, a SSD is "HOLY !@#$ THAT's FASTER!!!" than a HDD.


3. MB/s is the inverse of how we perceive speed (how much time we have to wait). So the bulk of the speed increase actually happens at the lower MB/s improvements. Imagine you needed to read 1000 MB of data.

100 MB/s (hard drive) = 10 sec/GB
250 MB/s (SATA 2 SSD) = 4 sec/GB
500 MB/s (SATA 3 SSD) = 2 sec/GB
1000 MB/s (early PCIe) = 1 sec/GB
2000 MB/s (current PCIe) = 0.5 sec/GB

You see how every time MB/s doubles, the reduction in wait time is halved? Going from a HDD to a lousy SATA 2 SSD saves you 4x as much time as going from a SATA 3 SSD to a modern PCIe SSD! Or put another way, switching from a HDD to

• 
  ■SATA 2 SSD gives you 63% the wait time reduction of a modern PCIe SSD.
  ■SATA 3 SSD gives you 83% the wait time reduction of a modern PCIe SSD.
  ■early PCIe SSD gives you 95% the wait time reduction of a modern PCIe SSD.
  ■modern PCie SSD gives you 100% the wait time reduction of a modern PCIe SSD.

So even at sequential accesses where SATA 3 actually makes a difference, the SSD on SATA 2 is giving you 75% the wait time reduction compared to a hard drive that you would've gotten with SATA 3. Not 50% like the MB/s figures would make you think.


4. The slower operation has the larger impact on wait time. If you're copying 1 GB of sequential data and 1 GB of 4k data, the total time it takes on a SSD with:

• 
  ■250 MB/s sequential speeds and 20 MB/s 4k speeds, total time is (1000/250)+(1000/20) = 54 seconds
  ■If you double sequential speeds to 500 MB/s, total time is now (1000/500)+(1000/20) = 52 seconds
  ■If you instead double 4k speeds 50 40 MB/s, total time is (1000/250)+(1000/40) = 29 seconds

So improving the slower operation yields a bigger reduction in wait time.


5. The important thing with a SATA 2 interface is to make sure you've enabled AHCI mode. AHCI includes NCQ - native command queuing. That's the ability of the drive to handle multiple file requests at the same time. SSDs are so fast that they can handle small file requests faster than the OS can make them. AHCI enables the OS to request multiple small files at the same time, and can result in about a 10x increase in small file read speed with 16-32 simultaneous requests. That's going to matter more than your laptop not having SATA 3.


tl;dr - People use the wrong benchmark when picking a SSD. The smallest MB/s results in the longest amount of time we have to wait. So what we really should be looking for is the SSD whose slowest MB/s benchmark (the 4k speeds) is the fastest. You can pretty much ignore the sequential MB/s speeds unless you regularly copy large movie files from one SSD to another or do real-time video editing.