Samsung 850 Evo performance problems with AHCI

[Keolv]

So I have an 850 Evo 120GB running on a SATA 2 AMD system with AHCI and a fairly old chipset (760G) on Windows 7 64 bit.
The IOPS are around 30k and the top speeds are about 250MB/s which is fine by me (SATA 2 after all), assuming everything else works fine. The drive is about 65% full. TRIM is turned on, I also have 10GB unallocated for over-provisioning in case I forget to free up space.
However, I've had severe driver problems. The first AHCI driver was causing extremely high CPU interrupts (25%), and after some testing I managed to find a proper driver. Now I have 0.25 - 1% interrupts at all time, which is still bad but ok.
The latest problem is that I seem to have a very low write speed and low write IOPS when I have the "Turn off Windows write cache buffer flushing on the device" checkbox unchecked. When I check it, everything is fine and I have around 250MB/s write and read speeds and 30k IOPS.
First question, what exactly are the implications of this box being checked ? Is data loss a big possibility ?
Second, what exactly is going on ? Is the motherboard somewhat not compatible with the SSD or I just get a really, really bad selection of AHCI drivers ? What is causing the high interrupts ?

 

[evilsofa]

From Samsung's SSD white paper:

While using the Windows Write-Cache Buffer feature, any data stored in the Cache Memory is lost or damaged in the event of a device failure or sudden loss of power. The Windows Write-Cache Buffer Flushing feature should be enabled to ensure data integrity. In particular, this feature is important if your SSD is being used in a server application. When using an SSD and HDD together, it is always recommended to have this feature ON. Because all configuration profiles use the “Write-Cache Buffer” feature, this system setting is toggled ON for all profiles. Samsung stronlgy recommens that Write-Cache Buffer Flushing be enabled at all the times. However, if you are more concered with performance than data integrity, you are welcome to disable this feature to eliminate extra commands being sent to the SSD to flush the data.

In summation, Samsung is saying you really want that option to be on, and you've found out why it needs to be on.

I can't comment on your AHCI driver issue. Others may be able to help you better if you report the motherboard's full make and model number, and exactly what AHCI drivers you have been using.

 

[Keolv]

So the standard AHCI Windows 7 driver has high CPU interrupts (25-30%) while doing a high IOPS activity.
The newest AMD AHCI driver has 30% Interrupts at all times.
The fairly old AMD AHCI driver from the motherboard's website has 2-3% interrupts max, but unless I check "Turn off Windows write cache buffer flushing on the device" it has 91 write IOPS, and after I check it, it's back to 30k write IOPS.