A Closer Look at AHCI vs RAID

There has been somewhat of a debate over what the correct / optimal setting should be for the NVMe SSDs in the XPS 13 9350 and XPS 15 9550. The reasons for picking one or the other have changed over time, but there has never been one conclusive answer. My goal here is to try to find that answer, or at the very least compile some relevant information.

Many people received or are receiving their XPS machines in with the SSD set in RAID mode. However, I have personally heard from a few people that after sending their machines back to dell for maintenance, they have been shipped back in AHCI mode. Peculiar.

According to information on the Intel’s site about the Rapid Storage Technology driver (the one that is needed during Windows re-installation when in RAID mode) they state that the driver “supports NVMe devices”. When in RAID mode, consulting the machine’s boot menu will show you the controller type listed for the drive is “NVMe” as seen in this picture. This is a good sign, and the correct controller for the our PCIe SSDs. However, the storage controllers listed in Device Manager (in RAID mode) are Intel Chipset SATA RAID Controller and Microsoft Storage Spaces Controller, as seen here.

For those of you who don’t know, AHCI is a different controller type. NVMe is a newer controller designed specifically for PCIe-based solid state drives with several advantages over the older AHCI controller. I’m not going to go into detail about what makes them different, just know that NVMe is the controller that we want for our SSD.

My goal was evaluate the effect of choosing AHCI mode versus RAID mode when performing a re-installation of Windows 10. I would additionally evaluate the use of using Samsung’s 950 Pro SSD driver, a close relative of the PM951, to evaluate its stability and impact on performance when used in conjunction with AHCI mode.

To measure performance differences I used the ATTO Disk Benchmark v3.05 utility and CrystalDiskMark v5.1.2 utility. Windows and its core components were fully updated. The XPS BIOS was updated to the most recent 1.3.3 version.

I first began by running the benchmarks on my current setup, which was still in RAID mode. My machine sports the 256GB PM951 that is rated up to 1000 MB/s reads and 280 MB/s writes according to Samsung’s website. The results of the benchmarks showed 1570 MB/s reads and 315 MB/s writes. You can see the results here. These results are likely high due to the synthetic nature of the benchmarks. When compared to other users on userbenchmark.com my SSD was similar, but landed in the upper 96th percentile with 1348 MB/s reads and 298MB/s writes. You can see my test results here.

I then went about the process of backing up my computer on an external hard drive, setting the computer to AHCI mode and re-installed Windows 10.

Before beginning the process of installing updates and whatnot, I wanted to check the storage controllers in Device Manager. Sure enough, the Intel Chipset SATA RAID Controller was replaced by the Standard NVM Express Controller. In order to maintain parity, I attempted to then restore my computer from the system image I had just made earlier in the day. I crossed my fingers that it wouldn’t reset the laptop to RAID mode, but seeing as that’s a BIOS option, I assumed it wouldn’t. WELL it didn’t work, I assume because of the disparity between the drivers.

I made sure Windows was completely up-to-date after about an hour of restarts, and ran the benchmarks. You can see one of the results here. I was on a time crunch and didn’t bother installing screen capture software or tracking down where PrtScr was saving to. I also forgot to take a pic of the 32 GB runs, but believe me it was the same.

BUT THEN through some stroke of magic doo-hickory I was able to re-image my laptop with the backup I had saved – all of my programs and settings were back to the way they were during the first benchmark. I installed the Samsung 950 Pro SSD controller (which only works on AHCI mode) to see if it had any benefit over the standard Windows NVMe controller. After the install, the Samsung NVMe Controller replaces the Standard NVM Express Controller in Device Manager, as seen here. I ran the two benchmarks that you can see here.

So as you can see from the benchmarks I’ve run there is no significant difference in SSD performance between AHCI and RAID mode settings. Using Samsung’s 950 Pro driver off-label has no noticeable impact. However, these results only apply to the 256GB PM951 that I tested. If you have results that differ or if I did something wrong, please feel free to share.