Single drive with RAID?

antares · 28 Jun 2016

This doesn't make sense. My new Dell 15 XPS (9550) laptop has the BIOS SATA setting set to RAID (not AHCI) and yet it has a single internal OS NVMe PCIe SSD. Any idea why it's not set to AHCI? thanks

Slartybart · 29 Jun 2016

antares said:

This doesn't make sense. My new Dell 15 XPS (9550) laptop has the BIOS SATA setting set to RAID (not AHCI) and yet it has a single internal OS NVMe PCIe SSD. Any idea why it's not set to AHCI? thanks

Everything you need to know about NVMe, the insanely fast future for SSDs | PCWorld

Enthusiasts will want to take a hard look at Intel’s 750. Most recent high-end motherboards will get firmware upgrades to support NVMe so you can boot from the drive. Most legacy mainstream boards will probably not. But our talks with Intel and other vendors indicate that the flood gates have opened, and you should see a torrent of NVMe support later in the year.

Until then, there are viable ways around your PC’s storage bottleneck, if indeed you consider 500MBps a bottleneck.
One is RAID 0.
While a single SATA port is limited to 600Gbps, combining four makes for 2.4GBps of bandwidth.
In real life, the DMI bus behind the SATA throttles this to 2GBps, and SATA/RAID overhead reduces that to about 1.4GBps, but it’s still a hefty improvement.

antares · 29 Jun 2016

Thanks for your reply. I just installed Windows 8.1 in legacy mode (not UEFI) with Secure Boot disabled, and it won't boot in RAID mode, only when I set the BIOS setting to AHCI. I'd like to use it in RAID mode to take advantage of better bandwidth, any hint? Thanks

Slartybart · 29 Jun 2016

I don't know the answer - look around in the tutorial section or search the forum

Another member might drop in and have the answer - hang tight.

DeaconFrost · 29 Jun 2016

I think you are misunderstanding what RAID is. Your BIOS may be set to RAID, which is the pointless decision by Dell for all laptops we buy from them, but you aren't actually using RAID unless you have multiple drives. The A in RAID stands for Array, which is not possible with a single drive. There's no increased bandwidth to be had with a single drive.

If your system bots with the BIOS set to AHCI, then there's no reason to put any more thought in to this. Leave it as AHCI and enjoy using that insanely fast drive.

fdegrove · 29 Jun 2016

Hi,

All of the above and maybe load a proper NVMe driver for that M2 ssd ?
Not that it will make much difference performance wise.

As stated above, there is no such thing a single RAID drive, Raid always requires a pair of drives at the least.

Redundancy array of inexpensive drives if I remember correctly.

One set of drives set up as raid 0 would have 0 redundancy but roughly twice the throughput of a single drive and the capacity of a single drive.

As with everything electronics there no such thing as a free lunch.

Cheers,

Slartybart · 29 Jun 2016

Many systems with Hybrid drives use IRST to configure software RAID 0 - it's not real RAID, it's taking advantage of the caching in RAID protocol.

I found these post that might be of more interest to antares than a debate on RAID

I would look at this one a little deeper if it were my machine.
Dell XPS 15 9550 M.2 PCIe NVMe SSD - neither RAID nor AHCI | NotebookReview

Dell XPS 15 (9550) list of hardware and software problems | NotebookReview

antares · 30 Jun 2016

Thanks everyone for your replies. Let me put this straight. I do know what RAID stands for, RAID0 requires at least 2 drives. My Dell 15 XPS (9550) has a SINGLE NVMe SSD drive (only ONE drive). Yet it came with Windows 10 preinstalled and as I discovered in the factory BIOS settings, the SATA mode was set to RAID. So now comes again the question I made in my initial post: how can a system with a SINGLE drive function in RAID mode? Because as I see it, a single drive can only boot in AHCI mode, not RAID.
But as I said above, since the laptop does boot under RAID (factory setting), there must be an advantage setting it to RAID instead of AHCI. If so, I'd like to change the SATA mode back to RAID, but since my WIn8.1x64 OS was already installed in another PC under AHCI, it won't boot (it boots only in AHCI), I googled and tried this tip, but I still get BSOD when booting under RAID mode.

fdegrove · 30 Jun 2016

Hi,

In the case of the notebook the SATA settings are irrelevant to the boot process since nothing is attached to it bar perhaps a DVD drive. You boot from the NVMe drive.
Why it is factory set to RAID in bios is a mystery to me. And no, I don't see any advantage to set it to RAID mode.

but since my WIn8.1x64 OS was already installed in another PC under AHCI, it won't boot (it boots only in AHCI), I googled and tried this tip, but I still get BSOD when booting under RAID mode.

That other PC has a single drive attached to the primary SATA controller ? If so then yes, it could BSOD when set to RAID mode in bios.

Cheers,

DeaconFrost · 30 Jun 2016

I don't know why Dell sets RAID as the default, but there's no benefit or reason for it. It will boot if it is installed under that setting because that is just the controller setting...has nothing to do with the amount of drivers connected. You could always run a single drive on a real RAID controller without a problem. There just wasn't ever a point in doing so and still isn't.

With a single drive, you want it to be set to AHCI. I'll say it again, that there's is nothing you are missing out on and no reason to spend any time worrying about it. If the system is booting as is, go on enjoying it. There's no advantage, as we've confirmed this with Dell. When we get new systems in, we upgrade to the latest BIOS and set the controller to AHCI.