Question Does Windows or Linux boot from m2 ssd installed on a NVMe PCIe Adapter?

On X99 chipset, only the CPU-attached PCIe slots are PCIe 3.0. The chipset attached ones, including to the onboard x2 M.2 slot if so equipped, are only PCIe 2.0--which means ~1GB/s for x2, or 2GB/s for x4. If you plug a 3.0 card into a 2.0 slot then it will negotiate the fastest common speed, which is 2.0. You'd have to use the specific PCIe 3.0 slots to go faster, even if that means using up an x16 slot for your x4 card.

NVMe boot requires BIOS support. If it's a name-brand X99 board with years of BIOS updates, then it's likely some of the later ones do support booting from a NVMe drive in a PCIe adapter, even if the board did not come with any M.2 slot. If it's an Aliexpress X99 board with no BIOS support at all, I wouldn't count on even an onboard M.2 slot to be bootable, but it could still be used for secondary storage.

Heck, I wouldn't count on an Aliexpress X99 board to even actually have an X99 chipset, seeing how most are only dual-channel RAM rather than quad.
 
I never buy any devices from Aliexpress after receiving a knockoff phone(said to be a high end phone but turning out to be a low end one, almost cannot be used). My x99 motherboard, which bought from ebay, died after 3 months with rarely using. Since, I won't buy any China brand motherboards no matter how they look very nice in appearance. I am not saying all these motherboard are no good but I don't want to go through a lot of trouble again(replacing motherboard and installing OS and programs again).

x99 motherboards integrated with m2 ssd slot but the speed seems to be the same as sata 3. Therefore, I want to buy a pcie adapter to see if it is much faster.
 
It's a good board model. Gigabyte added NVMe 1.0 support to all of their X99 board BIOSes back in 2015 and it works for many people, but don't expect every modern drive to be compatible.

Multiple I/O queues was not supported until 1.2.1 in 2016 when it went from 1 queue per device (like SCSI, with multiple queue depths in it) to 64k. 1.3 added Namespace Priority which is like QoS to prioritize what's most important for reducing latency on. Drives are up to v2.1 now.

If you will be buying new drives, then just buy from a place that accepts returns and you won't risk much. Otherwise it's safer to first Google for what drives other people have had success with in their Gigabyte X99 boards of any model.
 
Don't know what to say.
Some do, some don't.

If I may ask, why are you buying parts for such an old platform?
Because I have the old parts that fit into x99 motherboard. This Gigabyte board is still good nowadays. Server CPUs are better than desktop counterparts.

The onboard m2 socket seems to be a socket 3. Why is its speed only up to 1GBs?
 
It's a good board model. Gigabyte added NVMe 1.0 support to all of their X99 board BIOSes back in 2015 and it works for many people, but don't expect every modern drive to be compatible.

Multiple I/O queues was not supported until 1.2.1 in 2016 when it went from 1 queue per device (like SCSI, with multiple queue depths in it) to 64k. 1.3 added Namespace Priority which is like QoS to prioritize what's most important for reducing latency on. Drives are up to v2.1 now.

If you will be buying new drives, then just buy from a place that accepts returns and you won't risk much. Otherwise it's safer to first Google for what drives other people have had success with in their Gigabyte X99 boards of any model.
I think the NVMe 1.3 or 1.4 is backward compatible with older hardware. The speed, of course, just as fast as or slightly faster than sata3.