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Possible to mirror 2x m.2 NVMe drives with different PCIe bus bandwidths

Western Digital Drives - The Preferred Drives of FreeNAS and TrueNAS CORE

BitByteBit

Neophyte
Joined
Jul 22, 2021
Messages
10
Hi community

I'm hoping to have two m.2 NVMe SSDs in a mirrored vdev, however, my motherboard has 1x PCIe 3.0 x2 slot on board and then I'd add the second m.2 SSD via a PCIe to m.2 NVMe adapter in a PCIe 3.0 x4 expansion slot.
Will this cause issues that one drive only has 2 PCIe lanes while the other drive has full 4 PCIe lanes?
I realize the mirror will most likely run at the speed of the slowest drive (i.e. if both drives only had 2 PCIe lanes), which I'm O.K. with if it's going to work without any unforeseen stability problems.

Keen for any feedback before I order the drives.
Thank you.
 

HoneyBadger

Mushroom! Mushroom!
Joined
Feb 6, 2014
Messages
3,340
Much the same as mirroring a drive on a SATA3 with a SATA2 port, you'll effectively be slowed down to the lower of the two protocol limits.

But in this case, PCIe 3.0 is good for 1GB/s per lane in each direction. Unless your SSDs will be able to deliver more than 2GB/s - and then your network connection can also keep up - I wouldn't worry.
 

BitByteBit

Neophyte
Joined
Jul 22, 2021
Messages
10
Thanks HoneyBadger, and yes from what I've read, 2 PCIe 3.0 lanes can give support around 1.96 GB/s (https://en.wikipedia.org/wiki/PCI_Express) so even though my NVMe drives have read and write above 2 GB/s I'll be limited to just below 2G GB/s theoretically, which I'm fine with :)

Just wanted to make sure there were no other reasons I wasn't aware of as to not go this approach.
 
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