Question PCIe 4.0 NVMe drive in a PCIe 3.0 system - is the speed loss proportional to the drive's 4.0 speed?


Ok, so, I know that PCIe 3.0 caps out at about 3,500MBps/3,000MBps read/write speeds.

So, let's say we have two different PCIe 4.0 NVMe drives. Drive 1 has speeds of 7,000/6,000 MBps R/W, but drive 2 is a more budget drive, and has speeds of 5,000/4,000 MBps R/W.

What will happen if each of these is put in a system that only supports PCI3 3.0? Will they have the same peak speeds, saturating the PCI3 3.0, thus 3,500/3,000? Or will they be more or less proportional to their 4.0 speeds, thus, drive 1 will max out the 3.0 system at 3,500/3,000, but drive 2 will only hit maybe 2,500/2,000 MBps?


Depends on the work load I imagine, but for the sequential data writes and reads that achieve those numbers they will both be capped to the maximum bandwidth of the M.2 slot.

It doesn't really matter, you rarely hit those speeds in normal use. Basically need to be transferring files from one drive to the other. Even 10Gbps networking doesn't go that fast.


Do not be much swayed by vendor synthetic SSD benchmarks.
They are done with apps that push the SSD to it's maximum using queue lengths of 30 or so.
Most desktop users will do one or two things at a time, so they will see queue lengths of one or two.
What really counts is the response times, particularly for small random I/O. That is what the os does mostly.
For that, the response times of current SSD's are remarkably similar. And quick. They will be 50X faster than a hard drive.
In sequential operations, they will be 2x faster than a hard drive, perhaps 3x if you have a sata3 interface.
6X with a pcie interface.
Larger SSD's are preferable. They have more nand chips that can be accessed in parallel. Sort of an internal raid-0 if you will.
Also, a SSD will slow down as it approaches full. That is because it will have a harder time finding free nand blocks
to do an update without a read/write operation.
Larger ssd devices have more endurance.

I would not worry much about pcie 3/4/5 issues.
Buy the capacity you need from a quality vendor.

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