SSDs have become fast, except in the cloud

How much faster would the network need to get, in order to meet (or at least approach) the speed of a local SSD? are we talking about needing to 2x or 3x the speed, or by factors of hundreds or thousands?

The Samsung 990 in my desktop provides ~3.5 GB/s streaming reads, ~2 GB/s 4k random-access reads, all at a latency measured at around 20-30 microseconds. My exact numbers might be a little off, but that's the ballpark you're looking at, and a 990 is a relatively cheap device.

10GbE is about the best you can hope for from a local network these days, but that's 1/5th the bandwidth and many times the latency. 100GbE would work, except the latency would still mean any read dependencies would be far slower than local storage, and I'm not sure there's much to be done about that; at these speeds the physical distance matters.

In practice I'm having to architecture the entire system around the SSD just to not bottleneck it. So far ext4 is the only filesystem that even gets close to the SSD's limits, which is a bit of a pity.

Around 4x-10x depending on how many SSDs you want. A single SSD is around the speed of a 100 Gbps Ethernet link.

SATA3 is 6 Gbit, so each VM on a machine multiplied by 6 Gbit. For NVMe, probably closer to 4-5x that. You’d need some serious interconnects to get a server rack access to un-bottlenecked SSD storage.

The problem isn't necessarily speed, it's random access latency. What makes SSDs fast and "magical" is their low random-access latency compared to a spinning disk. The sequential-access read speed is merely a bonus.

Networked storage negates that significantly, absolutely killing performance for certain applications. You could have a 100Gbps network and it still won't match a direct-attached SSD in terms of latency (it can only match it in terms of sequential access throughput).

For many applications such as databases, random access is crucial, thus why nowadays' mid-range consumer hardware often outperforms hosted databases such as RDS unless they're so overprovisioned on RAM that the dataset is effectively always in there.

100Gbps direct shouldn't be too bad, but it might be difficult to get anyone to sell it to you for exclusive usage in a vm...

Um... why the hell does the network care whether I am doing random or sequential access? Your left that part out of your argument.

Ah sorry, my bad. You are correct that you can fire off many random access operations in parallel and get good throughput that way.

The problem is that this is not possible when the next IO request depends on the result of a previous one, like in a database where you must first read the index to know the location of the row data itself.

OK thanks yes that makes sense. Pipelining problems are real.

Networking doesn't have to have high latency. You can buy network hardware that is able to provide sub-microsecond latency. Physical distance still matters, but 10% of typical NVMe latency gets you through a kilometer of fiber.

(The network indeed doesn't care, but bandwidth of dependent rather than independent accesses depends on latency)