SSDs have become fast, except in the cloud

This was a huge technical problem I worked on at Google, and is sort of fundamental to a cloud. I believe this is actually a big deal that drives peoples' technology directions.

SSDs in the cloud are attached over a network, and fundamentally have to be. The problem is that this network is so large and slow that it can't give you anywhere near the performance of a local SSD. This wasn't a problem for hard drives, which was the backing technology when a lot of these network attached storage systems were invented, because they are fundamentally slow compared to networks, but it is a problem for SSD.

According to the submitted article, the numbers are from AWS instance types where the SSD is "physically attached" to the host, not about SSD-backed NAS solutions.

Also, the article isn't just about SSDs being no faster than a network. It's about SSDs being two orders of magnitude slower than datacenter networks.

It's because the "local" SSDs are not actually physically attached and there's a network protocol in the way.

I think you're wrong about that. AWS calls this class of storage "instance storage" [0], and defines it as:

> Many Amazon EC2 instances can also include storage from devices that are located inside the host computer, referred to as instance storage.

There might be some wiggle room in "physically attached", but there's none in "storage devices located inside the host computer". It's not some kind of AWS-only thing either. GCP has "local SSD disks"[1], which I'm going to claim are likewise local, not over the network block storage. (Though the language isn't as explicit as for AWS.)

[0] https://aws.amazon.com/ec2/instance-types/

[1] https://cloud.google.com/compute/docs/disks#localssds

If the SSD is installed in the host server, doesn't that still allow for it to be shared among many instances running on said host? I can imagine that a compute node has just a handful of SSDs and many hundreds of instances sharing the I/O bandwidth.

How do these machines manage the sharing of one local SSD across multiple VMs? Is there some wrapper around the I/O stack? Does it appear as a network share? Geniuinely curious...

In say VirtualBox you can create a file backed on the physical disk, and attach it to the VM so the VM sees it as a NVMe drive.

In my experience this is also orders of magnitude slower that true direct access, ie PCIe pass-through, as all access has to pass through the VM storage driver and so could explain what is happening.