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I have to take some decisions about migrating from dedicated servers to the cloud.
Everything is fine except I don't quite understand how to compare physical SSD to the cloud because depending on the numbers it seems like virtual SSDs are just less performant (I hope that this is not true)

On the dedicated server we have following SSD:

SSD PM883 SATA III 2,5" 960 GB MZ-7LH960HAJR

550 Mb/s Read, 520 Mb/s Write,

Max. Random Read 4K: 98k IOPS,

Max. Random Write 4K: 25k IOPS.

In the cloud infratstructure we have following options:

  • 500 IOPS per 500GB (so for 1TB it will be 1000 IOPS)
  • 5000 IOPS per 500GB (how actually this "X per Y GB" supposed to work?)
  • 15000 IOPS per 500GB

So the most expensive virtual SSD would be less performant than this physical SSD? Unfortunately, I was unable to get a clear answer from the provider support

I hope there is something I didn't get and virtual SSDs are more performant

Thanks for any help

uneven
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2 Answers2

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the thing is the physical drive in your machine is exclusive to it. in the cloud the drives are shared by many virtual machines so it will be slower than a dedicated drive(of the same speed) because its resources, not only the space but the speeds/bandwidths too, are being divided/balanced.

its not (always) a question of connection because if the drives are on the same machine as the cpus the hypervisor have "virtual fiber channel connection" at 10GB/s (more than any sata ssd, even nvme pcie 3.0). and even if the drives are in other machines, most clouds may have at least 10GB network inside them.

bigubr
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That depends... Let's start with your dedicated server. Try to put your DB on virtual machine. There is context switching between host, and virtual machine at processor, so in terms of iops - virtual solution is much slower. On the other hand, XEN Hypervisor - AWS uses that solution - can attach particular virtual machine to host interrupt, so it works almost like on bare metal. I hope that is the case.

Now let's leave dedicated server. Virtual solution, in the cloud, means that your hard drive is network attached. To me it means that virtual iops are theoretical, based on SSD. I'm sure they do good job, but to me that is simply less reliable.

What to do? Pick biffy machine, or... at AWS you can ask them to have it exclusively. That way you have full io. Pick machine with fast network. Learn how they work - here is EBS doc, for AWS.

Besides all of that. Agree that this is a question for severfault. Also look for developers forums dedicated to a provider you are planning to use / using. To me that is even better source. For example - this one... well, this one is not answered, also questions are quite old. Looks like AWS/EBS has separate storage, and by scaling size, or asking for iops you are using more, and more disks, and that is how they achieve particular, virtual iops level. So... looks like we have to live with virtual iops, almost as good as real ones...

Michał Zaborowski
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