Apples and oranges, right? You need ten thousand hard drives to match the IOPS of one SSD and even with the 10000 disks your service latency will still be three orders of magnitude worse. The other advantage of an SSD is in terms of bytes per volume, in case rack density matters to you.

Putting 24TB parts in a Backblaze Storage Pod 6.0 would presumably allow 1440TB in a 4U rack mount server.[1] In practice, how would you reach the same density with SSDs? (I haven't looked into it, just curious if you know that SSD options more dense than that exist, and whether they are equally openly documented.)

[1] https://www.backblaze.com/blog/open-source-data-storage-serv...

There aren’t many full open-source solutions available, but you can buy 100TB 3.5” SSDs for data center use.[1] At Storage Pod densities, that’s 6000TB in 4U. I’m not sure if some other factor comes in that limits density, but that’s a first-order estimate.

[1] https://nimbusdata.com/products/exadrive/pricing/

Heat dissipation may be a concern, but, if we are talking 6PB of flash storage, we can pretty much consider custom liquid cooling is an option.

There are 1U servers with 32 EDSFF slots, which were advertised to reach 1PB with 32TB SSDs. But 16TB is more common, and that's still 2PB in 4U that you can buy today without getting into exotic pricing.

If 2PB SSD is not exotic pricing.. I’m glad I can work with off the shelf stuff.

You can put 36 15TB NGSFF SSDs into a 1U height enclosure and only 12cm deep. SSD volumetric density is a lot higher than disk and has been for a few years now.

SSDs are smaller, so you can pack more of them in the same volume. Not aware of really open designs (maybe in the OpenCompute project there are some), but just from a quick look at Supermicros homepage:

Something like https://www.supermicro.com/en/products/system/1U/1029/SSG-10... can fit 32 SSDs à 16 TB in a 1U system (Intel at some point announced 32 TB models in the same format, but I'm unsure if they ever were available).

This style can fit 48x 16TB in 2U: https://www.supermicro.com/en/products/system/2U/2028/SSG-20...

The trouble with such dense SSD capacity is more being able to interface them fast enough with the host and the outside world.

Well, there are 100tb 3.5" SSDs available...

Let me guess - if you have to ask for the price, you can't afford it ...

price isn't that bad... only $40K[0]

[0]https://www.newegg.com/nimbus-data-dc-100tb/p/2U3-002M-00004...

Gotta love that $29.00 shipping charge. It's not like they could have afforded to offer free shipping at those prices :)

You need both SSD and HDD, the latter for archival storage and data replication. And that only works at data center scale.

IOPS differences are exaggerated. You can generally beat consumer-grade SSDs with just four HDDs.

That depends very much on the workload. Four hard drives can deliver in aggregate a sequential bandwidth that exceeds any one consumer SATA SSD, or the sustained write bandwidth of most consumer NVMe SSDs. But when people are discussing IOPS, the usual implication is that they're talking about non-sequential access of relatively small block sizes. For those workloads, the difference between consumer SSDs and hard drives are still measured in orders of magnitude.

So, what workloads did you have in mind when you said "generally"?