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Comment by tines | Hacker News Reader

What even is a mainframe, exactly?

A modern mainframe is a system specialized in extremely high transaction throughput at extremely low cost-per-transaction, while guaranteeing data durability and computational correctness.

Sounds like a computer. Why would getting access to one to play around encourage people to buy one? In other words, it sounds like a quantitative difference rather than a qualitative one.

My understanding is that Z mainframes have a number of unique features to support those use cases. Stuff like hot swapping CPUs, and hundreds of IO coprocessors to avoid the main cores from getting blocked. Don't think they're just rebranded x86 machines, but not an expert.

True. I find it interesting how difficult that they find it to put the benefits into words that don't sound like a brochure.

Surely there should be some specifications behind this? Benchmarks?

I worked on them a bit while at IBM.

You can open a terminal and come back in a month and it will still be there. Unlikely kunernetes where containers regularly go down.

You can obviously achieve high reliability with Kunernetes, but you'll need queues and retry logic, that you don't need with SystemZ.

> You can open a terminal and come back in a month and it will still be there. Unlikely kunernetes where containers regularly go down.

You mixing apples and oranges there, by comparing kubernetes workloads to mainframes. Kubernetes isn't really designed to serve long-persistent workloads of that fashion. Although tbf I've had VM's that last for years and real hardware (albeit Sun) that's had over a decade of uptime, so I'm not sure what all the fluff is about.

Both have a goal of high reliability. They are both solving the problem of running you application on a single server.

Kubernetes runs across multiple nodes to achieve that, that don't need to be in the same datacentre, though.

Is this where we are right now as a society: a terminal which doesn't die in a month is presented as an achievement?

I assume the achievement part is that any CPU can be replaced transparently to the terminal running?

I've not worked with a mainframe, but I've worked with an IBM storage system that worked on similar principles: We could connect our systems via dual controllers, to separate bays of controllers on the storage array. You could pull whole bays of controller cards and the system would stay up. You could pull whole bays of hard drives, and the system would stay up. You could pull power supplies and it'd remain up. You could swap RAM and CPUs in the servers managing it without shutting them down, but you could also pull one of those servers, and it'd remain up. If stuff started failing, and IBM engineer would show up because the system would call home. This was around 25 years ago.

It wasn't cheap, but it made a typical "modern" high availability setup look like a crude homemade toy.

But as impressive as it was, there are just very few places where that impressiveness provides enough value to justify the cost. And having to deal with IBM.

Yes, Hard drives can die, power supply can die, another user can write an infinite loop and it will not kill your terminal.

Not a big deal today, but in the 80s hard drives died more frequently and a bank losing just 5 seconds of transactions could cost them millions.

Anandtech ran a story when the z16's CPU was announced:

https://www.anandtech.com/show/16924/did-ibm-just-preview-th...

Hah, try getting ECC memory from a PC supplier. Surprisingly difficult and expensive when it should have been default twenty years ago.

ECC memory is widely available, you can buy it on amazon, and if you have a contract with a server manufacturer they'll be happy to sell it to you.

It's just not compatible with consumer CPUs (well, ddr5 is, but ddr4 and lower wasn't) because Intel was voluntarily segmenting the market to upcharge for server CPUs.

Sounds like a working solution is not as “widely available” as you suggest. Apple/Amd play similar games.

I see it as similar to an on prem mini AWS cloud, with dedicated hardware for things like WAF, storage, compute, networking, lambdas and databases.

I've heard it said a couple of times that working in cloud is closer to working on a mainframe than most want to admit.

That’s a good analogy. They are both platforms. The accelerators for things like Java, MQ, databases, are built in to the platform.

Yep, cloud is really just an infinitely scalable mainframe.

The IBM mainframe: How it runs and why it survives[0] from last fall.

From way back in 2004[1], Commercial Fault Tolerance: A Tale of Two Systems compares the reliability / philosophy of IBM mainframes with Tandem Servers.

[0] https://arstechnica.com/information-technology/2023/07/the-i...

[1] http://www.engr.newpaltz.edu/~bai/CSE40534/bar04.pdf

Dave Plummer recently did a tour: https://www.youtube.com/watch?v=ouAG4vXFORc

It's definitely intriguing if it were.. I don't know, available to tinker with?

The other definition you got isn't bad, but I think it misses the point for this discussion: it is a rack or multi rack scale computer with built-in and centrally managed features for redundancy, failover, job allocation, scaling, etc.

As an example, in many main frames, you can configure them with a spare set of CPUs, and if one of your CPUs fails, the replacement is brought online automatically and transparently, the code you write doesn't have to know about anything related to the failure.

One neat thing is that the hot spares can be used during boot to shorten the time to availability. Not sure LinuxONE boxes are bought like that, but for the Z-series you can buy the machine with more capacity than you pay for and pay for it to be enabled at a later point.

I'm astonished that people on Hacker News can ask such a question.

It's essentially a very large single clocked machine.

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