[pjmlp]

There is something like live patching.

One reason mainframes and micros are still around us, is that you can change almost everything between hardware and software without downtime.

It is also available in commercial surviving UNIXes, and as paid for feature in some Linux distros, although not to the extent that those grandparent systems are capable of.

[da_chicken]

The problem with live patching is twofold.

First, you might not reload everything in memory, so it will be patched on disk but not in process.

Second, you have not tested that the system can boot to a functional system. Say you have done live patching for 5 years and never rebooted, and then you have a power loss or hardware failure/upgrade that takes the system down. When you try to bring it back up, it doesn't work. Which configuration change in the past 5 years caused that? Which backup do you use?

And, yeah, everything is hot swappable on VAX. Those machines also cost 6+ figures, and often require a service contract that includes a permanent on site tech.

[kjs3]

And, yeah, everything is hot swappable on VAX.

Only the last generation or 2 of the highest end VAXen had any significant hot swap (VAX 9000/400 and later, which sold very poorly). The vast majority of VAX machines didn't. Even hot-swapping DSSI disks was at best iffy.

When someone whose been there talks about VAX 'high availability', they're usually talking about VAX/VMS clustering. Very cool and generally effective approach to the problem. That was one big issue with the end-game VAXen: clustering a couple of 6-figure mid-range machine was often considered a better solution than all-in on one 7- to 8-figure VAX 'mainframe'.

often require a service contract that includes a permanent on site tech.

I don't recall that being common with DEC service contracts. Most of the sites I know of that had dedicated DEC techs were either very large installs or had...other...drivers (e.g. tech had to have a TS clearance to work on the machines).

[Squeeeez]

How would you implement no-downtime hot swap with only one item?

[kjs3]

By implementing hot-swap into the one item? Am I missing something in this question?

[da_chicken]

Executing hardware hot-swap typically means telling the system that a component is going down. Then the system moves those resources to the other component to gracefully allow you to remove it without a restart.

Like it's not a case where you just yank out a CPU as you like as though it were a spindle in a RAID-6 array. Especially if there's only one CPU. The state machine can't maintain state if the only component that tracks and maintains state goes missing.

[coldtea]

>First, you might not reload everything in memory, so it will be patched on disk but not in process.

You design for this with generational tagged objects or something similar.

[mx7zysuj4xew]

Which is moot, because of the system is important enough you'll have an automatic failover to another system running on standby

All this "we must reboot to test" is bullshit excuses by unqualified workers

[z3t4]

Had an accidental reboot, and it could not boot. Had redundancy, but the other server had failed silently days prior. Solved it with three way redundancy and extra monitoring. Systems fail in many ways at the same time. If you do not test it, there is a chance it wont work. Controlled failure is preferred over unknowns, like rebooting once in a while just to make sure it works.

[close04]

Ah, spoken with the confidence of a freshly minted qualified worker :). Anything you don’t test is a wish, not a production system. You either know that your systems work end to end because you tested periodically, or you pray they will.

How do you know the automatic failover works? How do you know the standby system works?

I’ve seen many a “qualified workers” getting sent packing because they never fully tested the prod system because they just knew everything will work, and never tested the backup systems because qualified workers do the job right the first time, no need for backup.

[X0Refraction]

Not sure I'm following honestly. Your primary goes down and it fails over to the secondary (which becomes the primary), but if you can't boot how do you then get another secondary ready to fail over to again when the new primary inevitably fails?

[fragmede]

You patch it in memory and on disk. What you put on disk is the patch though, so when you restart, the original unpatched version is booted, and then the same live patch is applied. This is how Ksplice worked. It has the advantage that there isn't a config file in /etc to get changed out from under it, so the second problem did not apply.

[da_chicken]

Ksplice can do that because the kernel is only in memory in one place an it never sleeps. It has to orchestrate a process that's always running, which is complex, but it's never more than one.

Now try patching glibc like that. Not only does almost every thread have it in memory, several of them will have it in process, and some of them will have it swapped to disk while the thread sleeps. You're going to quickly decide that you actually just want a little bit of downtime or else you want to stand up a redundant system. There's a reason that some live patching systems explicitly exclude glibc and similar libraries.

[fragmede]

The corporate named "Ksplice for userspace" did exactly that, patch glibc.

https://blogs.oracle.com/linux/new-userspace-patching-with-o...

[pjmlp]

Yes, some things actually cost money, especially if they aren't easy to implement.

[silvestrov]

A Danish bank found out that this can bite you in the ass.

When you hotpatch the system for years then you have no idea if the system can boot up or it will fail somewhere in the booting process.

i.e. you can only trust what you regularly test.

[Suzuran]

Mainframes can LPAR dynamically. When you want to test if your production system will IPL cleanly, you clone your production environment to an isolated LPAR and IPL it. No impact to production and you get your test.

[dredmorbius]

US telcos as well.

There were several switch failures in the 1980s / 1990s in which systems which had been upgraded in place without a full restart failed. (IIRC, one burnt down, literally.)

Engineers were uncertain as to whether or not a cold-boot restart was even possible.

Account concerning an AT&T system upgrade sourcing Risks Digest (Vol 9, Issue 62, February 26, 1990) by the recently deceased Peter G. Neumann: <https://telephoneworld.org/landline-telephone-history/the-cr...>.

[pjmlp]

Interesting, it there any public info on the case?

Not doubting it, only curious about some kind of postmorten.

[silvestrov]

In Danish: https://danskebank.com/da/news-og-insights/nyhedsarkiv/press...

or translated: https://danskebank-com.translate.goog/da/news-og-insights/ny...

TLDR: power supply failed completely and DB2 failed running recovery operations due to multiple old/existing software bugs.

[pjmlp]

Thanks for hunting it down.

[ErroneousBosh]

> One reason mainframes and micros are still around us, is that you can change almost everything between hardware and software without downtime.

We have some Sun V880s at work and I'm fairly sure the only part you cannot change with the power on and system running is the motherboard itself.

And I would not be surprised if some ex-Sun Gandalf Beard "well akshully"s this comment.

[linksnapzz]

Hot swapping the failed half of a bonded NIC pair on a v880 was a treat…

[ErroneousBosh]

I come from an era when unplugging the RAM pack could blow every chip on the ZX80's board, so hot-swappable PCIe cards are just absolute fucking black magic to me.

[linksnapzz]

Yeah, I almost had a heart attack the first time I saw someone do a 'cfgadm unconfigure' && 'cfgadm disconnect'; then pop open the side of a prod box, press a button and pull a card out.

"See, oracle's still running!"

Things like that used to be how one distinguished enterprise hw & sw vs. PCs w/ delusions of grandeur.

[mesrik]

You should't need mainframe for 100% (or five nines if that's fine) service uptime.

You can build that way cheaper with 2-3 proper clustered load balancer units, 2-3 application servers behind those and those using persistent storage (databases,ldap, files) which allow writing multiple nodes simultaneously.

I used to work uni that we had few services from 2012 to 2025 my retirement with zero downtime. One time my manager with tech background tried to add PBR in hurry using WebUI and did not understand cli syntax and caused close to require reboot, but I was able to fix it from cli rolling back previous config and rebooting one unit at time. Upgrading software major version up to each unit supported level wasn't hard, upgrade node it joins back cluster, upgrade another node and it joins cluster, all done. Few times I had to fix manually config for some less important test backend servers that I had forgotten to change before upgrade. No big deal. No major outages during all that 13 years time happened. Some redirecting policy and action syntax was first hard to understand and learn like GeoIP, but I was very surprised how darn reliable and nice they to use and maintain.

The LB's were (Citrix) Netscalers in clustering mode (all nodes process traffic concurrently), which allowed live update one node at time without losing any connectivity through them. That wouldn't have been possible devices in just HA mode.

We had just 2 beefy units which worked very well for us, but you can have 2-32 of them in cluster and managing thousands of servers behind them if you need that. Netscalers are FreeBSD derived where quite a bit of the TCP/IP stack was rewritten adding support many some quite odd features std FreeBSD doesn't have. Much of that is IP/ethernet multicast features, PBR's, Traffic Domains (VRF's) and of many service and monitoring processes which sync cluster (or HA) and if node fails another can continue straight from there without any loss of traffic to clients being proxied.

Though I think most people in this forum are familiar with with haproxy, pound and web-server software provided reverse proxying.

A car analogy if previous were your fancy sport sedan Netscaler and F5 BigIP are formula F1 class cars ie. quite different beasts altogether.

e: And proper LB's are not just for HTTPS etc. but very nice proxying many other protocols were they TCP, UDP or something else. We did done VPN's and something like Cisco AP'S CAPWAP (DTLS ie SSL over UDP). e: typo.

[pjmlp]

> You should't need mainframe for 100% (or five nines if that's fine) service uptime.

Hence my second paragraph.

Thanks for sharing the story.

[Scramblejams]

I’ve long wanted that amazing uptime and virtualization and huge I/O and all that cool stuff mainframes offered, but on the desktop or in the closet, with modern CPUs.

I think I’m gonna hafta keep waiting...