Literally just had my pihole hard crash this weekend due to a bad update to FTL. Apparently they had a major version upgrade and didn't bother to read the notes so I had to do a full OS reinstall.

Back up your configs people. Had to dig through documentation to find the sqlite file and then parse through it like some sort of animal.

I didn't have a problem on my Pi-hole for a very long time too. OP has that probably because s/he's using it as a DHCP server as well.

Right, I didn't have any issues running it on a pi for years too. The problems came when I started messing with things. So, really my advice is to help save people from ideas like mine.

I decided one day to take a bunch of old laptops and create a proxmox cluster out of them. It worked great, but I didn't have a use for them, I was just playing. So, I decided to retire the pi and put the pihole on the cluster. HA for the win!

I did that and came woke up a few days later to my family complaining that they had no internet. I found the pihole container on a different node and it wouldn't start. Turns out with proxmox you need separate storage for HA to work. I had assumed that it would be similar to jboss clustering which I'm familiar with, and the container would be on all the nodes and only one actice at a time, with some syncing between nodes. Nope.

What's worse is the container refused to move back to the origional node AND wouldn't start. The pi was stored away at this point so I figured it would be easier to just create a new container, but duh, no internet. Turn off dns settings on the router, bam have internet.

Eventually set up the old pi again, and it took me a while to figure out what I had done wrong with proxmox. But while I was figuring it out it was nice to have the backup.

Now I always have two running on different hardware, just in case.

I've a single Pihole instance running on a RPi 4 and have experienced not a single instance of any of the 3 probs you mention. Except, of course, the very few minutes it takes for a reboot which I can schedule and am aware when it's happening...

Yeah, I believe it can vary depending on how you host it.

In my experience whenever I brought down the PiHole instance (Docker Compose) I would lose all internet access, which is expected since I’m essentially taking away my devices one and only library, so to mitigate this I spun up PiHole on another device and set that as my secondary (backup) DNS resolver.

This way I can take a container down, update it and all without losing resolution to the internet.

Raspberry Pies (is that how you pluralize it?), and especially their SD cards are not the most reliable pieces of hardware. I've already had a few die on me.

As for how annoying outages are, I guess that depends on how many people and services you have on your network relying on a functioning DNS. I am running two pihole instances on separate hardware in a keepalived virtual IP setup, with a replicated configuration. Sounds complicated, but it's really easy.

It's just nice to be able to reboot or perform maintenance on my pihole knowing it won't impact DNS, and not having to worry about interrupting my girlfriend streaming her Netflix series or whatever. For example, just a couple of weeks ago I converted my bare-metal pihole installation to a dockerized one, which was a couple of hours of work, without any DNS downtime at all.

Yep, if you have somewhere to put a docker container or VM you can have redundancy.

Right, I never said two raspberry pis, I meant two instances. Like one pi and a container run elsewhere.

Secondary DNS is not for redundancy!

The way secondary DNS works is that a client distributes DNS requests across the primary and secondary DNS servers. So if you have pihole as your primary DNS and, say, 8.8.8.8 as your secondary DNS, you're sending half of your DNS requests to google unfiltered. And if your pihole DNS goes down, half of your DNS queries time out.

The way to have redundancy with DNS is with a standby server that takes over the IP of the primary server if it goes down. You can do this with keepalived.

And what do you set that secondary DNS entry to? Operating systems may use both, so you need the secondary to point to a pi hole or else you're letting ads through randomly.

I have two piholes - they serve different DHCP ranges (e.g. 1-100 and 101-250), and option 6 references each other.

You specifically shouldn’t run two DHCP servers on the same network. It can cause IP conflicts when two servers assign the same address to different devices. Because the device doesn’t care which DHCP server gave it an address; It just listens to whichever one happens to respond first. And each DHCP server will have its own table of reserved/in-use addresses. And if those tables don’t match, IP conflicts can occur.

Device 1 connects to the network, and requests an IP address. DHCP server 1 checks its table of available addresses, and responds with “your address is 192.168.1.50.” It marks that address as in-use, so it won’t assign it to anything else in the meantime. Device 2 connects to the network, and requests an address. DHCP server 2 checks its table of available addresses (which doesn’t match server 1’s table) and responds with “your address is 192.168.1.50.” Now you have two devices occupying the same IP address, which breaks all kinds of things.

The largest reason to run two is because DNS queries are split amongst the primary and secondary DNS servers. If you only have a primary pihole, you’ll still occasionally get ads when devices use their secondary DNS servers.

Great, I recommend having two Adguard Home instances.

I just use their free public option. It's basically as good as pihole. With pihole I still got some ads. I still get some like this.