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Specifying Ceph Monitors when Mounting CephFS Shares

Brett answers a question with another analogy! This time, he talks about the recommended number of Ceph monitors to specify when mounting CephFS shares.

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Hey, everyone Brett Kelly here for another Tuesday tech tip with 45Drives. This week, we're keeping it light, going back to our tech tip roots of a quick tech tip.

For those who have mounted a CephFS share before might have wondered, do I have to specify all the monitors? Do have to specify any of the monitors? What happens if I only put one?

For those who haven't done this, well, don't worry. Stick around for fun analogy at the end and might like to inject some life into this. But let's answer that question. Let's get into it.

OK, so. When you mount a CephFS share, you have to specify the monitors, your Ceph monitors, the IPS of them. That's how it knows how to connect and get in. But just last week, a couple of guys on our team asked the question again and well, you could always read the docs, but that's no fun, right? So, I answered the question and then kind of on the spot was like, oh, here's a good analogy with it.

So first of all, to answer the question, you technically only need to put one monitor address in your in your mount command. You can put “,” separated up to however many monitors you have. That is the recommended way. But the best way to really remember that and hit that home is wrap it up an analogy.

So, picture this, ladies and gentlemen, you go you go to a bar and it's packed and the lineups through the roof, and it's going to be hours to get in. But, you know one of the bouncers. (A monitor persay.) But, you know, one of these bouncers. And you go you go talk to them and say, “hey, let me get in.” They’re your buddy and they let you in and everything's all right. This is like mounting with one monitor. Talk to them, you get in, everything's good.

OK, now let's imagine that his shifts over and that bouncer goes home. Do you have to leave the bar? No, you're in. you're all good. Mountings CephFS works the same way. Once you get let in by that first monitor, you're in. The client is aware of all the members of the cluster so you are in. Your bouncer left, no one is going to go in and pull you and tell you to leave.

But where you can get into trouble is if you've got to go for a breather, say you want to have a smoke, meet up with a friend or whatever. So, you leave, you unmount your client, you reboot your client machine, whatever. You leave the bar, have your cigarette, you go back in, but you're your bouncer is gone. Your monitors down for whatever reason. You're not getting in anymore. That's your one ticket in. So, you're screwed.

And that's why technically you can get into a Ceph cluster with only one monitor in the mount command, but really you should have a list of all the bouncers. You should have all three of your monitors specified there. So, if you ever have to step out, have a smoke (reboot your client machine), you can always get right back in case that first guy left for whatever reason.

All right. Well, that's all you really have to keep in mind when you mount your CephFS clients. Remember the bar analogy. So hope you enjoyed that. Hope to learned something. And we'll catch you guys next week.

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