Minio

Official minio documentation available: https://docs.min.io/

Minio philosophy

Minio clusters are configured with a fixed size once, and cannot be resized afterwards. It is thus important to make a good conservative estimate about the amount of data you are going to store. The original philosophy was a set-up-and-forget strategy. You set up N disks and then you have a N/2 redundancy. Once half of your disks are dead (or a bit earlier) it is time to set up a new cluster and migrate everything using Minio’s mirroring facilities. Of course this is not ideal because the longer you have your cluster, the less space it will be able to reliably store. Hence minio has gotten some healing capabilities, which we will cover how to use in the next section. It is possible to replace broken disks with fixed disks, and then heal.

However, it is not possible to shrink or increase the cluster size. If the cluster is starting to get full, you will need to set up a parallel bigger cluster, mirror everything to the new cluster, swap the DNS entries to the new one, and then decomission the old one.

Hurdles from the trenches: disk usage statistics; directories vs. disks

I have done some more go code reading and have solved more minio mysteries. tl;dr: if you want to be safe, run minio on disks, not mounted directories.

Minio will detect if a directory is a mount point and if it is call statfs(2) to figure out the amount of available blocks. If it’s not a mount directory, it will just call du . in a for loop and update some counter (which sounds like a bad strategy to me).

https://github.com/minio/minio/blob/e6d8e272ced8b54872c6df1ef2ad556092280224/cmd/posix.go#L320-L352

so the answer is: if you use minio, e.g. with mountpoints, it will silently do the right thing and if you configure it to use two directories on the same mount, it will silently do something slightly incorrect (rather than crash loudly). (du is not a very reliable way to get Used metrics.)

So the Used metric in mc info is lying. We should instead look at the Total and Available metric which are derived from statfs. Those are already exposed by node-exporter anyway.

Moral of the story: we’re probably getting weird numbers because we’re not using disks but we’re using folders instead. We should use disks.

When doing a healing procedure, minio will give you feedback per bucket item. If a bucket item hapens to be say 500 Megabytes, it might look like the progress is stuck, but it just means it doesn’t update until the entire 500mb file is healed. Don’t worry and have a bit of patience.