Who said anything about storage servers? I'm using zfs on laptops and desktops right now because I want data checksums and a filesystem that doesn't have a history of breaking horribly (I dropped btrfs after the second time it hosed my rootfs). Given the license issue with zfs - and in particular, the technical fallout like needing dkms - I'd be very pleased to replace it.
They also fucked up their parity RAID repair so that the next disk failure on a different disk will destroy the data, argued that the pathological B-tree performance reported by a user couldn't be real, because they didn't understood the tree depth would only be logarithmic to the base of the fanout if the entries are the same size. insert shocked pikachu face
That failure mode is still there although it's been mitigated a bit. I hit it a few months ago but was able to recover after a few hours of doing various stuff.
It's really a shame, though. btrfs is permanently unfinished, and zfs has the wrong license. End result is people stick with ext4 or xfs most of the time anyway.
Otoh, good luck bringing a CoW filesystem back from 100%. Delete a file? Sure, let me just make a copy of all the metadata that was pointing at it using... the zero blocks I have left.
Which is why ZFS reserves "slop space" to make sure that doesn't happen, instead of defaulting to making it easy for users to corner themselves like that.
I have been hearing noise recently that btrfs is risky and unstable but (knocks on wood) i've been running it for years now with zero issues. What am I missing?
Failures in storage software are very rare. Which makes it very hard to test... (you need to run it a lot, for a very long time if you hope to find errors by chance).
Also, some failure modes are worse than others. The failures known as DI (data integrity) are the worst. Even though they aren't expected to happen to everyone at a certain frequency (because, again, mature storage software is comparatively very reliable), even a single DI error that happened to any user sets up a major alarm.
In the storage industry, the running joke is that after first DI in your product you lose funding, after the second DI you loose the product.
And it did happen to Btrfs quite a bit... I've seen it with my own eyes when a system didn't come back after power failure. (But I'm in the business of testing software storage products, so, it's less surprising that it happened to me).
So... it's perfectly plausible that you have never seen Btrfs fail, and it's been more error prone than eg. EXT4. The error rate is low enough so that if you don't actively try to cause the error you will never experience one. But, over a large group of diverse use patterns, the rate is still worse than expected.
It still has an issue with getting stuck in a bad state if you let the free space drop too low (e.g. under 5% / 2GB). It might still be recoverable but it takes a lot of fiddling and work to do that.
btrfs is almost 20 years old now, so lots of people only used it back when it was newer and far buggier.
In my experience, btrfs is actually more reliable than other filesystems due to its checksumming abilities, but when it does fail, it's much harder to fix than with other filesystems (which will often try to continue on even when stuff is broken).
