this post was submitted on 07 Dec 2023
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As with every software/product: they have different features.
ZFS is not really hip. It's pretty old. But also pretty solid. Unfortunately it's licensed in a way that is maybe incompatible with the GPL, so no one wants to take the risk of trying to get it into Linux. So in the Linux world it is always a third-party-addon. In the BSD or Solaris world though ....
btrfs has similar goals as ZFS (more to that soon) but has been developed right inside the kernel all along, so it typically works out of the box. It has a bit of a complicated history with it's stability/reliability from which it still suffers (the history, not the stability). Many/most people run it with zero problems, some will still cite problems they had in the past, some apparently also still have problems.
bcachefs is also looming around the corner and might tackle problems differently, bringing us all the nice features with less bugs (optimism, yay). But it's an even younger FS than btrfs, so only time will tell.
ext4 is an iteration on ext3 on ext2. So it's pretty fucking stable and heavily battle tested.
Now why even care? ZFS, btrfs and bcachefs are filesystems following the COW philisophy (copy on write), meaning you might lose a bit performance but win on reliability. It also allows easily enabling snapshots, which all three bring you out of the box. So you can basically say "mark the current state of the filesystem with tag/label/whatever 'x'" and every subsequent changes (since they are copies) will not touch the old snapshots, allowing you to easily roll back a whole partition. (Of course that takes up space, but only incrementally.)
They also bring native support for different RAID levels making additional layers like mdadm unnecessary. In case of ZFS and bcachefs, you also have native encryption, making LUKS obsolete.
For typical desktop use: ext4 is totally fine. Snapshots are extremely convenient if something breaks and you can basically revert the changes back in a single command. They don't replace a backup strategy, so in the end you should have some data security measures in place anyway.
*Edit: forgot a word.
Btw COW isn't necessarily (and isn't at least for ZFS) a performance trade-off. Data isn't really copied, new data is simply written elsewhere on the disk (and the old data is not marked as free space).
Ultimately it actually means "the data behaves as though it was copied," which can be achieved in many ways. There are many ways to do that without actually copying.
So let me give an example, and you tell me if I understand. If you change 1MB in the middle of a 1GB file, the filesystem is smart enough to only allocate a new 1MB chunk and update its tables to say "the first 0.5GB lives in the same old place, then 1MB is over here at this new location, and the remaining 0.5GB is still at the old location"?
If that's how it works, would this over time result in a single file being spread out in different physical blocks all over the place? I assume sequential reads of a file stored contiguously would usually have better performance than random reads of a file stored all over the place, right? Maybe not for modern SSDs...but also fragmentation could become a problem, because now you have a bunch of random 1MB chunks that are free.
I know ZFS encourages regular "scrubs" that I thought just checked for data integrity, but maybe it also takes the opportunity to defrag and re-serialize? I also don't know if the other filesystems have a similar operation.
Not OP, but yes, that's pretty much how it works. (ZFS scrubs do not defrgment data however).
Fragmentation isn't really a problem for several reasons.
Some (most?) COW filesystems have mechanisms to mitigate fragmentation. ZFS, for instance, uses a special allocation strategy to minimize fragmentation and can reallocate data during certain operations like resilvering or rebalancing.
ZFS doesn't even have a traditional defrag command. Because of its design and the way it handles file storage, a typical defrag process is not applicable or even necessary in the same way it is with other traditional filesystems
Btrfs too handles chunk allocation effeciently and generally doesn't require defragmentation, and although it does have a defrag command, it's almost never used by anyone, unless you have a special reason to (eg: maybe you have a program that is reading raw sectors of a file, and needs the data to be contiguous).
Fragmentation is only really an issue for spinning disks, however, that is no longer a concern for most spinning disk users because:
Enterprise users also almost always use a RAID (or similar) setup, so the same as above applies. They also use filesystems like ZFS which employs heavy caching mechanisms, typically backed by SSDs/NVMes, so again, fragmentation isn't really an issue.
Cool, good to know. I'd be interested to learn how they mitigate fragmentation, though. It's not clear to me how COW could mitigate the copy cost without fragmentation, but I'm certain people smarter than me have been thinking about the problem for my whole life. I know spinning disks have their own set of limitations, but even SSDs perform better on sequential reads over random reads, so it seems like the preference would still be to not split a file up too much.