> I'm not an expert (but, so most people are no experts in this area), but i have a feeling that ZFS needs huge amounts of memory (compared to ext). It does nifty stuff, for sure. But do i need all of that? Hardly.

ZFS needs more memory than ext4, but reports for just how much memory ZFS needs is grossly over estimated. At least for desktop usage - file servers are a different matter and thus that's where those figures come from.

To use a practical example, I've ran ZFS + 5 virtual machines on 4GB RAM and not had any issues what-so-ever.

> For example, i am wondering why i would want to put compression into the file system.

A better question would be, why wouldn't you? It happens transparently and causes next to no additional overhead. But you can have that disabled in ZFS (or any other file system) if you really want to.

> Or deduplication.

Deduplication is disabled on ZFS by default. It's actually a pretty nichely used feature despite it's wider reporting.

> Or the possibility to put some data on SSD and other on HDD. If i have a server and user data, it should be up to the application to do this stuff. That should be more efficient, because the app actually knows how to handle the data correctly and efficiently. It would also be largely independent of the file storage.

I don't get your point here. ZFS doesn't behave any differently to ext in that regard. Unless you're talking about SSD cache disks, in which case that's something you have to explicitly set up.

> I've seen some cases where we had a storage system, pretending to do fancy stuff and fail on it. And debugging such things is a nightmare (talking about storage vendors here, though). But for example, a few years ago we had major performance problems because a ZFS mount was at about 90% of space. It was not easy to pinpoint that. After the fact it's clear, and nowadays googling it would probably give enough hints. But in the end I would very much like that my filesystem does not just slow down depending on how much filled it is. Or how much memory i have.

ZFS doesn't slow down if the storage pools are full; the problem you described there sounds more like fragmentation, and that affects all file systems. Also the performance of all file systems is also memory driven (and obviously storage access times). OS's cache files in RAM (this is why some memory reporting tools say Windows or Linux is using GB's RAM even when there's little or no open applications - because they don't exclude cached memory from used memory). This happens with ext4, ZFS, NTFS, xfs and even FAT32. Granted ZFS has a slightly different caching model to the Linux kernel's, but file caching is something that is free memory driven and applies to every file system. This is why file servers are usually speced with lots of RAM - even when running non-ZFS storage pools.

I appreciate that you said none of us are experts on file systems, but it sounds to me like you've based your judgement on a number of anecdotal assumptions; in that the problems you raised are either not part of ZFS's default config, or are limitations present in any and all file systems out there but you just happened upon them in ZFS.

> i am wondering if it is just too much. For example, nowadays you have a lot of those stateless redundant S3 compatible storage backends. Or use Cassandra, etc. Those already copy your data multiple times. Even if they run on ZFS, you don't gain much.

While that's true, you are now comparing Apples to oranges. But in any case, it's not best practice to be running a high performance database on top of ZFS (nor any of CoW file system). So in those instances ext4 or xfs would definitely be a better choice.

FYI, I also wouldn't recommend ZFS for small capacity / portable storage devices nor many real time appliances. But if file storage is your primary concern, then ZFS definitely has a number of advantages over ext4 and xfs which aren't over-complicated nor surplus toys (eg snapshots, CoW journalling, online scrubbing, checksums, datasets, etc).