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Comment by krob | Hacker News Reader

Biggest hurdle in using zfs is that it consumes a huge amount of ram and uses considerable CPU cycles when compared with xfs or ext4. It's like running a massive MySQL server on top of another MySQL server if ever that was possible. Follow Reddit datahorders subreddit people speak all the time about how powerful freenas machine must be just to give decent transfer speeds over samba. Yes freenas will run without lot of CPU speed and ram but performance will suffer greatly.

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ZFS uses lot of RAM only if you enable deduplication (and once you enable it, you cannot disable it). Without it, the RAM and CPU time requirements are very modest.

That’s not at all correct, without deduplication enabled I have observed massive memory usage across BSD, Linux and ZFS on macOS.

That's probably the ARC, the in-memory caching structure, which gives up memory when applications need it. The weird thing on Linux (and maybe the others) is that the kernel marks the memory as "used" and not "cache", so tools like top/htop show high memory usage, even though ZFS will shrink its ARC when any actual applications require it.

That's the ARC. The ARC max size will default to (at least on FreeBSD) total ram -1GB[1]. Because the ARC is read cache it will shrink if the system needs more memory. Unused ram is wasted ram.

[1]https://www.freebsd.org/doc/handbook/zfs-advanced.html

Minor correction: you can disable it, but this doesn't change the status of the data already deduped. This is in fact true also when you enable it: current data is unaffected by the change, you need to move/copy it.

The biggest issue is that there is no official tool to have a list of files that are currently deduped (I had to write a script that parse the output of zdb) so if you're not sure you have to move all the data out of the dataset.

I don't know about that. Essentially there's this range from "use a lot of storage" to "use a lot of computing power". Most traditional file systems use a lot of storage and that's that. ZFS let's you pick a point on the scale it will operate at.

We use ZFS at work. ZFS wants enough memory to be happy. One learns usually learns how much memory is too little by not giving it enough memory. Basically ZFS is great if you want the benefits of ZFS, and are willing to deal with the tradeoffs.

What do you mean by trade-offs? My experience with it has been pretty much all upside.

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