Hacker News Reader

Comment by fer | Hacker News Reader

I have high hopes for bcachefs, but so far the benchmarks[0] are a quite disappointing. I understand it'll have overhead since it does many things, but I'd expect it to perform closer to btrfs or zfs, but it's consistently abysmal (which affects zfs at times, too).

[0] https://www.phoronix.com/review/linux-617-filesystems

View on HN

https://www.phoronix.com/review/bcachefs-617-dkms/2

has the benchmarks of the dkms module

Why is the DKMS module so much faster than the original one? Just wondering lol

I think it has improvements that were never upstreamed to the kernel, based on the developer's comments elsewhere[0].

[0]: https://www.phoronix.com/forums/forum/software/general-linux...

It's hard to take those benchmarks too seriously. ZFS, btrfs and I guess bcachefs - which I've never used and don't have any opinion on - do things XFS and EXT4 don't and can't do.

I know more about ZFS than the others. It wasn't specified here whether ZFS had ashift=9 or 12; it tries to auto-detect, but that can go wrong. ashift=9 means ZFS is doing physical I/O in 512 bytes, which will be an emulation mode for the nvme. Maybe it was ashift=12. But you can't tell.

Secondly, ZFS defaults to a record size of 128k. Write a big file and it's written in "chunks" of 128k size. If you then run a random read/write I/O benchmark on it with a 4k block size, ZFS is going to be reading and writing 128k for every 4k of I/O. That's a huge amplification factor. If you're using ZFS for a load which resembles random block I/O, you'll want to tune the recordsize to the app I/O. And ZFS makes this easy, since child filesystem creation is trivially cheap and the recordsize can be tuned per filesystem.

And then there's the stuff things like ZFS does that XFS / EXT4 doesn't. For example, taking snapshots every 5 minutes (they're basically free), doing streaming incremental snapshot backups, snapshot cloning and so on - without getting into RAID flexibility.

I don't think any of that means the benchmarks shouldn't be taken seriously. GP didn't say they expect Bcachefs to perform like EXT4/XFS, they said they expected more like Btrfs or ZFS, to which it has more similar features.

On the the configuration stuff, these benchmarks intentionally only ever use the default configuration – they're not interested in the limits of what's possible with the filesystems, just what they do "out of the box", since that's what the overwhelming majority of users will experience.

Anyone who uses zfs out of the box in a way substitutable with xfs, shouldn't. So I guess they serve a purpose that way. But that argument doesn't need any numbers at all.

> Anyone who uses zfs out of the box in a way substitutable with xfs, shouldn't.

Substitutable how? Like, I'm typing this on a laptop with a single disk with a single zpool, because I want 1. compression, 2. data checksums, 3. to not break (previous experiments with btrfs ended poorly). Obviously I could run xfs, but then I'd miss important features.

> If you're using ZFS for a load which resembles random block I/O, you'll want to tune the recordsize to the app I/O.

You probably don't want to do that because that'll result in massive metadata overhead, and nothing tells you that the app's I/O operations will be nicely aligned, so this cannot be given as general advice.

Those benchmarks were messed up. Notice bcachefs is the only one using 512b block size. That's going to massively increase overhead.

> I very much doubt that's the main issue - the multithreaded sqlite performance makes me wonder if something's up with our fsync performance. I'll wait to see the results with the DKMS version, and if the numbers are still bad I'll have to see if I can replicate it and start digging.

https://www.phoronix.com/forums/forum/software/general-linux...

Reply on news.ycombinator.com