There’s been some Friday night kernel drama on the Linux kernel mailing list… Linus Torvalds has expressed regrets for merging the Bcachefs file-system and an ensuing back-and-forth between the file-system maintainer.
ZFS doesn’t support tiered storage at all. Bcachefs is capable of promoting and demoting files to faster but smaller or slower but larger storage. It’s not just a cache. On ZFS the only option is really multiple zpools. Like you can sort of do that with the persistent L2ARC now but TBs of L2ARC is super wasteful and your data has to fully fit the pool.
Tiered storage is great for VMs and games and other large files. Play a game, promote to NVMe for fast loadings. Done playing, it gets moved to the HDDs.
You’re misrepresenting L2ARC and it’s a silly comparison to claim to need TBs of L2ARC and then also say you’d copy the game to nvme just to play it on bcachefs. That’s what ARC does. RAM and SSD caching of the data in use with tiered heuristics.
I know, that was an example of why it doesn’t work on ZFS. That would be the closest you can get with regular ZFS, and as we both pointed out, it makes no sense, it doesn’t work. The L2ARC is a cache, you can’t store files in it.
The whole point of bcachefs is tiering. You can give it a 4TB NVMe, a 4TB SATA SSD and a 8 GB HDD and get almost the whole 16 TB of usable space in one big filesystem. It’ll shuffle the files around for you to keep the hot data set on the fastest drive. You can pin the data to the storage medium that matches the performance needs of the workload. The roadmap claims they want to analyze usage pattern and automatically store the files on the slowest drive that doesn’t bottleneck the workload. The point is, unlike regular bcache or the ZFS ARC, it’s not just a cache, it’s also storage space available to the user.
You wouldn’t copy the game to another drive yourself directly. You’d request the filesystem to promote it to the fast drive. It’s all the same filesystem, completely transparent.
Looks dead on arrival to me, so much complexity for “performance” but the filessystem is outclassed by everything else in existence. If there was any real performance from this complexity it could have cool niche use cases but this is very disappointing https://www.phoronix.com/review/bcachefs-linux-67/2
