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Linus Torvalds Begins Expressing Regrets Merging Bcachefs (www.phoronix.com)
submitted 2 years ago by to c/linux@lemmy.ml
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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.

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[–] 7 points 2 years ago (9 children)

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.

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  • [–] 3 points 2 years ago* (last edited 2 years ago) (8 children)

    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.

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  • [–] 4 points 2 years ago (7 children)

    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.

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  • [–] 1 point 2 years ago (6 children)

    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

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  • [–] 2 points 2 years ago* (3 children)

    Brand new anything will not show up with amazing performance, because the primary focus is correctness and features secondary.

    Premature optimisation could kill a project's maintainability; wait a few years. Even then, despite Ken's optimism I'm not certain we'll see performance beating a good non-cow filesystem; XFS and EXT4 have been eeking out performance for many years.

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  • [–] 1 point 2 years ago (2 children)

    Cow is an excuse for writing performance, though the read is awful too currently

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  • [–] 1 point 2 years ago (1 child)

    A rather overly simplistic view of filesystem design.

    More complex data structures are harder to optimise for pretty much all operations, but I'd suggest the overwhelmingly most important metric for performance is development time.

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