idea: consider abolishing hardlinks

[allisonkarlitskaya]

There is a lot of complexity that we carry in order to faithfully represent hardlinks in a filesystem that is read-only and already features data de-duplication. Maybe it's time to make our lives easier...

@cgwalters what do you think? We've discussed this in the past....

@alexlarsson might also have an opinion on this

[cgwalters]

I haven't dug into this section of composefs-rs and I have no opinion on its complexity at the moment.

Presumably what you're specifically getting at is that the composefs-EROFS wouldn't represent the hardlinks, but splitstream would still naturally reassemble them right?

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There's definitely huge problems in this area. ostree has from the start always forcibly hardlinked things that are content identical (which we then backpedaled with an exception for zero sized files). Obviously from my PoV getting away from that is great.

Going the opposite and always breaking links I think could lead to some corner cases. We have hardlinks for the rpmdb in some cases in rpm-ostree for legacy reasons. The current ostree-container base image format leans into hardlinks as a way to ship the object checksums, but obviously that can go away too (though it'd be nice to have a canonical way to ship the fsverity digests, which I guess is either something zstd:chunked/estargz like or maybe a special xattr?).

However, another big problem in this space is while composefs is read-only, some common use cases will have writable uppers w/overlayfs and it's possible that some workload was actually relying on the hardlink semantics of changing one changing all names.

[alexlarsson]

podman/OCI explicitly needs hardlinks. We have a hardlinking backend of data sharing in containers/storage, but it cannot be enabled by default due to the different behaviour with not respecting hardlinks.

[allisonkarlitskaya]

podman/OCI explicitly needs hardlinks. We have a hardlinking backend of data sharing in containers/storage, but it cannot be enabled by default due to the different behaviour with not respecting hardlinks.

So maybe we close this?