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The two sides of reflink()

The two sides of reflink()

Posted May 12, 2009 1:22 UTC (Tue) by giraffedata (subscriber, #1954)
In reply to: The two sides of reflink() by nix
Parent article: The two sides of reflink()

If they weren't in the kernel, they'd need to be in some userspace library (ew).

They work better in user space -- there's more flexibility there and the basic concept of a directory has nothing to do with resource allocation between users, which is what the kernel is for. Many OSes do them outside the kernel. The only reason they have to be in the kernel in Unix is that the kernel deletes files implicitly based on directory references. And as I've been saying, we'd be better off without that.

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The two sides of reflink()

Posted May 12, 2009 19:59 UTC (Tue) by nix (subscriber, #2304) [Link]

Putting directories outside the kernel also means that a whole pile of
things POSIX guarantees become, as near I can tell, impossible to provide.
I can't see any way to keep cross-directory rename() atomic, for instance.

Also it's a grotesque security hole: now you can't keep stuff secret by
hiding it in unreadable directories anymore.

Periodically there are proposals to introduce an open()-by-inode-number
syscall. They are always shot down. I don't know what sort of system
you're thinking of, but it isn't Unix.

(And if you're going to go that route, make the inums 1024 bits long and
bingo, you've got a capability-based system.)

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