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[#] Wed Aug 26 2020 13:53:13 EDT from IGnatius T Foobar @ Uncensored

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Fun discovery of the day ... at least for me.

cp(1), or at least the version of cp found on Linux/Linux, has a relatively new option "--reflink" to create a copy-on-write (COW) copy when used on a filesystem that supports it (such as btrfs).

This is going to be really useful for creating low-overhead clones of things without having to create subvolumes and snapshots. Right away I'm going to use this to build groups of virtual machines, because I'm sure btrfs can do "linked clones" better than VMware can.

The btrfs people and the NetApp people should get together and standardize an NFS command to do this remotely.

[#] Wed Aug 26 2020 17:22:10 EDT from LoanShark @ Uncensored

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I wonder if that plays well with overlayfs

[#] Wed Aug 26 2020 17:52:36 EDT from IGnatius T Foobar @ Uncensored

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I don't know. From what I can tell, when the option first appeared, it was just "--reflink" and the man page said "works with btrfs". Recently it changed [ https://linux.die.net/man/1/cp ] to "--reflink=<when>" and "<when>" can be "always" or "auto" with the default being "always" if neither option is specified, and btrfs is no longer mentioned, so they're obviously trying to generalize it.

It looks like they're trying to make the COW-copy heuristics as close as they can to sparse file heuristics, and probably generalizing the kernel interface for it too.

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