Use ioctl_ficlonerange(2) instead of copy_file_range(2)

[AkihiroSuda]

The cp --reflink command uses the FICLONERANGE ioctl() command, but not copy_file_range(); in fact, no utilities use copy_file_range(), though it should be the default.

https://lwn.net/Articles/754506/

And there is also ioctl_fideduperange(2. really confusing.. :(

[stevvooe]

Why? Are you suggesting we change? It sounds like containerd makes the correct choice over cp.

[AkihiroSuda]

Does current containerd really dedupe?

[AkihiroSuda]

Sorry copy_file_range seems actually doing reflink (at least on XFS): #128

[AkihiroSuda]

But it looks like potentially there is case where copy_file_range does not do deduplication while ioctl_ficlonerange does, on some non-XFS filesystems?

https://twitter.com/lordcyphar/status/1039417947033194496