The guests typically then have a limited root capability that does not
included making device nodes so they really do not have access to the
device, only the filesystem.
With the limits on the root capabilities, the binaries can surely be made
read-only even for the guest roots, so no reflinks are needed for the
Sure, but if you make the binaries read only you no longer have independent guest systems that can be administered without knowledge of the host or other guests. In other words, if I now want to upgrade the apache server in one guest, I can't since the binary is read only to my guest root user. With COW, no problem, as a guest admin I do not even know that my apache binary is shared with others. It is only relevant to the host (the host unifies the various guest binaries, not the guest).
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