COW Links

COW Links

Posted Apr 1, 2004 18:25 UTC (Thu) by JoeBuck (subscriber, #2330)
Parent article: COW Links

I think that there's a good argument for making COW links appear to have Posix semantics by default, so that applications that don't know about COW links work correctly, but those that have been modified to know about them can do better.

The model to use is to think of COW links as an optimization of file copying, and then to use them all the time (e.g. tell cp to use COW links if the copy is to the same FS). The model, to a naive program, is that a COW link is a complete copy of the file. This means that a naive du program will show the COW links as taking up space that they don't, and diff and cmp's shortcuts, where they know that two files are identical if they have the same inode, won't work. But the key point is that all programs will continue to work correctly. Then a new interface can be added to detect the COW link, which will permit programs like du, cmp, diff etc. to work more efficiently. If we don't assure that COW links obey well-understood POSIX semantics, it is likely that new ways to attack security will be found (because some program that doesn't know about COW links might be made to malfunction if one is present), and if we depart from known semantics, that just opens up a can of worms (how do hard links to COW links to symbolic links behave, etc). If a COW link is treated by all of the existing POSIX operations as if it were a distinct, separate file, these questions answer themselves.

I hope that by the time COW links reach production, discussions take place with the BSD people so that portable free software can take advantage of them.

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