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COW LinksCOW LinksPosted Apr 2, 2004 23:05 UTC (Fri) by giraffedata (subscriber, #1954)Parent article: COW Links
This is really quite an assault on the integrity of the file interface definition. If I understand what's proposed, it means that when you write to a file, if you happened to originally find that file via a directory entry that has/had the "cow" attribute, then before your write goes through, the system creates a new file, copies the entire contents of that old file to the new one, and then magically changes some (which?) file pointers and some (which?) open instances to refer to the new file instead of the old one. Or looked at another way, you have two different files all along, but most of the internal data structures and code think there's only one. The implications of that are so messy, I don't even want to start going through all the scenarios. The overlay filesystem mentioned above is a cleaner alternative. Another cleaner alternative is just having multiple files share the same blocks in copy-on-write fashion. Things like du and diff can be inconvenienced by these, but at least mainstream file access is still sane.
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