Messages in this thread | | | Date | Thu, 11 Jan 2007 17:09:43 +1100 | From | Nick Piggin <> | Subject | Re: O_DIRECT question |
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Linus Torvalds wrote: > > On Wed, 10 Jan 2007, Linus Torvalds wrote: > >>So don't use O_DIRECT. Use things like madvise() and posix_fadvise() >>instead. > > > Side note: the only reason O_DIRECT exists is because database people are > too used to it, because other OS's haven't had enough taste to tell them > to do it right, so they've historically hacked their OS to get out of the > way. > > As a result, our madvise and/or posix_fadvise interfaces may not be all > that strong, because people sadly don't use them that much. It's a sad > example of a totally broken interface (O_DIRECT) resulting in better > interfaces not getting used, and then not getting as much development > effort put into them. > > So O_DIRECT not only is a total disaster from a design standpoint (just > look at all the crap it results in), it also indirectly has hurt better > interfaces. For example, POSIX_FADV_NOREUSE (which _could_ be a useful and > clean interface to make sure we don't pollute memory unnecessarily with > cached pages after they are all done) ends up being a no-op ;/ > > Sad. And it's one of those self-fulfilling prophecies. Still, I hope some > day we can just rip the damn disaster out.
Speaking of which, why did we obsolete raw devices? And/or why not just go with a minimal O_DIRECT on block device support? Not a rhetorical question -- I wasn't involved in the discussions when they happened, so I would be interested.
O_DIRECT is still crazily racy versus pagecache operations. Chris Mason's recent patches to attempt to fix it, while actually doing quite a fine job, are very intrusive and complex for such a sad corner case.
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