Re: O_DIRECT question

[Nick Piggin <nickpiggin@yahoo.com.au>]

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.

-- 
SUSE Labs, Novell Inc.
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