Re: O_DIRECT question

[Phillip Susi]

dean gaudet wrote:
> it seems to me that if splice and fadvise and related things are 
> sufficient for userland to take care of things "properly" then O_DIRECT 
> could be changed into splice/fadvise calls either by a library or in the 
> kernel directly...

No, because the semantics are entirely different.  An application using 
read/write with O_DIRECT expects read() to block until data is 
physically fetched from the device.  fadvise() does not FORCE the kernel 
to discard cache, it only hints that it should, so a read() or mmap() 
very well may reuse a cached page instead of fetching from the disk 
again.  The application also expects write() to block until the data is 
on the disk.  In the case of a blocking write, you could splice/msync, 
but what about aio?


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