Also, keep in mind writes to O_DIRECT devices have to wait for the data
to get on the platters rather than into the kernel cache.
---------------------------------------------------------------------------
Tom Lane wrote:
> "Jim C. Nasby" <jim@nasby.net> writes:
> >> DB2 and Oracle, from memory, allow users to pass hints to the planner to
> >> use/not use file system caching.
>
> > Might it make sense to do this for on-disk sorts, since sort_mem is
> > essentially being used as a disk cache (at least for reads)?
>
> If sort_mem were actually being used that way, it might be ... but it
> isn't, and so I doubt O_DIRECT would be an improvement. It seems more
> likely to force disk I/O that otherwise might not happen at all, if the
> kernel happens to have sufficient buffer space on hand.
>
> I'll concede though that a large sort would probably have the effect of
> blowing out the kernel's disk cache. So while O_DIRECT might be a net
> pessimization as far as the sort itself is concerned, it would probably
> be more friendly to the rest of the system, by leaving disk buffers free
> for more productive uses. It'd all depend on your workload ...
>
> regards, tom lane
>
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