Re: O_DIRECT in freebsd

On 29 Oct 2003, Doug McNaught wrote:

> Christopher Kings-Lynne <chriskl@familyhealth.com.au> writes:
> 
> > FreeBSD 4.9 was released today.  In the release notes was:
> > 
> > 2.2.6 File Systems
> > 
> > A new DIRECTIO kernel option enables support for read operations that
> > bypass the buffer cache and put data directly into a userland
> > buffer. This feature requires that the O_DIRECT flag is set on the
> > file descriptor and that both the offset and length for the read
> > operation are multiples of the physical media sector size.
> > 
> > Is that of any use?
> 
> Linux and Solaris have had this for a while.  I'm pretty sure it's
> been discussed before--search the archives.  I think the consensus
> was that it might be useful for WAL writes, but would be a fair amount
> of work and would introduce portability issues...

I would think the biggest savings could come from using directIO for 
vacuuming, so it doesn't cause the kernel to flush buffers.

Would that be just as hard to implement?