Tom Lane wrote:
> Bruce Momjian <[EMAIL PROTECTED]> writes:
> > Basically, I think we need free-behind rather than O_DIRECT.
> There are two separate issues here --- one is what's happening in our
> own cache, and one is what's happening in the kernel disk cache.
> Implementing our own free-behind code would help in our own cache but
> does nothing for the kernel cache.


> My thought on this is that for large seqscans we could think about
> doing reads through a file descriptor that's opened with O_DIRECT.
> But writes should never go through O_DIRECT.  In some scenarios this
> would mean having two FDs open for the same relation file.  This'd
> require moderately extensive changes to the smgr-related APIs, but
> it doesn't seem totally out of the question.  I'd kinda like to see
> some experimental evidence that it's worth doing though.  Anyone
> care to make a quick-hack prototype and do some measurements?

True, it is a cost/benefit issue.  My assumption was that once we have
free-behind in the PostgreSQL shared buffer cache, the kernel cache
issues would be minimal, but I am willing to be found wrong.

  Bruce Momjian                        |
  [EMAIL PROTECTED]               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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