Re: Free WAL caches on switching segments

I looked this over and I am unsure what this does for us that isn't
already accomplished using the wal_sync_method settings.  See xlog.c for
a description of O_DIRECT and when it is used.

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ITAGAKI Takahiro wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> wrote:
>
> > > > > Here is a small patch to prevent undesired WAL file caching by kernel.
> > > > > posix_fadvise(POSIX_FADV_DONTNEED) attempts to free cached pages and
> > > > > the kernel will discard them in preference to other data caches.
> > > >
> > > > On plenty of platforms, this won't even compile ...
> >
> > Yes, but we probably have to have a configure test to see if
> > posix_fadvise exists too.  I will keep this for 8.2.
>
> I think we can use _POSIX_ADVISORY_INFO to test if posix_fadvise exists.
> Also, I added the check on whether WAL archiving is enabled, because
> archivers might use the caches to read the WAL segment.
>
>
> By the way, should we put posix_fadvise on the separated place with renaming
> pg_fadvise? If we use posix_fadvise in other purposes, for example,
> read-ahead control, the separation would be good to keep codes clean.
>
> ---
> ITAGAKI Takahiro
> NTT Cyber Space Laboratories
>

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