Re: posix_fadvise missing in the walsender

On Mon, Feb 18, 2013 at 2:16 AM, Heikki Linnakangas
<hlinnakangas@vmware.com> wrote:
> On 17.02.2013 14:55, Joachim Wieland wrote:
>>
>> In access/transam/xlog.c we give the OS buffer caching a hint that we
>> won't need a WAL file any time soon with
>>
>>      posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
>>
>> before closing the WAL file, but only if we don't have walsenders.
>> That's reasonable because the walsender will reopen that same file
>> shortly after.
>>
>> However the walsender doesn't call posix_fadvise once it's done with
>> the file and I'm proposing to add this to walsender.c for consistency
>> as well.
>>
>> Since there could be multiple walsenders, only the "slowest" one
>> should call this function. Finding out the slowest walsender can be
>> done by inspecting the shared memory and looking at the sentPtr of
>> each walsender.
>
>
> I doubt it's worth it, the OS cache generally does a reasonable job at
> deciding what to keep. In the non-walsender case, it's pretty clear that we
> don't need the WAL file anymore, but if we need to work any harder than a
> one-line posix_fadvise call, meh.

If that's the case, why have the advisory call at all?  The OS is
being lied too (in some cases)...

merlin