Free WAL caches on switching segments

Hi,

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.

I think it works just like as O_DIRECT in terms of cache control.
O_DIRECT may be a better solution than posix_fadvise, but
posix_fadvise may be used on platforms where O_DIRECT is not supported.

pgbench results are as follows:
wal_sync_method
- open_sync : 156.0 tps
- fdatasync : 126.3 tps
- fdatasync+fadvise : 161.2 tps
(8.1beta1 on Linux 2.6.8-24)

I'll appreciate any comments and advices,
Takahiro

*** xlog.c Mon Aug 29 11:51:19 2005
--- xlog-with-fadvise.c Tue Aug 30 15:08:24 2005
***************
*** 1352,1357 ****
--- 1352,1358 ----
Assert(npages == 0);
if (openLogFile >= 0)
{
+ posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
if (close(openLogFile))
ereport(PANIC,
(errcode_for_file_access(),
***************
*** 1535,1540 ****
--- 1536,1542 ----
if (openLogFile >= 0 &&
!XLByteInPrevSeg(LogwrtResult.Write, openLogId, openLogSeg))
{
+ posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
if (close(openLogFile))
ereport(PANIC,
(errcode_for_file_access(),