Re: O_DIRECT for WAL writes

On Mon, 2005-05-30 at 11:24 -0400, Tom Lane wrote:
> Wouldn't count on it :-(. One thing I'm particularly worried about is
> buffer cache consistency: does the kernel guarantee to flush any buffers
> it has that overlap the O_DIRECT write operation?

At least on Linux I believe the kernel guarantees consistency between
O_DIRECT and non-O_DIRECT operations. From googling, it seems AIX also
provides consistency, albeit not for free[1]:

To avoid consistency issues, if there are multiple calls to open
a file and one or more of the calls did not specify O_DIRECT and
another open specified O_DIRECT, the file stays in the normal
cached I/O mode. Similarly, if the file is mapped into memory
through the shmat() or mmap() system calls, it stays in normal
cached mode. If the last conflicting, non-direct access is
eliminated, then the file system will move the file into direct
I/O mode (either by using the close(), munmap(), or shmdt()
subroutines). Changing from normal mode to direct I/O mode can
be expensive because all modified pages in memory will have to
be flushed to disk at that point.

-Neil

[1]
http://publib16.boulder.ibm.com/pseries/en_US/aixbman/prftungd/diskperf9.htm