Re: Allowing WAL fsync to be done via O_SYNC

* Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> [010315 14:54] wrote:
> Alfred Perlstein <bright(at)wintelcom(dot)net> writes:
> > How many files need to be fsync'd?
>
> Only one.
>
> > If it's more than one, what might work is using mmap() to map the
> > files in adjacent areas, then calling msync() on the entire range,
> > this would allow you to batch fsync the data.
>
> Interesting thought, but mmap to a prespecified address is most
> definitely not portable, whether or not you want to assume that
> plain mmap is ...

Yeah... :(

Evil thought though (for reference):

mmap(anon memory) returns addr1
addr2 = addr1 + maplen
split addr1<->addr2 on points A B and C
mmap(file1 over addr1 to A)
mmap(file2 over A to B)
mmap(file3 over B to C)
mmap(file4 over C to addr2)

It _should_ work, but there's probably some corner cases where it
doesn't.

--
-Alfred Perlstein - [bright(at)wintelcom(dot)net|alfred(at)freebsd(dot)org]