Re: Providing anonymous mmap as an option of sharing memory

Tom Lane wrote:
> Shridhar Daithankar <shridhar_daithankar(at)myrealbox(dot)com> writes:
>
>>I was looking thr. the source and thought it would be worth to seek
>>opinion on this proposal.
> This has been discussed and rejected before. See the archives.

I went thr. this for details.

http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/TODO.detail/mmap

There seem to be two objections to mmap.

1. If a backend from last crashed running postmaster exists then it might have
file etc. open and that is in general not such a good idea

2. For replacing stdio for data and WAL files with mmap, mmap does not guarantee
order of IO which defeats WAL.

I covered only first point in my post. IMO it is not such a unsolvable problem.
If a postmaster crashes hard but leaves a backend running, would it clean pid
file etc? I don't think so. So if a postmaster can start on a 'pid-clean' state,
then it is guaranteed to be no childs left around.

There were issues where linux not supporting MAP_SHARE and MAP_ANONYMOUS
simaltenously but they are quite old messages, from 1998, talking of linux
2.0.x. I don't think it is still true anymore but need to check.

Too bad, freeBSD M_NOSYNC is not a standard otherwise even for point 2, it could
have been considered.

Did I miss something?

Shridhar