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Re: PERFORMANCE IMPROVEMENT by mapping WAL FILES

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Janardhana Reddy <jana-reddy(at)mediaring(dot)com(dot)sg>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: PERFORMANCE IMPROVEMENT by mapping WAL FILES
Date: 2001-09-26 15:15:00
Message-ID: 919.1001517300@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Janardhana Reddy <jana-reddy(at)mediaring(dot)com(dot)sg> writes:
>           By   mapping  the WAL files by each backend  in to its address
> space  using "mmap"  system call ,

There are a lot of problems with trying to use mmap for Postgres.  One
is portability: not all platforms have mmap, so we'd still have to
support the non-mmap case; and it's not at all clear that fsync/msync
semantics are consistent across platforms, either.  A bigger objection
is that mmap'ing a file in one backend does not cause it to become
available to other backends, thus the entire concept of shared buffers
breaks down.

If you think you can make it work, feel free to try it ...

			regards, tom lane

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