Re: PERFORMANCE IMPROVEMENT by mapping WAL FILES

I have just  completed the functional testing  the WAL using mmap  , it is
working  fine,  I  have tested  by commenting out the  "CreateCheckPoint "
functionality so that  when i kill the postgres and restart it will redo all the records from the
WAL log file  which is updated  using mmap.    Just i need  to  clean code and to do some stress testing.By the end of
thisweek i should able to  complete  the stress test  and
 
generate the patch file .   As Tom Lane mentioned  i see the  problem in portability  to all platforms,
     what i propose is to use mmap for only WAL  for some platforms like linux,freebsd etc . For  other platforms we
canuse the existing method by
 
slightly modifying thewrite()  routine to write only the modified part of the page.

Regards
jana

>
>
> OK, I have talked to Tom Lane about this on the phone and we have a few
> ideas.
>
> Historically, we have avoided mmap() because of portability problems,
> and because using mmap() to write to large tables could consume lots of
> address space with little benefit.  However, I perhaps can see WAL as
> being a good use of mmap.
>
> First, there is the issue of using mmap().  For OS's that have the
> mmap() MAP_SHARED flag, different backends could mmap the same file and
> each see the changes.  However, keep in mind we still have to fsync()
> WAL, so we need to use msync().
>
> So, looking at the benefits of using mmap(), we have overhead of
> different backends having to mmap something that now sits quite easily
> in shared memory.  Now, I can see mmap reducing the copy from user to
> kernel, but there are other ways to fix that.  We could modify the
> write() routines to write() 8k on first WAL page write and later write
> only the modified part of the page to the kernel buffers.  The old
> kernel buffer is probably still around so it is unlikely to require a
> read from the file system to read in the rest of the page.  This reduces
> the write from 8k to something probably less than 4k which is better
> than we can do with mmap.
>
> I will add a TODO item to this effect.
>
> As far as reducing the write to disk from 8k to 4k, if we have to
> fsync/msync, we have to wait for the disk to spin to the proper location
> and at that point writing 4k or 8k doesn't seem like much of a win.
>
> In summary, I think it would be nice to reduce the 8k transfer from user
> to kernel on secondary page writes to only the modified part of the
> page.  I am uncertain if mmap() or anything else will help the physical
> write to the disk.
>
> --
>   Bruce Momjian                        |  http://candle.pha.pa.us
>   pgman@candle.pha.pa.us               |  (610) 853-3000
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