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 this week 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 can use the existing method by
slightly modifying the
write() routine to write only the modified part of the page.
> OK, I have talked to Tom Lane about this on the phone and we have a few
> 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(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
> + If your life is a hard drive, | 830 Blythe Avenue
> + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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