From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Curt Sampson <cjs(at)cynic(dot)net>, "J(dot) R(dot) Nield" <jrnield(at)usol(dot)com>, PostgreSQL Hacker <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Buffer Management |
Date: | 2002-06-25 14:56:58 |
Message-ID: | 200206251456.g5PEuwO15564@candle.pha.pa.us |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
Tom Lane wrote:
> Curt Sampson <cjs(at)cynic(dot)net> writes:
> > On Tue, 25 Jun 2002, Tom Lane wrote:
> >> The other discussion seemed to be considering how to mmap individual
> >> data files right into backends' address space. I do not believe this
> >> can possibly work, because of loss of control over visibility of data
> >> changes to other backends, timing of write-backs, etc.
>
> > I don't understand why there would be any loss of visibility of changes.
> > If two backends mmap the same block of a file, and it's shared, that's
> > the same block of physical memory that they're accessing.
>
> Is it? You have a mighty narrow conception of the range of
> implementations that's possible for mmap.
>
> But the main problem is that mmap doesn't let us control when changes to
> the memory buffer will get reflected back to disk --- AFAICT, the OS is
> free to do the write-back at any instant after you dirty the page, and
> that completely breaks the WAL algorithm. (WAL = write AHEAD log;
> the log entry describing a change must hit disk before the data page
> change itself does.)
Can we mmap WAL without problems? Not sure if there is any gain to it
because we just write it and rarely read from it.
--
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
From | Date | Subject | |
---|---|---|---|
Next Message | Thomas Lockhart | 2002-06-25 14:58:49 | Re: SQL99, CREATE CAST, and initdb |
Previous Message | Bruce Momjian | 2002-06-25 14:55:54 | Re: Buffer Management |