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Thread: Disable WAL backup pages when fsync is off

Disable WAL backup pages when fsync is off

From
Bruce Momjian
Date:
The following patch disables WAL backup pages when fsync is off.

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073
Index: src/backend/access/transam/xlog.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v
retrieving revision 1.205
diff -c -c -r1.205 xlog.c
*** src/backend/access/transam/xlog.c    30 Jun 2005 00:00:50 -0000    1.205
--- src/backend/access/transam/xlog.c    3 Jul 2005 02:52:57 -0000
***************
*** 593,599 ****
                  {
                      /* OK, put it in this slot */
                      dtbuf[i] = rdt->buffer;
!                     if (XLogCheckBuffer(rdt, &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                      {
                          dtbuf_bkp[i] = true;
                          rdt->data = NULL;
--- 593,601 ----
                  {
                      /* OK, put it in this slot */
                      dtbuf[i] = rdt->buffer;
!                     /* If fsync is off, no need to backup pages. */
!                     if (enableFsync &&
!                         XLogCheckBuffer(rdt, &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                      {
                          dtbuf_bkp[i] = true;
                          rdt->data = NULL;

Re: Disable WAL backup pages when fsync is off

From
Tom Lane
Date:
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> The following patch disables WAL backup pages when fsync is off.

This is a good idea why?

            regards, tom lane

Re: Disable WAL backup pages when fsync is off

From
Bruce Momjian
Date:
Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > The following patch disables WAL backup pages when fsync is off.
>
> This is a good idea why?

There is no need to write backup pages if fsync is off.  Our TODO has:

    * Turn off after-change writes if fsync is disabled

      If fsync is off, there is no purpose in writing full pages to WAL

Basically, with fsync off, you are saying you don't care about crash
recovery, so why write those pages?

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

Re: Disable WAL backup pages when fsync is off

From
Bruce Momjian
Date:
Russell Smith wrote:
> On Sun, 3 Jul 2005 03:51 pm, Tom Lane wrote:
> > Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > > The following patch disables WAL backup pages when fsync is off.
> >
> > This is a good idea why?
> >
> If it is, why do we write wal at all if fsync is off?

Good question.  WAL is to recover the file system files in case of a
crash.  WAL is used in cases where PostgreSQL crashes, but the operating
system is still running.  WAL brings the file system files back to a
consistent state containing all completed transactions.

fsync() is for forcing the data to disk so an operating system crash is
recoverable.  If that isn't happening, the cleanup of partial page
writes is hardly important because the system is not consistent.

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073