Tom Lane wrote:
> Bruce Momjian <bruce@xxxxxxxxxx> writes:
> >>> I asked on IRC and was told it is true, and looking at the C code it
> >>> looks true. ?What synchronous_commit = false does is to delay writing
> >>> the wal buffers to disk and fsyncing them, not just fsync, which is
> >>> where the commit loss due to db process crash comes from.
>
> >> Ah, I see. Thanks.
>
> > I am personally surprised it was designed that way; I thought we would
> > just delay fsync.
>
> That would require writing and syncing to be separable actions. If
> you're using O_SYNC or similar, they aren't.
Ah, very good point. I have added a C comment to clarify why this is
the current behavior; attached and applied.
--
Bruce Momjian <bruce@xxxxxxxxxx> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ None of us is going to be here forever. +
Index: src/backend/access/transam/xact.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/transam/xact.c,v
retrieving revision 1.291
diff -c -c -r1.291 xact.c
*** src/backend/access/transam/xact.c 13 May 2010 11:39:30 -0000 1.291
--- src/backend/access/transam/xact.c 29 Jun 2010 18:33:47 -0000
***************
*** 1028,1034 ****
if (XactSyncCommit || forceSyncCommit || haveNonTemp)
{
/*
! * Synchronous commit case.
*
* Sleep before flush! So we can flush more than one commit records
* per single fsync. (The idea is some other backend may do the
--- 1028,1034 ----
if (XactSyncCommit || forceSyncCommit || haveNonTemp)
{
/*
! * Synchronous commit case:
*
* Sleep before flush! So we can flush more than one commit records
* per single fsync. (The idea is some other backend may do the
***************
*** 1054,1060 ****
else
{
/*
! * Asynchronous commit case.
*
* Report the latest async commit LSN, so that the WAL writer knows to
* flush this commit.
--- 1054,1065 ----
else
{
/*
! * Asynchronous commit case:
! *
! * This enables possible committed transaction loss in the case of a
! * postmaster crash because WAL buffers are left unwritten.
! * Ideally we could issue the WAL write without the fsync, but
! * some wal_sync_methods do not allow separate write/fsync.
*
* Report the latest async commit LSN, so that the WAL writer knows to
* flush this commit.
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