Re: fsync and wal_sync_method

On Wed, Sep 11, 2013 at 6:11 PM, ascot(dot)moss(at)gmail(dot)com
<ascot(dot)moss(at)gmail(dot)com>wrote:

> Hi,
>
> I am using pg 9.2.4 and ubuntu 12.04 to set up a pair of pg-Master and
> pg-Slave with streaming replication.
>
> The archive_command is enabled and the "rsync" is used in pg-Master to
> save all its archived WAL files to the 3rd machine for backup purpose, by
> default, both fsync and wal_sync_method are commented out in
> postgresql.conf:
>
> archive_command = '(cp %p /usr/pg_arcxlog/%f && rsync -aq %p pg(at)pg_slave
> :/usr/pg_arcxlog/%f'
> #fsync = on # turns forced
> synchronization on or off
> #wal_sync_method = fsync # the default is the first option
>
> Thus I think the latest WAL might not be flushed to disk from time to time
> in pg-Master, therefore the WAL saved in the 3rd server might not contain
> the latest WAL. In order to avoid possible data lost, should 'fsync" be
> turned ON in pg-Master and pg-Slave? if yes what would be the best
> settings of "wal_sync_method" for these two servers respectively?
>
> Please advise.
> regards
>

I think you are confusing fsync and switching WAL segments for archiving
purposes. Here's a link to the docs to get you started:
http://www.postgresql.org/docs/9.2/static/runtime-config-wal.html#GUC-ARCHIVE-TIMEOUT

fsync is enabled unless you disable it, the fact that it is commented out
means that it is set to its default ("on"). wal_sync_method at its default
is probably fine, but if you are interested, look at the pg_test_fsync
tool: http://www.postgresql.org/docs/9.2/static/pgtestfsync.html