Re: fsync or fdatasync

The original poster was wrong about the default.

We use fdatasync where available, and fsync when it is not.  We also use
O_SYNC on open if it is available.

---------------------------------------------------------------------------

Gaetano Mendola wrote:
>
> "Tom Lane" <tgl@sss.pgh.pa.us> wrote in message
> news:11753.1031590251@sss.pgh.pa.us...
> > "Gaetano Mendola" <mendola@bigfoot.com> writes:
> > > apparently the default value for wal_sync_method is fsync,
> > > and apparently the best method is fdatasync.
> >
> > Best on what platform, and according to what evidence?
>
> Well, the man say ( Linux ):
>
>
> fdatasync flushes all data buffers of a file to disk (before the system call
> returns).  It resembles fsync but is
>        not required to update the metadata such as access time.
>
>        Applications that access databases or log files often write a tiny
> data fragment (e.g., one line in a  log  file)
>        and  then  call  fsync immediately in order to ensure that the
> written data is physically stored on the harddisk.
>        Unfortunately, fsync will always initiate two write operations: one
> for the newly written data and another one in
>        order to update the modification time stored in the inode. If the
> modification time is not a part of the transac�
>        tion concept fdatasync can be used to avoid unnecessary inode disk
> write operations.
>
>
> So, what is wrong here ? Seems clear that one write is better than two.
>
> Ciao
> Gaetano
>
>
>
>
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