Re: ext3 writing of data before metadata in ordered mode

[Josef Bacik <josef@redhat.com>]

On Sun, Oct 25, 2009 at 02:33:59PM -0700, Joel Fernandes wrote:
> In data=ordered mode the ext3_ordered_commit_write function marks the
> buffers as dirty, how then does the JBD ensure that the data is
> written before the metadata?  Once the data buffers are marked as
> dirty, JBD doesn't have control anymore over when the data is written
> is actually written to disk right? Because the actually writing of the
> data is handled by the page wtriteback mechanism (pdflush) right?
> 
> I might be missing something here, thanks for your time and patience.
> 

ordered mode means we don't care when the data gets flushed out, just so long as
it happens before we do metadata.  So we mark the buffer as dirty, which is
appropriate, so that if pdflush decides that it needs to start flushing dirty
data it can.  We also add the buffer to the transactions t_sync_datalist list so
we know all of the data buffers that were modified in this transaction.  So when
we go to commit the transaction we go through this list writing out all of the
dirty buffers on that list.  If we hit a buffer that is not dirty we know its
already been written out and we can move on to the next one.  Then after all
this is done we go through the list of metadata that was modified in that
transaction, write out the journal entries, and then mark the metadata as dirty
so it can be written out at some point in the future.  Let me know if that makes
sense.  Thanks,

Josef