Re: ext3 writing of data before metadata in ordered mode

[Joel Fernandes <agnel.joel@gmail.com>]

Hi Josef, Your analysis makes perfect sense. Thank you so much.

Another question, what could explain the slowness in data=ordered
mode? I believe everything is asynchronous right? various lists are
maintained, and kjournald keeps checking theses lists and flushing
data before metadata written and marked dirty as you said. Is the
slowness because the flushing of data is done earlier than required
unlike when done by pdflush which waits for a certain amount of time?

Regards,
-Joel

On Mon, Oct 26, 2009 at 6:19 AM, Josef Bacik <josef@redhat.com> wrote:
> 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
>