Re: ext3 journal commit performance

[Muthian Sivathanu]

> >
> > I have a question on ext3 journal commit code. 
> When a
> > transaction is committed in the ordered mode, ext3
> > first issues the data writes, waits for them to
> > finish, then issues the journal writes, waits for
> them
> > to finish, and then writes out the commit record.
> >
> > It appears that the first wait (for the data
> blocks)
> > is unnecessary because all that is required is
> that
> 
> Wrong. If you perform two buffered writes
> back-to-back
> will you guarantee that they are both on the disk
> when
> the second finishes? Not on your life! They can
> (read will)
> be reordered depending upon the closest seek. So it
> is
> mandatory that one wait to make sure that both
> writes
> occur in order.
> 
> 

Sorry if I was unclear.  I did not say that waiting
for the metadata will guarantee commit of the data as
well.  My point is you can wait for both of them
_together_ after issuing both of them to disk, instead
of serializing them at the issue stage itself.

Muthian

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