Re: [RFC PATCH] ext3 writepage() journal avoidance

[Badari Pulavarty <pbadari@us.ibm.com>]

Andrew Morton wrote:

> Badari Pulavarty <pbadari@us.ibm.com> wrote:
>
> > I am trying to speed up ext3 writepage() by avoiding
> > journaling in non-block allocation cases. Does this
> > look reasonable ? So far, my testing is fine. What am 
> > I missing here ?
>
> Nothing.  ext3's writepage(), prepare_write() and commit_write() do often
> needlessy open and close transactions when we're doing overwrites.  It's
> something I've meant to look at for a few years, on and off.
>
> I'd expect that prepare_write() and commit_write() are more important than
> writepage().
>
>
>
> It might be better to test PageMappedToDisk() rather than walking the
> buffers.  It's certainly faster and it makes optimisation of
> prepare_write() and commit_write() easier to handle.
>
> I'm not sure that PageMappedToDisk() gets set in all the right places
> though - it's mainly for the `nobh' handling and block_prepare_write()
> would need to be taught to set it.  I guess that'd be a net win, even if
> only ext3 uses it..
>
> Then again, we might be able to speed up block_prepare_write() if
> PageMappedToDisk(page).
Makes sense. I will take a look.

> If we go this way we need to be very very careful to keep PG_mappedtodisk
> coherent with the state of the buffers.  Tricky.  We need to think about
> whether block_truncate_page() should be clearing PG_mappedtoisk if we did a
> partial truncate.
>
> Don't forget that ext3 supports journalled-mode files on ordered- or
> writeback-mounted filesystems, via `chattr +j'.  

Wow !! Never knew that. I assume we switch mapping->a_ops for this inode ?

> Please be sure to test the
> various combinations which that allows when playing with the write paths -
> it can trip things up.
>
> Also be sure to test nobh-mode.

Sure. Thanks for your reply and valuable suggestions. :)

Thanks,
Badari

>



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