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

[Andrew Morton <akpm@osdl.org>]

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).

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'.  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.