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

>