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

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Andrew Morton wrote:

Badari Pulavarty <[email protected]> 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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