linux-kernel - Re: [PATCH] Improve buffered streaming write ordering

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Message-ID: <20081007100257.GA30745@skywalker>
Date:	Tue, 7 Oct 2008 15:32:57 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] Improve buffered streaming write ordering

On Tue, Oct 07, 2008 at 05:05:54AM -0400, Christoph Hellwig wrote:
> On Tue, Oct 07, 2008 at 02:15:31PM +0530, Aneesh Kumar K.V wrote:
> > +static int ext4_write_cache_pages(struct address_space *mapping,
> > +		      struct writeback_control *wbc, writepage_t writepage,
> > +		      void *data)
> > +{
> 
> Looking at this functions the only difference is killing the
> writeback_index and range_start updates.  If they are bad why would we
> only remove them from ext4?

I am also not updating wbc->nr_to_write.

ext4 delayed allocation writeback is bit tricky. It does

a) Look at the dirty pages and build an in memory extent of contiguous
logical file blocks. If we use writecache_pages to do that it will
update nr_to_write, writeback_index etc during this stage.

b) Request the block allocator for 'x' blocks. We get the value x from
step a.

c) block allocator may return less than 'x' contiguous block. That would
mean the variables updated by write_cache_pages need to corrected. The
old code was doing that. Chris Mason suggested it would make it easy
to use a write_cache_pages which doesn't update the variable for ext4.

I don't think other filesystem have this requirement.

-aneesh
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