linux-kernel - Re: [PATCH] Revert "writeback: limit write_cache

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Message-ID: <20100629020420.GE24343@mail.oracle.com>
Date:	Mon, 28 Jun 2010 19:04:20 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	ocfs2-devel@....oracle.com, Tao Ma <tao.ma@...cle.com>,
	Dave Chinner <dchinner@...hat.com>,
	Christoph Hellwig <hch@....de>, Mark Fasheh <mfasheh@...e.com>
Subject: Re: [PATCH] Revert "writeback: limit write_cache_pages integrity
 scanning to current EOF"

On Tue, Jun 29, 2010 at 11:56:15AM +1000, Dave Chinner wrote:
> > 	Regarding XFS, how do you handle catching the tail of an
> > allocation with an lseek(2)'d write?  That is, your current allocation
> > has a few blocks outside of i_size, then I lseek(2) a gigabyte past EOF
> > and write there.  The code has to recognize to zero around old_i_size
> > before moving out to new_i_size, right?  I think that's where our old
> > approaches had problems.
> 
> xfs_file_aio_write() handles both those cases for us via
> xfs_zero_eof().  What it does is map the region from the old EOF to
> the start of the new write and zeroes any allocated blocks that are
> not marked unwritten that lie within the range. It does this via the
> internal mapping interface because we hide allocated blocks past EOF
> from the page cache and higher layers.

	Makes sense as an approach.  We deliberately do this through the
page cache to take advantage of its I/O patterns and tie in with JBD2.
Also, we don't feel like maintaining an entire shadow page cache ;-)

Joel

-- 

Life's Little Instruction Book #356

	"Be there when people need you."

Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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