From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] block_write_full_page: switch synchronous writes to use
 WRITE_SYNC_PLUG
Date: Tue, 7 Apr 2009 16:09:44 -0700
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To: Theodore Tso <tytso@mit.edu>
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On Tue, 7 Apr 2009 18:19:33 -0400
Theodore Tso <tytso@mit.edu> wrote:

> Now that we have a distinction between WRITE_SYNC and WRITE_SYNC_PLUG,
> use WRITE_SYNC_PLUG in __block_write_full_page() to avoid unplugging
> the block device I/O queue between each page that gets flushed out.
> 
> The upstream callers of block_write_full_page() which wait for the
> writes to finish call wait_on_buffer(), wait_on_writeback_range()
> (which ultimately calls sync_page(), which calls
> blk_run_backing_dev(), which will unplug the device queue), and so on.
> 

<sob>

> 
> We should get this applied to avoid any performance regressions
> resulting from commit a64c8610.
> 
>  fs/buffer.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 977e12a..95b5390 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1646,7 +1646,8 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
>  	struct buffer_head *bh, *head;
>  	const unsigned blocksize = 1 << inode->i_blkbits;
>  	int nr_underway = 0;
> -	int write_op = (wbc->sync_mode == WB_SYNC_ALL ? WRITE_SYNC : WRITE);
> +	int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
> +			WRITE_SYNC_PLUG : WRITE);
>  
>  	BUG_ON(!PageLocked(page));

So how does WRITE_SYNC_PLUG differ from WRITE, and what effect does
this change have upon kernel behaviour?