Re: [PATCH v2] ext4: directly leave out of ext4_find_delalloc_range() if filesystem mount with "nodelalloc"

[Ted Ts'o <tytso@mit.edu>]

On Thu, Dec 08, 2011 at 02:59:54PM +0800, Robin Dong wrote:
> From: Robin Dong <sanbai@taobao.com>
> 
> We found performance regression when using bigalloc with "nodelalloc"  (1MB cluster size):
> 
> 1. mke2fs -C 1048576 -O ^has_journal,bigalloc /dev/sda
> 2. mount -o nodelalloc /dev/sda /test/
> 3. time dd if=/dev/zero of=/test/io bs=1048576 count=1024
> 
> The "dd" will cost about 2 seconds to finish, but if we mke2fs without "bigalloc",
> "dd" will only cost lesss than 1 second.
> 
> The reason is:  when using ext4 with "nodelalloc", it will call ext4_find_delalloc_cluster() nearly
> everytime it call ext4_ext_map_blocks(), and ext4_find_delalloc_range() will also scan all pages
> in cluster because no buffer is "delayed".
> A cluster has 256 pages (1MB cluster), so it will scan 256 * 256k pags when creating a 1G file. That
> severely hurts the performance.
> 
> Therefore, we return out from ext4_find_delalloc_range() when using "nodelalloc".
> 
> Signed-off-by: Robin Dong <sanbai@taobao.com>

Thanks, applied.

					- Ted