On Thu, Oct 06, 2011 at 11:39:54AM +0800, Li Zefan wrote:
> Follow those steps:
>
> # mount -o autodefrag /dev/sda7 /mnt
> # dd if=/dev/urandom of=/mnt/tmp bs=200K count=1
> # sync
> # dd if=/dev/urandom of=/mnt/tmp bs=8K count=1 conv=notrunc
>
> and then it'll go into a loop: writeback -> defrag -> writeback ...
>
> It's because writeback writes [8K, 200K] and then writes [0, 8K].
>
> I tried to make writeback know if the pages are dirtied by defrag,
> but the patch was a bit intrusive. Here I simply set writeback_index
> when we defrag a file.
>
> Signed-off-by: Li Zefan <lizf@xxxxxxxxxxxxxx>
Tested-by: David Sterba <dsterba@xxxxxxx>
> ---
> fs/btrfs/ioctl.c | 7 +++++++
> 1 files changed, 7 insertions(+), 0 deletions(-)
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 970977a..7a10f94 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -1047,6 +1047,13 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
> if (!max_to_defrag)
> max_to_defrag = last_index - 1;
>
> + /*
> + * make writeback starts from i, so the defrag range can be
> + * written sequentially.
> + */
> + if (i < inode->i_mapping->writeback_index)
> + inode->i_mapping->writeback_index = i;
> +
> while (i <= last_index && defrag_count < max_to_defrag) {
> /*
> * make sure we stop running if someone unmounts
> -- 1.7.3.1
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