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>
---
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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