On 08/28/2012 01:12 AM, Josef Bacik wrote:

On Mon, Aug 27, 2012 at 10:52:20AM -0600, Liu Bo wrote:

quoted

This is based on Josef's "Btrfs: turbo charge fsync".

The above Josef's patch performs very good in random sync write test,
because we won't have too much extents to merge.

However, it does not performs good on the test:
dd if=/dev/zero of=foobar bs=4k count=12500 oflag=sync

The reason is when we do sequencial sync write, we need to merge the
current extent just with the previous one, so that we can get accumulated
extents to log:

A(4k) --> AA(8k) --> AAA(12k) --> AAAA(16k) ...

So we'll have to flush more and more checksum into log tree, which is the
bottleneck according to my tests.

But we can avoid this by telling fsync the real extents that are needed
to be logged.

With this, I did the above dd sync write test (size=50m),

         w/o (orig)   w/ (josef's)   w/ (this)
SATA      104KB/s       109KB/s       121KB/s
ramdisk   1.5MB/s       1.5MB/s       10.7MB/s (613%)

Signed-off-by: Liu Bo <redacted>
---
 fs/btrfs/extent_map.c |   20 ++++++++++++++++++++
 fs/btrfs/extent_map.h |    2 ++
 fs/btrfs/inode.c      |    1 +
 fs/btrfs/tree-log.c   |    6 +++---
 4 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 1fe82cf..ac606f0 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c

@@ -203,6 +203,8 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 			em->block_start = merge->block_start;
 			merge->in_tree = 0;
 			if (merge->generation > em->generation) {
+				em->mod_start = em->start;
+				em->mod_len = em->len;

Shouldn't this be

em->mod_start = merge->start;
em->mod_len += merge_len;

They just do the same thing.

There is already a 
em->start = merge->start;
em->len += merge_len

quoted

 				em->generation = merge->generation;
 				list_move(&em->list, &tree->modified_extents);
 			}

@@ -222,6 +224,7 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 		rb_erase(&merge->rb_node, &tree->map);
 		merge->in_tree = 0;
 		if (merge->generation > em->generation) {
+			em->mod_len = em->len;

And this should be em->mod_len += em->len?

No, em->len has already contained the merge's len.

thanks,
liubo