[PATCH] Btrfs : send, truncate first to enhance many small files

[robbieko <robbieko@synology.com>]

From: Robbie Ko <robbieko@synology.com>

The commands generated by send contain the following step:
1. mkfile o1851-19-0
2. rename o1851-19-0 -> alsa-driver/alsa-kernel/isa/es1688/es1688.c
3. set_xattr alsa-driver/alsa-kernel/isa/es1688/es1688.c - name=user.xattr data_len=4 data=test
4. write alsa-driver/alsa-kernel/isa/es1688/es1688.c - offset=0, len=10458
5. truncate alsa-driver/alsa-kernel/isa/es1688/es1688.c size=10458
6. chown alsa-driver/alsa-kernel/isa/es1688/es1688.c - uid=1024, gid=100
7. chmod alsa-driver/alsa-kernel/isa/es1688/es1688.c - mode=0644
8. utimes alsa-driver/alsa-kernel/isa/es1688/es1688.c

After writing file content, it will truncate file to the correct size.
Btrfs truncate will flush last page if size does not align to sectorsize,
and this will cause receive process to wait until flush finishes.
In order to avoid waiting flushing data.This patch changes the order so
that truncate command is sent before write command.

Overall performance improves by 102 percent when sending 790000 small files.
original: 32m45.311s
patch: 16m8.387s

Signed-off-by: Robbie Ko <robbieko@synology.com>
---
 fs/btrfs/send.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 20d3300..7ae2347 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -5857,10 +5857,6 @@ static int finish_inode_if_needed(struct send_ctx *sctx, int at_end)
 					goto out;
 			}
 		}
-		ret = send_truncate(sctx, sctx->cur_ino, sctx->cur_inode_gen,
-				sctx->cur_inode_size);
-		if (ret < 0)
-			goto out;
 	}
 
 	if (need_chown) {
@@ -6044,6 +6040,15 @@ static int changed_inode(struct send_ctx *sctx,
 					sctx->left_path->nodes[0], left_ii);
 		}
 	}
+	if (result == BTRFS_COMPARE_TREE_NEW ||
+	    result == BTRFS_COMPARE_TREE_CHANGED) {
+		if (S_ISREG(sctx->cur_inode_mode)) {
+			ret = send_truncate(sctx, sctx->cur_ino, sctx->cur_inode_gen,
+						sctx->cur_inode_size);
+			if (ret < 0)
+				goto out;
+		}
+	}
 
 out:
 	return ret;
-- 
1.9.1