commit 362290ea03b52e4ce4edb43453a51a3b1f1cd33f
Author: Filipe David Borba Manana <fdmanana@gmail.com>
Date:   Sat Apr 5 22:34:45 2014 +0100

    Btrfs: send, don't issue unnecessary file truncate commands
    
    Every time a file is created or updated, send (either incremental or not) always
    issues a file truncate command. In several commons scenarios this truncate is not
    needed and yet it was still issued all the time. The unnecessary cases where the
    truncate command is not needed are:
    
    * We have a new file, and its last extent isn't a hole, so that its last write
      command ends at the file's size;
    
    * For an incremental send, we updated in place an existing file, without changing
      its size;
    
    * For an incremental send, we updated the file, increased its size, and the last
      file extent item doesn't correspond to a hole. In this case the last write
      command against the file ends at the file's new size;
    
    * For an incremental send, we didn't update the file's data at all, we only changed
      its mode, group or access/update timestamps.
    
    Therefore don't send truncate commands for these cases, reducing the stream's size
    and saving time.

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index fa378c7ebffd..46f52b4bbc21 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -120,6 +120,7 @@ struct send_ctx {
 	u64 cur_inode_mode;
 	u64 cur_inode_rdev;
 	u64 cur_inode_last_extent;
+	u64 cur_inode_max_write_end;
 
 	u64 send_progress;
 	u64 total_data_size;
@@ -4494,6 +4495,8 @@ static int send_hole(struct send_ctx *sctx, u64 end)
 			break;
 		offset += len;
 	}
+	sctx->cur_inode_max_write_end = max(offset,
+					    sctx->cur_inode_max_write_end);
 tlv_put_failure:
 	fs_path_free(p);
 	return ret;
@@ -4529,6 +4532,10 @@ static int send_write_or_clone(struct send_ctx *sctx,
 		len = btrfs_file_extent_num_bytes(path->nodes[0], ei);
 	}
 
+	if (offset >= sctx->cur_inode_size) {
+		ret = 0;
+		goto out;
+	}
 	if (offset + len > sctx->cur_inode_size)
 		len = sctx->cur_inode_size - offset;
 	if (len == 0) {
@@ -4560,6 +4567,8 @@ static int send_write_or_clone(struct send_ctx *sctx,
 		}
 		ret = 0;
 	}
+	sctx->cur_inode_max_write_end = max(offset + len,
+					    sctx->cur_inode_max_write_end);
 out:
 	return ret;
 }
@@ -4982,6 +4991,7 @@ static int finish_inode_if_needed(struct send_ctx *sctx, int at_end)
 	u64 right_gid;
 	int need_chmod = 0;
 	int need_chown = 0;
+	int need_truncate = 1;
 	int pending_move = 0;
 	int refs_processed = 0;
 
@@ -5022,9 +5032,13 @@ static int finish_inode_if_needed(struct send_ctx *sctx, int at_end)
 		need_chown = 1;
 		if (!S_ISLNK(sctx->cur_inode_mode))
 			need_chmod = 1;
+		if (sctx->cur_inode_max_write_end == sctx->cur_inode_size)
+			need_truncate = 0;
 	} else {
+		u64 old_size;
+
 		ret = get_inode_info(sctx->parent_root, sctx->cur_ino,
-				NULL, NULL, &right_mode, &right_uid,
+				&old_size, NULL, &right_mode, &right_uid,
 				&right_gid, NULL);
 		if (ret < 0)
 			goto out;
@@ -5033,6 +5047,13 @@ static int finish_inode_if_needed(struct send_ctx *sctx, int at_end)
 			need_chown = 1;
 		if (!S_ISLNK(sctx->cur_inode_mode) && left_mode != right_mode)
 			need_chmod = 1;
+
+		if (old_size == sctx->cur_inode_size &&
+		    sctx->cur_inode_max_write_end <= sctx->cur_inode_size)
+			need_truncate = 0;
+		else if (sctx->cur_inode_size > old_size &&
+			 sctx->cur_inode_max_write_end == sctx->cur_inode_size)
+			need_truncate = 0;
 	}
 
 truncate_inode:
@@ -5052,10 +5073,13 @@ 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_truncate) {
+			ret = send_truncate(sctx, sctx->cur_ino,
+					    sctx->cur_inode_gen,
+					    sctx->cur_inode_size);
+			if (ret < 0)
+				goto out;
+		}
 	}
 
 	if (need_chown) {
@@ -5110,6 +5134,7 @@ static int changed_inode(struct send_ctx *sctx,
 	sctx->cur_ino = key->objectid;
 	sctx->cur_inode_new_gen = 0;
 	sctx->cur_inode_last_extent = (u64)-1;
+	sctx->cur_inode_max_write_end = 0;
 
 	/*
 	 * Set send_progress to current inode. This will tell all get_cur_xxx