[PATCH 2/4] btrfs-progs: Use mirror_num start from 1 to avoid unnecessary retry

[Qu Wenruo <wqu@suse.com>]

[BUG]
If the first copy of a tree block is corrupted but the other copy is
good, btrfs-progs will report the error twice:
  checksum verify failed on 30556160 found 42A2DA71 wanted 00000000
  checksum verify failed on 30556160 found 42A2DA71 wanted 00000000

While kernel only report it once, just as expected:
  BTRFS warning (device dm-3): dm-3 checksum verify failed on 30556160 wanted 0 found 42A2DA71 level 0

[CAUSE]
We use mirror_num = 0 in read_tree_block() of btrfs-progs.

At first glance it's pretty OK, but mirror num 0 in btrfs means ANY
good copy. Real mirror num starts from 1.
In the context of read_tree_block(), since it's read_tree_block() to do
all the checks, mirror num 0 just means the first copy.

So if the first copy is corrupted, btrfs-progs will try mirror num 1
next, which is just the same as mirror num 0.
After reporting the same error on the same copy, btrfs-progs will
finally try mirror num 2, and get the good copy.

[FIX]
The fix is way simpler than all the above analyse, just starts from
mirror num 1.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 disk-io.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/disk-io.c b/disk-io.c
index 797b9b79ea3c..369592eb7b5c 100644
--- a/disk-io.c
+++ b/disk-io.c
@@ -325,7 +325,7 @@ struct extent_buffer* read_tree_block(struct btrfs_fs_info *fs_info, u64 bytenr,
 	struct extent_buffer *eb;
 	u64 best_transid = 0;
 	u32 sectorsize = fs_info->sectorsize;
-	int mirror_num = 0;
+	int mirror_num = 1;
 	int good_mirror = 0;
 	int num_copies;
 	int ignore = 0;
@@ -381,7 +381,7 @@ struct extent_buffer* read_tree_block(struct btrfs_fs_info *fs_info, u64 bytenr,
 			ignore = 1;
 			continue;
 		}
-		if (btrfs_header_generation(eb) > best_transid && mirror_num) {
+		if (btrfs_header_generation(eb) > best_transid) {
 			best_transid = btrfs_header_generation(eb);
 			good_mirror = mirror_num;
 		}
-- 
2.21.0