[PATCH v2] btrfs: Remove unnecessary code from __btrfs_rebalance

[Nikolay Borisov <nborisov@suse.com>]

The first step fo the rebalance process, ensuring there is 1mb free on
each device. This number seems rather small. And in fact when talking
to the original authors their opinions were:

"man that's a little bonkers"
"i don't think we even need that code anymore"
"I think it was there to make sure we had room for the blank 1M at the
beginning. I bet it goes all the way back to v0"
"we just don't need any of that tho, i say we just delete it"

Clearly, this piece of code has lost its original intent throughout
the years. It doesn't really bring any real practical benefits to the
relocation process. Additionally, this patch makes the balance process
more lightweight by removing a pair of shrink/grow operations which
are rather expensive for heavily populated filesystems. This is mainly due to 
shrink requiring relocating block groups, involving heavy use of the btree.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Suggested-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
Changes since v1: 
 * Improved changelog by adding information about reduced runtimes and explaining
 where they would come from.

 I did measurements of btrfs balance with and without the patch with 
 funclatency from bcc tools but didn't observe large differences, but this was 
 on a ligthly populated filesystem. 

 fs/btrfs/volumes.c | 53 ----------------------------------------------
 1 file changed, 53 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index d49baad64fe6..19cc31de1e84 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3699,17 +3699,11 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_balance_control *bctl = fs_info->balance_ctl;
 	struct btrfs_root *chunk_root = fs_info->chunk_root;
-	struct btrfs_root *dev_root = fs_info->dev_root;
-	struct list_head *devices;
-	struct btrfs_device *device;
-	u64 old_size;
-	u64 size_to_free;
 	u64 chunk_type;
 	struct btrfs_chunk *chunk;
 	struct btrfs_path *path = NULL;
 	struct btrfs_key key;
 	struct btrfs_key found_key;
-	struct btrfs_trans_handle *trans;
 	struct extent_buffer *leaf;
 	int slot;
 	int ret;
@@ -3724,53 +3718,6 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
 	u32 count_sys = 0;
 	int chunk_reserved = 0;
 
-	/* step one make some room on all the devices */
-	devices = &fs_info->fs_devices->devices;
-	list_for_each_entry(device, devices, dev_list) {
-		old_size = btrfs_device_get_total_bytes(device);
-		size_to_free = div_factor(old_size, 1);
-		size_to_free = min_t(u64, size_to_free, SZ_1M);
-		if (!test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state) ||
-		    btrfs_device_get_total_bytes(device) -
-		    btrfs_device_get_bytes_used(device) > size_to_free ||
-		    test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state))
-			continue;
-
-		ret = btrfs_shrink_device(device, old_size - size_to_free);
-		if (ret == -ENOSPC)
-			break;
-		if (ret) {
-			/* btrfs_shrink_device never returns ret > 0 */
-			WARN_ON(ret > 0);
-			goto error;
-		}
-
-		trans = btrfs_start_transaction(dev_root, 0);
-		if (IS_ERR(trans)) {
-			ret = PTR_ERR(trans);
-			btrfs_info_in_rcu(fs_info,
-		 "resize: unable to start transaction after shrinking device %s (error %d), old size %llu, new size %llu",
-					  rcu_str_deref(device->name), ret,
-					  old_size, old_size - size_to_free);
-			goto error;
-		}
-
-		ret = btrfs_grow_device(trans, device, old_size);
-		if (ret) {
-			btrfs_end_transaction(trans);
-			/* btrfs_grow_device never returns ret > 0 */
-			WARN_ON(ret > 0);
-			btrfs_info_in_rcu(fs_info,
-		 "resize: unable to grow device after shrinking device %s (error %d), old size %llu, new size %llu",
-					  rcu_str_deref(device->name), ret,
-					  old_size, old_size - size_to_free);
-			goto error;
-		}
-
-		btrfs_end_transaction(trans);
-	}
-
-	/* step two, relocate all the chunks */
 	path = btrfs_alloc_path();
 	if (!path) {
 		ret = -ENOMEM;
-- 
2.17.1