@@ -1606,6 +1606,24 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
up_write(&space_info->groups_sem);
goto next;
}
+ if (bg->used == 0) {
+ /*
+ * It is possible that we trigger relocation on a block
+ * group as its extents are deleted and it first goes
+ * below the threshold, then shortly after goes empty.
+ *
+ * In this case, relocating it does delete it, but has
+ * some overhead in relocation specific metadata, looking
+ * for the non-existent extents and running some extra
+ * transactions, which we can avoid by using one of the
+ * other mechanisms for dealing with empty block groups.
+ */
+ if (!btrfs_test_opt(fs_info, DISCARD_ASYNC))
+ btrfs_mark_bg_unused(bg);
+ spin_unlock(&bg->lock);
+ up_write(&space_info->groups_sem);
+ goto next;
+ }
spin_unlock(&bg->lock);
/* Get out fast, in case we're unmounting the filesystem */
As we delete extents from a block group, at some deletion we cross below the reclaim threshold. It is possible we are still in the middle of deleting more extents and might soon hit 0. If the block group is empty by the time the reclaim worker runs, we will still relocate it. This works just fine, as relocating an empty block group ultimately results in properly deleting it. However, we have more direct ways of removing empty block groups in the cleaner thread. Those are either async discard or the unused_bgs list. In fact, when we decide whether to relocate a block group during extent deletion, we do check for emptiness and prefer the discard/unused_bgs mechanisms when possible. Not using relocation for this case reduces some modest overhead from empty bg relocation: - extra transactions - extra metadata use/churn for creating relocation metadata - trying to read the extent tree to look for extents (and in this case finding none) Signed-off-by: Boris Burkov <boris@bur.io> --- fs/btrfs/block-group.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)