Btrfs: throttle delayed refs better

On one of our gluster clusters we noticed some pretty big lag spikes.  This
turned out to be because our transaction commit was taking like 3 minutes to
complete.  This is because we have like 30 gigs of metadata, so our global
reserve would end up being the max which is like 512 mb.  So our throttling code
would allow a ridiculous amount of delayed refs to build up and then they'd all
get run at transaction commit time, and for a cold mounted file system that
could take up to 3 minutes to run.  So fix the throttling to be based on both
the size of the global reserve and how long it takes us to run delayed refs.
This patch tracks the time it takes to run delayed refs and then only allows 1
seconds worth of outstanding delayed refs at a time.  This way it will auto-tune
itself from cold cache up to when everything is in memory and it no longer has
to go to disk.  This makes our transaction commits take much less time to run.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 3cebb4aeddc7..ca6bcc33d033 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1360,6 +1360,7 @@ struct btrfs_fs_info {
 
 	u64 generation;
 	u64 last_trans_committed;
+	u64 avg_delayed_ref_runtime;
 
 	/*
 	 * this is updated to the current trans every time a full commit
@@ -3172,6 +3173,8 @@ static inline u64 btrfs_calc_trunc_metadata_size(struct btrfs_root *root,
 
 int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
 				       struct btrfs_root *root);
+int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
+				       struct btrfs_root *root);
 void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
 int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 			   struct btrfs_root *root, unsigned long count);