If no one disagree, I'll try to implement it using list.
In fact, after an easy patch and some tests, it doesn't bring much
performance regression, and delayed_ref_nodes are still mergeable.
Thanks,
Qu
-------- Original Message --------
Subject: Is rbtree really needed to restore ref_root in
btrfs_delayed_ref_head?
From: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
To: linux-btrfs <linux-btrfs@xxxxxxxxxxxxxxx>, David Sterba
<dsterba@xxxxxxx>, Chris Mason <clm@xxxxxx>, Josef Bacik <jbacik@xxxxxx>
Date: 2015年03月24日 15:21
Hi all and maintainers.
I'm investigating several qgroup bugs, and find out current delayed ref
implement has several possible problem which may lead to qgroup bugs.
Although my previous RFC patchset
(http://www.mail-archive.com/linux-btrfs@xxxxxxxxxxxxxxx/msg42458.html)
is trying to resolve some qgroup problems, some deep problem in
delayed-ref seems blocking further fix.
[Problem]
Seq in ref_node doesn't really make sense
For example, in Liu Bo's fstests btrfs/017, all DROP_DELAYED_REF
ref_node will have the same sequence number.
But qgroup routine, especially with my RFC
patchset(http://www.mail-archive.com/linux-btrfs@xxxxxxxxxxxxxxx/msg42458.html),
needs the exact insert order to do accurate excl/rfer calculation.
My first idea was to reintroduce the minor sequence number in ref_node,
but soon I realized that we could have a better idea with [FIX].
[FIX]
Why not dual index ref_node with list only?
Current implement using rb-tree of ref_root is only
update_existing_ref(), which is in fact merging ref_nodes with same
(bytenr, parent) tuple.
But in fact, we have merge_refs() and doesn't need to do such thing at
insert time.
IMHO use list to index ref_node should be a quite qgroup friendly
implement, where qgroup codes can get the perfect insert sequence it needs.
Delayed-ref is somewhat fundamental piece of btrfs, so I send the mail
before writing the patch.
It would be quite nice if anyone can point if there is anything wrong
before I wasting several days to write a meaningless patch.
Thanks,
Qu
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