• Josef Bacik's avatar
    Btrfs: rework qgroup accounting · fcebe456
    Josef Bacik authored
    Currently qgroups account for space by intercepting delayed ref updates to fs
    trees.  It does this by adding sequence numbers to delayed ref updates so that
    it can figure out how the tree looked before the update so we can adjust the
    counters properly.  The problem with this is that it does not allow delayed refs
    to be merged, so if you say are defragging an extent with 5k snapshots pointing
    to it we will thrash the delayed ref lock because we need to go back and
    manually merge these things together.  Instead we want to process quota changes
    when we know they are going to happen, like when we first allocate an extent, we
    free a reference for an extent, we add new references etc.  This patch
    accomplishes this by only adding qgroup operations for real ref changes.  We
    only modify the sequence number when we need to lookup roots for bytenrs, this
    reduces the amount of churn on the sequence number and allows us to merge
    delayed refs as we add them most of the time.  This patch encompasses a bunch of
    architectural changes
    
    1) qgroup ref operations: instead of tracking qgroup operations through the
    delayed refs we simply add new ref operations whenever we notice that we need to
    when we've modified the refs themselves.
    
    2) tree mod seq:  we no longer have this separation of major/minor counters.
    this makes the sequence number stuff much more sane and we can remove some
    locking that was needed to protect the counter.
    
    3) delayed ref seq: we now read the tree mod seq number and use that as our
    sequence.  This means each new delayed ref doesn't have it's own unique sequence
    number, rather whenever we go to lookup backrefs we inc the sequence number so
    we can make sure to keep any new operations from screwing up our world view at
    that given point.  This allows us to merge delayed refs during runtime.
    
    With all of these changes the delayed ref stuff is a little saner and the qgroup
    accounting stuff no longer goes negative in some cases like it was before.
    Thanks,
    Signed-off-by: default avatarJosef Bacik <jbacik@fb.com>
    Signed-off-by: default avatarChris Mason <clm@fb.com>
    fcebe456
delayed-ref.c 25.4 KB