kept dirty to reduce the number of on-disk metadata updates. The
sequence of operations is:
1) acquire the activemap lock;
2) update in-memory map;
3) if the list of keepdirty extents is changed, update on-disk metadata;
4) release the lock.
On-disk updates are not frequent in comparison with in-memory updates,
while require much more time. So situations are possible when one
thread is updating on-disk metadata and another one is waiting for the
activemap lock just to update the in-memory map.
Improve this by introducing additional, on-disk map lock: when
in-memory map is updated and it is detected that the on-disk map needs
update too, the on-disk map lock is acquired and the on-memory lock is
released before flushing the map.
Reported by: Yamagi Burmeister yamagi.org
Tested by: Yamagi Burmeister yamagi.org
Reviewed by: pjd
Approved by: re (marius)
MFC after: 2 weeks
a818a4ff09