Chapter 11. Advanced Copy Services 463
However, there is a lock for each grain. The lock can be in shared or exclusive mode. For 
multiple targets, a common lock is shared, and the mappings are derived from a particular 
source volume. The lock is used in the following modes under the following conditions:
/SM590000The lock is held in shared mode during a read from the target volume, which touches a 
grain that was not copied from the source.
/SM590000The lock is held in exclusive mode while a grain is being copied from the source to the 
target.
If the lock is held in shared mode and another process wants to use the lock in shared mode, 
this request is granted unless a process is already waiting to use the lock in exclusive mode.
If the lock is held in shared mode and it is requested to be exclusive, the requesting process 
must wait until all holders of the shared lock free it.
Similarly, if the lock is held in exclusive mode, a process wanting to use the lock in shared or 
exclusive mode must wait for it to be freed.
11.1.17  Event handling
When a FlashCopy mapping is not copying or stopping, the FlashCopy function does not 
affect the handling or reporting of events for error conditions that are encountered in the I/O 
path. Event handling and reporting are affected only by FlashCopy when a FlashCopy 
mapping is copying or stopping, that is, actively moving data.
This section describes these scenarios.
Node failure
Normally, two copies of the FlashCopy bitmap are maintained. One copy of the FlashCopy 
bitmap is on each of the two nodes that make up the I/O Group of the source volume. When a 
node fails, one copy of the bitmap for all FlashCopy mappings whose source volume is a 
member of the failing node’s I/O Group becomes inaccessible. 
FlashCopy continues with a single copy of the FlashCopy bitmap that is stored as non-volatile 
in the remaining node in the source I/O Group. The system metadata is updated to indicate 
that the missing node no longer holds a current bitmap. When the failing node recovers or a 
replacement node is added to the I/O Group, the bitmap redundancy is restored. 
Path failure (Path Offline state)
In a fully functioning system, all of the nodes have a software representation of every volume 
in the system within their application hierarchy.
Because the storage area network (SAN) that links IBM SAN Volume Controller nodes to 
each other and to the MDisks is made up of many independent links, it is possible for a subset 
of the nodes to be temporarily isolated from several of the MDisks. When this situation 
happens, the managed disks are said to be Path Offline on certain nodes.
Other nodes: Other nodes might see the managed disks as Online because their 
connection to the managed disks is still up.