Figure 7.29 Cluster Shared Volume in redirected mode
The coordinator node can be changed with minimal impact. A slight pause in I/O
occurs if you move the coordinator to another node, because the I/O is queued at each
node. However, the pause is unlikely to be noticed, which is critical given how
important the coordinator node is to CSV. Windows Server 2012 R2 introduced an
automatic rebalancing of coordinator roles for CSV volumes, which is why typically
you will see each node in the cluster having an equal number of disks for which it is
coordinator.
There are some considerations when you have multiple nodes directly writing to
blocks on the disk in some operations—for example, in disk maintenance such as
defragmentation or performing a backup. In Windows Server 2008 R2, manual
actions were required before performing these maintenance actions in addition to
ensuring that actions were taking on the coordinator node for the disk. Windows
Server 2012 optimized this process to place the CSV automatically in maintenance
mode when required, and it also moves the coordinator of the CSV between nodes as
required. In Windows Server 2012, the backup applications are all CSV aware to
ensure proper actions. In Windows Server 2012 R2, VSS got more intelligent; it
understands when the VSS snapshot is performed, even when running on a
noncoordinator node. The ChkDsk process was completely rewritten in Windows
Server 2012 to reduce the amount of volume offline time, from potentially hours to at
most 8 seconds, and on CSV there is no offline time at all because of the additional
redirection introduced by CSV. Behind the scenes, if ChkDsk needs to run against a
CSV volume, the handles between the physical disk and CSV are released, but CSV
maintains the handles that applications have to CSV. This means that once ChkDsk
has completed, within a few seconds the handles are restored between the disk and
CSV. CSV can then map the restored handles back to the original and persisted
handles of the applications to CSV, which means no break in access—only a slight
pause in I/O.
Cluster Shared Volumes in Windows Server 2008 R2 supported only Hyper-V virtual