8464ch02_Architecture.fm Draft Document for Review February 7, 2020 3:15 pm Therefore, the preferred method to keep nodes synchronized is by using a Network Time  Protocol (NTP) server. The NTP server can be a part of the grid-wide area network (WAN)  infrastructure, your intranet, or a public server on the internet (see Figure 2-8). Figure 2-8 Time coordination with NTP servers The NTP server address is configured into the system vital product data (VPD) on a  system-wide scope. Therefore, all nodes access the same NTP server. All clusters in a grid  should be able to communicate with the same NTP server that is defined in VPD. At a  minimum, the lowest two clusters by index must communicate with the NTP server. In the  absence of an NTP server, all clusters coordinate time with Cluster 0 or the lowest cluster  index designation. The lowest index designation is Cluster 0, if Cluster 0 is available. If not, it  uses the next available cluster. In a multi-cluster grid, only one cluster at a time can modify volume data or attributes which is  managed through the concept called “Volume Ownership. Any logical volume, or any copies of it, can be accessed by a host from any virtual device that  is participating in a common grid, even if the cluster that is associated with the virtual device  does not have a local copy. The access is subject to  volume ownership rules . At any point in  time, a logical volume is owned by only one cluster. The owning cluster controls access to the  data and the attributes of the volume.  Ownership can change dynamically. If a cluster must mount a logical volume on one of its  virtual devices and it is not the current owner of that volume, it must obtain ownership first.  When requested, the current owning TS7700 cluster transfers the ownership of the logical  volume as part of mount processing. This action ensures that the cluster with the virtual  device that is associated with the mount has ownership. Remember:  The volume ownership protects the volume from being accessed or modified  by multiple clusters simultaneously.  2.4.3 Data integrity by volume ownership  Ownership 62  IBM TS7700 Release 5.0 Guide