Creating host groups

You can group a set of hosts based on your business requirements. For example, you can group the hosts that run similar type of workloads. You can also group important hosts together so that the monitoring and recovery operations can be performed for the set of hosts together and quickly. In disaster situations, you can move a host group separately to the backup site.

Guidelines to manage host groups:

A host must already be added to the KSYS configuration settings and the host must have been paired with a backup host in the backup site.
Each host in a site must be a part of a host group. If a host is not added to any host groups, the host is automatically added to the Default_HG host group during the discovery operation.
If you add or remove hosts from a host group, you must run a discovery operation to manage or unmanage all the virtual machines from the recovery management. The modified host group displays the correct list of managed virtual machines only after a discovery operation.
If we remove all hosts from Default_HG, the disk group corresponding to Default_HG is not removed. The disk groups are retained with the removed hosts.
The corresponding hosts in the backup site that are paired with the active site hosts are grouped logically within the same host group. For example, if host1 in the active site is paired with host2 in the backup site and you create a host group hg1 with host1, then host2 is automatically added to the host group hg1.
Each host group must be associated with a separate disk group. The disks in the disk group must not be shared among different host groups. The disk groups are named in the following format:
```
VMRDG_{peer_domain_name}_{site_name}_{host_group_ID}
```
However, the disk group name must not exceed the maximum number of characters that is supported for a consistency group at storage level.
For SAN Volume Controller and DS8000® series of storage systems, host groups can span across a single type of storage. Multiple host groups can use same type of storage disks but the disks must not be shared among different host groups.

To add hosts to the KSYS configuration, complete the following steps in the KSYS LPAR:

To create a host group and add the existing hosts that you want to include in this host group, run the following command:
```
ksysmgr add host_group hg1 site=Site_1 hosts=Host_11,Host_12,Host_13
```
Note that all the hosts in the backup site that are paired with the specified hosts are also added to the host group.
Repeat Step 1 for all host groups that you want to create in the KSYS subsystem.

Optional: To verify the host groups that you added, run the following command:

ksysmgr query host_group hgname

An output that is similar to the following example is displayed:

Name:                hg1
Active Site Hosts:   Host_11
                     Host_12
                     Host_13
Backup Site Hosts:   Host_21
                     Host_22
                     Host_23
cpu_capacity:        none
memory_capacity:     none
skip_power_on:       No
Site:                Site_1
Vswitchmap:          Not currently set
Vlanmap:             Not currently set

To add or remove hosts from the existing host groups, run the following command:

ksysmgr modify host_group hg_name add | remove
      hosts=host1,host2... | file=filepath

Where, the file parameter is an XML file that contains a list of hosts. An example of the XML file is as follows:

<KSYSMGR><HOST><NAME>host1</NAME></HOST></KSYSMGR>
<KSYSMGR><HOST><NAME>host2</NAME></HOST></KSYSMGR>
<KSYSMGR><HOST><NAME>host3</NAME></HOST></KSYSMGR>

To modify the capacity-related attributes for all the hosts in a host group, run the following command:

ksysmgr modify host_group hg_name options
      [memory_capacity=(Whole Number > 1) | minimum | current_desired | none]
      [cpu_capacity=(Whole Number > 1) | minimum | current_desired | none]
      [skip_resource_check=yes|no]
      [skip_power_on=yes|no]

For more information about flexible capacity policies, see Configuring the flexible capacity policies.