A VRA is a Zerto Virtual Replication virtual machine that manages the replication of virtual machines across sites. A VRA must be installed on every hypervisor that hosts virtual machines that require protecting in the protected site and on every hypervisor that will host the replicated virtual machines in the recovery site. The VRA compresses the data that is passed across the WAN from the protected site to the recovery site. The VRA automatically adjusts the compression level according to CPU usage, including totally disabling it if needed.

The VRA is a custom, very thin, Linux-based virtual machine with a small footprint, disk – memory and CPU – and increased security since there are a minimum number of services installed.

Zerto recommends installing a VRA on every host so that if protected virtual machines are moved from one host in the cluster to another host in the cluster there is always a VRA to protect the moved virtual machines.

A VRA can manage a maximum of 1500 volumes, whether these are volumes being protected or recovered.

The priority assigned to a VPG dictates the bandwidth used. The Zerto Virtual Manager distributes bandwidth among the VRAs based on the VPG priority, and the VPGs with higher priorities are handled before writes from VPGs with lower priorities.

There are a number of tasks that you might need to perform on VRAs, including installing a new VRA on a host added to the hypervisor management tool or uninstalling VRAs and moving the data maintained by a VRA to another VRA when a host requires maintenance.

During normal operation, a VRA might require more disks than a single virtual machine can support. If this situation arises, the VRA creates new shadow VRA virtual machines, used by the VRA to maintain additional disks (a diskbox). A shadow VRA does not have an operating system and therefore does not have an IP address, or use VMware tools. A shadow VRA is created proactively at the recovery site and prior to the recovery VRA reaching the SCSI target limit imposed by VMware: 15 SCSI targets per SCSI controller and 4 SCSI controllers per virtual machine. This amounts to a limitation of 60 SCSI targets per virtual machine. Similar to a VRA, a shadow VRA must be left to Zerto Virtual Replication to manage, and must not be modified or removed for any reason.

The following VRA management options are described in this section:

Monitoring VRAs is described in “Monitoring Virtual Replication Appliances”, on page 242.