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How Site Recovery Manager Computes Datastore Groups
The composition of a datastore group is determined by the set of virtual machines that have files on the
datastores in the group, and by the devices on which those datastores are stored.
When you use array-based replication, each storage array supports a set of replicated devices. On Storage Area
Network (SAN) arrays that use connection protocols such as Fibre Channel and iSCSI, these devices are called
LUNs (logical storage units comprising one or more physical devices). On NFS arrays, they are typically
referred to as volumes. In every pair of replicated storage devices, one device is the replication source and the
other is the replication target. Data written to the source device is replicated to the target device on a schedule
controlled by the arrays' replication software. When you configure SRM to work with an SRA, the replication
source is at the protected site and the replication target is at the recovery site.
A datastore provides storage for virtual machine files. By hiding the details of physical storage devices,
datastores simplify the allocation of storage capacity and provide a uniform model for meeting the storage
needs of virtual machines. Because any datastore can span multiple devices, SRM must ensure that all devices
backing the datastore are replicated before it can protect the virtual machines that use that datastore. SRM
must ensure that all devices containing protected virtual machine files are replicated. During a recovery or
test, SRM must handle all such devices together. To achieve this goal, SRM aggregates datastores into datastore
groups to accommodate virtual machines that span multiple datastores. SRM regularly checks that datastore
groups contain all necessary datastores to provide protection for appropriate virtual machines. When
necessary, datastore groups are recalculated. For example, this may occur when new devices are added to a
virtual machine, and those devices are stored on a datastore that was not previously a part of the datastore
group.
A datastore group consists of the smallest set of devices required to ensure that if any of a virtual machine's
files is stored on a device in the group, all of the virtual machine's files are stored on devices that are part of
the same group. For example, if a virtual machine has disks on two different datastores, then both datastores
must be combined into a datastore group. Conditions that can cause datastores to be combined into a datastore
group include:
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A virtual machine has files on two different datastores.
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Two virtual machines share an RDM device on a SAN array, such as in the case of an MSCS cluster.
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Two datastores span extents corresponding to different partitions of the same device.
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A single datastore spans two extents corresponding to partitions of two different devices.
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Multiple devices belong to a consistency group. A consistency group is a collection of replicated devices
where every state of the target set of devices existed at some point in time as the state of the source set of
devices. Informally, the devices are replicated together such that when recovery happens using those
devices, software accessing the targets do not see the data in a state it is not prepared to deal with.
vSphere Replication
In vSphere Replication (VR), SRM uses vSphere replication technologies to replicate data to servers at the
recovery site.
vSphere Replication uses vSphere Replication Management Server (VRMS) to manage the VR infrastructure.
VR requires installing the VR Server (VRS) virtual appliance and VRMS virtual appliance, both of which can
be installed with SRM during the installation process. While VR does not require storage arrays, an VR storage
replication source and target can be any regular storage device, including, but not limited to, storage arrays.
Site Recovery Manager Administration Guide
12 VMware, Inc.