Administrator Guide

Live Volume and MPIO
47 Dell EMC SC Series: Synchronous Replication and Live Volume | CML1064
7 Live Volume and MPIO
By using Live Volume with a storage host that has access to both SC Series arrays in a Live Volume
configuration, multiple paths can be presented to the server across the arrays. All read and write I/O
ultimately flows to the primary Live Volume. Read and write I/O sent down paths to the primary Live Volume
are handled directly. Read and write I/O sent down paths to the secondary Live Volume are proxied to the
primary Live Volume via replication ports. Acknowledgements and read payloads are then sent back over the
replication ports, through the secondary Live Volume, and back down the originating path to the storage host.
ALUA requires special consideration to understand and optimize I/O paths for the Live Volume.
Note: Live Volume ALUA support was added in SCOS 7.3 for VMware vSphere and Microsoft Windows
operating systems only.
7.1 MPIO policies for Live Volume
For Live Volume arrays where a server has volume access through both the primary and secondary Live
Volume controllers (uniform storage presentation), it may be desirable to configure the MPIO policy to prevent
primary data access through the secondary Live Volume array. Instead, these paths would be reserved for
use as failover paths. Examples of these types of MPIO policies are Failover Only or Fixed. Use cases
include asynchronous replication link slowness or a QoS node that adds too much latency to proxied traffic.
A Round Robin MPIO policy may also be used with Live Volume. There may be slight deviations based on the
storage host, but in general, with the ALUA support added in SCOS 7.3, Round Robin utilizes all available
optimal paths to a device. MPIO paths leading to the primary Live Volume are advertised by the SC Series
array as optimal. In the event optimal paths are not available, Round Robin will then utilize non-optimal paths
that are available. MPIO paths leading to the secondary Live Volume are advertised by the SC Series array
as non-optimal.
Standalone servers or clustered servers accessing shared storage may use uniform storage presentation.
Uniform storage presentation provides host to Live Volume storage paths through both arrays. With the Live
Volume ALUA feature enabled, a Round Robin path selection policy can quickly and easily be configured with
uniform storage presentation. With the ALUA intelligence built into both the storage host and the arrays,
optimal MPIO paths will be used for I/O and the storage fabric will be both well utilized and well balanced.
For non-vSphere or Microsoft storage hosts in a uniform Live Volume configuration, all MPIO paths will be
reported as optimal. When using a Round Robin path selection policy, half of the I/O will go through the
primary Live Volume and the other half with be proxied through the replication link through the secondary Live
Volume. This may or may not be a good design choice. Also, the automatic role swap feature will not be able
to make an accurate determination of where the primary Live Volume should be, and the feature will be
ineffective if enabled.
Clustered servers accessing shared storage may also use non-uniform storage presentation. Non-uniform
means that each cluster node will access a Live Volume through one array or the other, but not both. Non-
uniform typically applies to a stretched cluster configuration where each node in the cluster accesses the Live
Volume through paths that are local in fabric proximity only. Each cluster node would have read/write access
to either the primary or the secondary Live Volume, but not both simultaneously. For the cluster nodes which
have access to the primary Live Volume, their front-end I/O will remain local in proximity. For the cluster
nodes which have access to the secondary Live Volume, their front-end I/O will be proxied to the primary Live
Volume through the replication link. Implementing Round Robin with non-uniform presentation is typically
preferred to provide automated port and fabric balancing, but fixed paths may also be used. Automatic role