Asynchronous Replication; Network Bandwidth Planning - HP StoreVirtual 4000 White Paper

Technical white paper | HP StoreVirtual 4000 Storage

Asynchronous replication

With Remote Copy, there are virtually no IP distance limitations for replicated solutions, the primary and recovery sites
may span hundreds or even thousands of kilometers, using different types of network access connectivity between
sites. With asynchronous replication, it is understood that the bandwidth between the two sites is limited. Writes to the
primary volume are acknowledged immediately back to the client, and then replicated to the remote volume as part of
a snapshot. The replication time required to replicate data between sites is determined by as part of a snapshot data
changed (i.e. size of the snapshot to be transferred) and bandwidth; these two factors also drive the appropriate Remote
Snapshot schedule.
Best practices
• To ensure Remote Copy resiliency, for customer with tight recovery point objectives (RPOs) and recovery time objectives
(RTOs), HP recommends considering WAN path redundancy and carrier diversity for WAN links between sites.
• For customers employing site-to-site VPN services, NAT, appropriate placement of firewall, public IP addressing, IPSec,
authentication, TCP and UDP port forwarding, are top of mind considerations.
• Ensure network time protocol (NTP) is properly configured and time zone synchronized between all storage nodes in
primary and secondary management groups.
• Use 802.1q tagged switched links to extend VLAN broadcast traffic between primary and secondary sites for non-routed
interlinks.
• For routed links between primary and secondary sites, make certain routers are configured with industry standard
routing protocols such as RIP and OSPF. HP recommends using OSPF as the routing protocol of choice given it has the
richest features over RIP.
• When QoS is enabled within your LAN for storage traffic make sure differentiated service code point (DSCP) or class of
service (COS) markings are honored by the service provider (across the WAN link) and applied from end to end.

Network bandwidth planning

Sufficient bandwidth between sites can present a challenge for StoreVirtual customers to meet their Remote Copy
objectives, as high-speed WAN links are typically pricey and constrains most small and medium-size customers to T1
bandwidth speeds for long-distance WAN connectivity. Remote Copy frequency and amount of data change on the
primary volume are key factors in calculating network bandwidth.
For example, the amount of data changed divided by the period of time between Remote Copy windows (frequencies)
determines the minimum bandwidth required for inter-site connectivity and determines if the RPO objective is met. If
our data change rate is 4 GB hourly and our replication window is every four hours, then network bandwidth required to
adhere to our Remote Copy schedule is at minimum a 10 Mb/s connection. See Table 6 as an example.
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