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Native synchronous block replication is consisted of a number of physical and software components. Each of these components will be discussed in the following sections. To fully configure synchronous replication, you must complete each of following:
Also discussed in this section will be the various storage resources which support synchronous replication, and the different replication modes and roles for synchronous replication. The following sections detail the functions, requirements, and interactions of these components. The configuration and management of these components in Unisphere will be discussed in the Unisphere Management subsection.
Native synchronous block replication is performed over the first Fibre Channel (FC) port configured on the system. Older model Dell Unity physical systems include two onboard Converged Network Adapter (CNA) ports which can be configured with either 16 Gb FC SFPs, 10 Gb Optical SFPs, or be used with Twinax cables. When configured as Fibre Channel, the lowest numbered port will be used for the synchronous replication interface on each SP. When the CNA is not configured for Fibre Channel, a 16 Gb Fibre Channel I/O module must be purchased, installed, and persisted on each of the SPs of the system. Dell Unity XT systems support both a 16 Gb and 32 Gb Fibre Channel I/O module. Support for the 32 Gb Fibre Channel I/O module was added in Dell UnityOS version 5.1. In any model type, the lowest numbered FC port on the system will be used as the synchronous block replication interface.
Starting with Dell UnityOS version 4.1, the Replication Capability field is displayed for the Fibre Channel ports. To view, go to Unisphere > System > System View > Enclosures, then select a Fibre Channel port to review. Figure 1 an example of the Enclosures tab on a Dell Unity 480 system, with the SP A I/O Module 0 FC Port 0 selected. The Replication Capability can be seen from the quick properties of the port.
Figure 1. Replication Capability
With Unisphere CLI, you can also easily locate the synchronous block replication port by running the /net/port/fc show –detail command. This command will return information for Fibre Channel ports of the system. A sample output of this command with only the relevant information displayed is shown below:
C:\>uemcli -d ip_address -u username -p password /net/port/fc show –detail
1: ID = spa_iom_0_fc0
Replication capability = Sync replication
4: ID = spb_iom_0_fc0
Replication capability = Sync replication
In the above example, the first Fibre Channel port on the system, labeled as I/O Module 0 FC Port 0 on both Storage Processors (SP) are used for synchronous block replication. These ports can be directly connected to the synchronous ports on the destination system or zoned to them using a switch. For zoning, the proper configuration is to zone the SPA replication port for the local and remote systems together, and in a different zone, zone the SPB replication ports together. It is recommended to dedicate the FC replication interfaces strictly for replication purposes, but it is not required. Hosts can also use these ports if required. For redundancy, it is best to configure SPA’s replication connection over one FC network, and SPB’s over another where possible.
The synchronous replication management port is a virtual port used by synchronous replication to pass management commands between systems. A synchronous replication management port is created on each SP and is configured on the management port of a storage processor. This port must be able to communicate with the destination system’s synchronous replication management port interfaces and does not support VLAN tagging. This port only needs to be configured when the system will participate in synchronous replication with a remote system. Synchronous replication management ports must be configured on the source and destination systems participating in synchronous replication.
When configuring replication, a trusted link must be created between two systems before any replication sessions can be made. A replication connection is a logical link that is created between systems that will participate in remote replication. The replication connection establishes a link for management, and the data path between a pair of systems. After the replication interfaces and the synchronous replication management ports are configured, you must create the replication connection between the systems. Once configured, all synchronous replication sessions will use the replication connections to transport data to the remote system.
Figure 2 below shows an example configuration of a replication connection between two systems. In this example, the Dell Unity XT systems are labeled “Production System” and “DR System”. All synchronous replication will occur between the Production System and the DR System. Each system is also configured with 32 Gb FC I/O Modules in slot 0, so I/O Module 0 port 0 on each SP is the synchronous replication port. After cabling each system to the network, configuring the synchronous replication management ports, and zoning the FC ports together, the replication connection can be established. All management related traffic will cross the LAN/WAN connections, and all data will be replicated across the FC Network synchronously. Once a replication connection is configured on one of the systems participating in replication, it is automatically created on the other system.
Verify and Update is a single operation that is used to update the replication connection information about the system it is issued on. This operation is performed on the replication connection itself, as opposed to an individual replication session. Verify and Update can be used to test a replication connection to a remote system or update the replication information if changes to the system have been made. Verify and Update should be issued to reestablish the replication connection to a remote system after an outage or when a configuration change to a replication interface on the system has been made. Another use case for using Verify and Update is if a Synchronous Replication Management Port IP Address has been changed on the system.
Note: A system can only be synchronously connected to one remote system at a time. FC connections to multiple Dell Unity systems will cause the remote connection verification to fail.
Figure 2. Native synchronous block replication connection example
A replication session uses a configured replication connection to transfer data from a source to a destination storage resource. If a replication session is created using Unisphere, a storage resource of the same size and type is created on the destination system and a full synchronization occurs between the source and destination storage resource. If replication is configured when creating a new resource, the synchronization operation is quick as no data needs to be copied across the link. When configuring synchronous block replication, snapshots and snapshot schedules are not replicated to the DR system.
The following steps outline a write operation to a storage resource with a synchronous replication session configured. In this example, assume the initial synchronization is complete.
When configuring replication, the source and destination storage resource must be of the same type. In Dell Unity, native synchronous replication is supported on the following storage resources:
On the system, replication for LUNs, thin clones, and VMware VMFS datastores function identically to each other. When configuring replication on a LUN, a single replication session is created, and the source and destination storage resources will be the same size and type. When configuring replication session on a thin clone, the destination storage resource will be a regular storage resource and not a clone. While synchronous replication is configured, you cannot extend the storage resource. Other options, such as the LUN’s name or tiering policy may be configured differently between systems.
In Dell Unity, a consistency group is a storage instance which contains one or more LUNs within a storage system. Consistency groups help organize the storage allocated for a particular host or hosts. consistency groups are treated as a single entity when they are replicated, meaning that a single replication session is created for the entire consistency group no matter how many LUNs it contains. When pausing or resuming replication on a consistency group, the entire group is affected by the replication operation.
Two storage resources are required for replication:
When a replication session is created in Unisphere, the destination storage resource is automatically created with the session. Upon creation, the destination resource is marked as a destination image. This restriction blocks Read/Write access on the destination storage resource. To view the data contained in the destination storage resource, you may take a snapshot of the resource and provide host access to it. Starting in UnityOS version 5.1, you may reuse a previously provisioned storage resource when reconfiguring a replication session.
In Unisphere, you can easily determine which storage resources are replicated and which are destination images from any of the storage resource pages, such as the LUNs tab on the Block page. While in this tab, select the gear icon, and hover over the Columns option to view the available columns that can be viewed. Select the check boxes for “Replication Type” and “Restricted Replication Access”. Replication Type displays what type of replication the storage resource is participating in, whether it is None, Remote, or Local. The Restricted Replication Access column will display “Yes” if the storage resource is labeled as a replication destination resource, or “No” if it is not. If a replication session is deleted, the destination storage resource will still be labeled as a replication destination image. The replication destination label must be edited manually over Unisphere CLI before the resource is allowed to receive I/O. For example, to remove the replication destination setting from a LUN, use the uemcli /stor/prov/luns/lun -id <value> set -replDest no command. For more information about removing the replication destination setting on other storage resource types, consult the Dell Unity: Unisphere Command Line Interface User Guide on Dell Online Support.
In Dell UnityOS versions 5.1 and later, Source, Destination, and All filtering buttons on the replication sessions page and various storage resource pages help the user easily identify replication source and destination resources/sessions without adding columns to the view. When All is selected, all resources/sessions on the current page are displayed. When Source is selected on a resource page, all resources that are the source of a replication session are displayed. Resources that are not replicated are also shown when Source is selected. When Source is selected on the replication sessions page, only replication session originating on the system are shown. When Destination is selected on a resource page, only resources that are the destination images of a replication session are shown. While on the sessions page, Destination will only the sessions replicating to the current system. Also, sessions that are part of local replication are displayed regardless of which view is selected.