Table Of Contents
Understanding DMM Topologies
Cisco MDS DMM is designed to support a variety of SAN topologies. The SAN topology influences the location of the SSM/MSM module and the DMM feature configuration. The following sections describe common SAN topologies and their implications for DMM:
Overview
Cisco DMM supports homogeneous SANs (all Cisco MDS switches), as well as heterogeneous SANs (a mixture of MDS switches and other vendor switches). In a heterogeneous SAN, you must connect the existing and new storage to Cisco MDS switches.
In both homogeneous and heterogeneous SANs, Cisco MDS DMM supports dual-fabric and single-fabric SAN topologies. Dual-fabric and single-fabric topologies both support single path and multipath configurations.
In a single path configuration, a migration job includes only the one path (which is represented as an initiator/target port pair). In a multipath configuration, a migration job must include all paths (which are represented as two initiator/target port pairs).
FC-Redirect
When a data migration job is in progress, all traffic (in both directions) sent between the server HBA port and the existing storage is intercepted and forwarded to the SSM/MSM, using the FC-Redirect capability.
FC-Redirect requirements for the SAN topology configuration include the following:
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The existing storage must be connected to a switch with FC-Redirect capability. FC-Redirect capability is available on MDS 9500 series and MDS 9200 series switches.
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The following examples show the server-to-storage packet flow when a data migration job is in progress. For clarity, the example shows the SSM/MSM and the existing storage connected to separate switches. The recommended practice is to connect the existing storage to the same switch as the SSM/MSM.
In Figure 6-1, the server HBA port is connected to switch A and the existing storage is connected to switch C. Both switches have FC Redirect capability. The SSM/MSM is installed on switch B. All three switches are running SAN-OS 3.2(1) or NX-OS 4.1(1b) or later.
Figure 6-1 Host Connected to FC-Redirect Switch
When the data migration job is started, FC-Redirect is configured on switch A to divert the server traffic to the SSM/MSM. FC-Redirect is configured on switch C to redirect the storage traffic to the SSM/MSM.
In Figure 6-2, the server HBA port is connected to switch A, which either does not have FC-Redirect capability or is not running SAN-OS 3.2(1) or NX-OS 4.1(1b) or later. The existing storage is connected to switch C, which has FC-Redirect capability. The SSM/MSM is installed on switch B. Switches B and C are running SAN-OS 3.2(1) or NX-OS 4.1(1b) or later.
When the data migration job is started, FC-Redirect is configured on switch C to redirect the server and storage traffic to the SSM/MSM. This configuration introduces additional network latency and consumes additional bandwidth, because traffic from the server travels an extra network hop (A to C, C to B, B to C). The recommended configuration (placing the SSM/MSM in switch C) avoids the increase in network latency and bandwidth.
Figure 6-2 Host not Connected to FC-Redirect Switch
DMM Topology Guidelines
When determining the provisioning and configuration requirements for DMM, note the following guidelines related to a SAN topology:
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Homogeneous SANs
A homogeneous SAN contains only Cisco MDS switches. Most topologies fit the following categories:
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For all of the above categories, we recommend that you locate the SSM/MSM in the switch closest to the storage devices. Following this recommendation ensures that DMM introduces no additional network traffic during data migrations.
Figure 6-3 shows a common SAN topology, with servers at the edge of the network and storage arrays in the core.
Figure 6-3 Homogeneous SAN Topology
In a homogeneous network, you can locate the SSM/MSM on any DMM-enabled MDS switch in the fabric. It is recommend that SSM/MSM is installed in the switch connected to the existing storage. The new storage should be connected to the same switch as the existing storage. If the SSM/MSM is on a different switch from the storage, additional ISL traffic crosses the network during the migration (all traffic between storage and server is routed through the SSM/MSM).
Heterogeneous SANs
When planning Cisco MDS DMM data migration for a heterogeneous SAN, note the following guidelines:
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Depending on the topology, you may need to make configuration changes prior to data migration.
DMM Method 3 Topology
DMM Method 3 is a derivative of the DMM Method 2 (also called Asynchronous DMM). DMM Method 3 supports the dedicated migration fabric and is designed to address the problem of migrating data from an array port that is connected to a dedicated SAN that is different from the production SAN.
Many IT organizations requirse data migration to a remote Data Center. Some organizations prefer to use a dedicated storage port (on the existing storage array) connected to a separate physical fabric. This fabric is called the migration or replication fabric because it is used for data migration as well as continuous data replication services.
The LUNs mapped to the existing storage port in the migration/remote SAN are also mapped to another storage port on the array that is connected to the production SAN and accessed by one or more servers. The servers may also access the storage from two production SANs for redundancy. In this topology, the migration SAN becomes the third SAN to which the existing storage array is connected. The new storage array is connected only to the migration SAN and may not have any ports on the production SAN(s). (See Figure 6-4)
Figure 6-4
DMM Method 3 Topology
In the above topology, DMM Method 3 should be used to migrate data from the existing storage to the new storage in the replication/migration SAN. DMM Method 3 requires an SSM/MSM in each of the production SANs (with support for a maximum of two production SANs) and an SSM/MSM in the migration SAN. The DMM Method 3 job has three SSM/MSMs unlike Method 1 and Method 2 which can have a maximum of two SSM/MSMs. In Method 3, the SSM/MSM in the migration SAN is responsible for executing the sessions in the DMM job, and copying the data from the existing storage to the new storage. The SSM/MSMs in the production SANs are responsible for tracking the server writes to the existing storage. No server writes are expected in the migration SAN.
Server writes in the Production SAN are logged by the SSM/MSM in that fabric by maintaining a Modified Region Log (MRL) for each LUN that is migrated. This MRL is the same as the MRL maintained in DMM Method 2. The SSM/MSM in the migration SAN does not maintain any MRL for the LUN because no server writes to the existing storage LUN are expected in the migration SAN. The SSM/MSM in the migration SAN is responsible for retrieving the MRLs for a LUN from both the production SANs and for performing a union of the MRLs to create a superset of all modified blocks on the LUN via paths from both production SANs. The SSM/MSM then copies all the modified regions from the existing storage LUN to the new storage LUN in the migration SAN. This process is repeated until the administrator is ready to finish the DMM job and perform a cutover. The finish operation on a Method 3 places all LUNs in the offline mode and performs a final pass over the combined MRL to synchronize the existing and new storage LUN in each session. This cutover process is the same process used in cutover operations in DMM Method 2.
Ports in a Server-Based Job
This section provides guidelines for configuring server-based migration jobs.
When creating a server-based migration job, you must include all possible paths from the host to the LUNs being migrated. All writes to a migrated LUN need to be mirrored in the new storage until the job is destroyed, so that no data writes are lost. Therefore, all active ports on the existing storage that expose the same set of LUNs to the server must be added to a single data migration job.
In a multipath configuration, two or more active storage ports expose the same set of LUNs to two HBA ports on the server (one initiator/target port pair for each path). Multipath configurations are supported in dual-fabric topologies (one path through each fabric) and in single-fabric topologies (both paths through the single fabric).
In a single path configuration, only one active storage port exposes the LUN set to the server. The migration job includes one initiator/target port pair (DMM does not support multiple servers accessing the same LUN set).
The following sections describe how to apply the rules to various configurations:
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Single LUN Set, Active-Active Array
In the example shown in Figure 6-5, the server accesses three LUNs over Fabric 1 using storage port ES1. The server accesses the same LUNs over Fabric 2 using storage port ES2.
Both storage ports (ES1 and ES2) must be included in the same data migration job, as both ports are active and expose the same LUN set.
Figure 6-5 Single LUN Set, Active-Active Array
You create a data migration job with the following configuration:
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Multiple LUN Set, Active-Active Arrays
In the example shown in Figure 6-6, the server accesses three LUNs over Fabric 1 using storage port ES1. The server accesses the same LUNs over Fabric 2 using storage port ES2. The server accesses three different LUNs over Fabric 1 using storage port ES3, and accesses the same LUNs over Fabric 2 using storage port ES4.
Figure 6-6 Multiple LUN Set, Active-Active Arrays
You need to create two data migration jobs, because the server has access to two LUN sets on two different storage ports. You need to include two storage ports in each data migration job, as they are active-active multipathing ports.
One migration job has the following configuration:
This job includes three data migration sessions (for LUNs 1,2, and 3).
The other migration job has the following configuration:
This job includes three data migration sessions (for LUNs 7,8, and 9).
Single LUN Set, Active-Passive Array
In an active-passive array, the LUNs exposed by a storage port may be active or passive.
Example 1: Each controller has two active ports
In the example shown in Figure 6-7, the server accesses a single LUN set. However, all LUNs are not active on a single storage port. The active-passive array in the example has two controllers, each with two ports. LUN 0 and LUN 1 are active on ES1 and ES2. LUN 2 and LUN 3 are active on ES3 and ES4.
Logically, the server sees two active LUN sets that are accessed from two different storage ports. Each storage port is paired for multipathing.
Figure 6-7 Example 1: Single LUN Set, Active-Passive Array
The server accesses LUN 0 and LUN 1 over Fabric 1 using storage port ES1. The server accesses the same LUNs over Fabric 2 using storage port ES2. The server accesses LUN 2 and LUN 3 over Fabric 1 using storage port ES3, and accesses the same LUNs over Fabric 2 using storage port ES4.
You need to create two data migration jobs, because the server has access to two LUN sets over two different storage ports. Each of the data migration jobs includes two storage ports, because both ports access the active LUNs on the storage.
Only the active LUNs and associated storage ports are included in each job. (LUNs 0 and 1 in one job and LUNs 1 and 2 in the other job).
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One migration job has the following configuration:
This job includes two data migration sessions (for LUNs 0 and 1).
The other migration job has the following configuration:
This job includes two data migration sessions (for LUNs 2 and 3).
Example 2: Each controller has only one active port
In the example shown in Figure 6-8, the server accesses a single LUN set. However, all LUNs are not active on a single storage port. The active-passive array in the example has two controllers, each with a single port. LUN 0 and LUN 1 are active on ES1. LUN 2 and LUN 3 are active on ES2.
Logically, the server sees two active LUN sets that are accessed from different storage ports.
Figure 6-8 Example 2: Single LUN Set, Active-Passive Array
The server accesses LUN 0 and LUN 1 over Fabric 1 using storage port ES1. The server accesses LUN 3 and LUN 4 over Fabric 2 using storage port ES2.
You need to create two data migration jobs, because the server has access to two LUN sets over two different storage ports. Each of the data migration jobs includes the ports from a single fabric.
One migration job has the following configuration:
The other migration job has the following configuration:
