Metro Mirror and Global Copy paths
This chapter describes the connections, physical links, and logical paths that are required to transfer data for Metro Mirror and Global Copy. The creation and management of these paths is the same for both Metro Mirror and Global Copy. This chapter uses the more general term Peer-to-Peer Remote Copy (PPRC) when describing features and uses that are common between Metro Mirror and Global Mirror.
This chapter includes the following topics:
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Terminology: The following pairs of terms are used interchangeably in this chapter:
•Primary and source
•Secondary and target
•Logical subsystem (LSS) and logical control unit (LCU)
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15.1 PPRC paths and links
Before a PPRC pair can be created, logical paths between the primary and secondary LSSs must be defined.
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Note: An LSS is a grouping of up to 256 logical volumes. You can define up to 255 LSSs in a DS8000. LSSs are either count key data (CKD) or fixed-block architecture (FB) and have affinity with one storage facility image server. Up to 128 LSSs can be managed by Server 0, and up to 127 LSSs can be managed by Server 1 (one LSS address is reserved).
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A logical PPRC path is defined between a primary LSS and a secondary LSS using a physical link between the storage systems. Establishing the PPRC logical paths creates an LSS to LSS relationship.
15.2 Fibre Channel physical links
The physical link includes the host adapter in the primary DS8000, the cabling, switches, or directors, any wide-band or long-distance transport devices (WDM, channel extenders, and WAN), and the host adapters in the secondary DS8000 storage system. Physical links are bidirectional and can be shared by both Metro Mirror and Global Copy.
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Fibre Channel support: For PPRC, the DS8000 supports Fibre Channel links only; you cannot use FICON links.
Metro Mirror paths: Consider defining all Metro Mirror paths that are used by one application environment on the same set of physical links if you intend to keep the data consistent. With this approach, the paths between multiple Metro Mirror volume relationships cannot fail at a different time. For more information, see 16.8.2, “Consistency group function: How it works” on page 128.
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A DS8000 Fibre Channel port can simultaneously be:
•A sender for a PPRC primary
•A receiver for a PPRC secondary
•A target for Fibre Channel Protocol (FCP) hosts I/O from Open Systems and Linux on z Systems
Each PPRC port provides connectivity for all LSSs within the DS8000 and can carry multiple logical PPRC paths. Although one FCP link can have sufficient bandwidth for many environments, the preferred practices are to:
•Configure at least two Fibre Channel links between each primary and secondary storage systems to:
– Provide redundancy for continuous availability if there is a physical path failure
– Provide multiple logical paths between the LSSs
•Dedicate Fibre Channel ports for PPRC usage, to ensure that there is no interference from host I/O activity. This action is especially essential with Metro Mirror, which is time critical and should not be impacted by host I/O activity.
See Chapter 17, “Metro Mirror and Global Copy implementation considerations” on page 139 for further information. IBM Support is available to assist you with determining the number of links that are used by a bandwidth analysis to ensure that the environment is able to effectively handle the workload.
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Sharing links: In general, you should not share the FCP links used for Metro Mirror with Global Mirror.
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PPRC links can be directly connected, or connected via switches.
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Channel extension: If you use channel extension technology devices for PPRC links, you should verify with the product’s vendor what environment (directly connected or connected with a SAN switch) is supported by the vendor and what SAN switch is supported.
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15.3 Logical paths
A PPRC logical path is a connecting path between the primary LSS and the secondary LSS. Physical links can carry multiple PPRC logical paths, as shown in Figure 15-1.
Figure 15-1 Logical paths
Logical PPRC paths are unidirectional, that is, they operate in only one direction. An LSS can be both a primary and a secondary for sets of PPRC logical paths. Also, logical paths in opposite directions can be defined on the same Fibre Channel physical link.
For bandwidth and redundancy, up to eight logical paths can be established between a primary LSS and a secondary LSS. Metro Mirror and Global Copy balance the workload across the available physical paths.
Figure 15-2 on page 114 shows an example of PPRC logical paths from primary H1 to secondary H2. With a 1:1 mapping of primary to secondary LSSs there are the following logical paths:
•LSS 1 in H1 to LSS 1 in H2
•LSS 2 in H1 to LSS 2 in H2
•LSS 3 in H1 to LSS 3 in H2
Alternatively, if the volumes in each of the LSSs of H1 map to volumes in all three target LSSs in H2, then there are nine logical paths over the physical link.
Whenever possible, you should use a 1:1 LSS mapping to simplify the management of the configuration.
Figure 15-2 Logical paths over a physical link for Metro Mirror
Metro Mirror FCP paths have certain architectural limits, which include:
•A primary LSS can maintain paths up to a maximum of 16 secondary LSSs. Each secondary LSS can be in a different DS8000.
•You can define up to eight physical port pairs per LSS-LSS relationship.
•An FCP port can host up to 1280 logical paths. These paths are the logical and directional paths that are made from LSS to LSS.
•An FCP physical link (the physical connection from one port to another port) can host up to 256 logical paths.
•An FCP port can accommodate up to 126 different physical links (DS8000 port to DS8000 port through the SAN).
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