Additional EMC SRDF Configurations

Metropolitan clusters using EMC SRDF can be set up as 1 by 1 or M by N configurations. The following sections show the rules that apply.

Setting up Hardware for 1 by 1 Configurations

Ensure that the Symmetrix disk arrays are correctly cabled using PV links to each node in the cluster that will run packages that access data on the Symmetrix.

See the SymCLI manual for instructions on creating the appropriate pseudo device files.

R1 and R2 devices must have been correctly defined and assigned to the appropriate nodes in the internal configurations that is downloaded by EMC support staff.
Figure 3-4 EMC R1 and R2 Definitions
R1 devices are locally protected (RAID 1 or RAID S).

R2 devices are locally protected (RAID 1, RAID S or BCV).




	NOTE: It is highly recommended that the R2 device is locally protected with RAID 1 or RAID S. If the R2 device is protected with BCV, and if it fails and there is a failover, the package cannot operate on the BCV device. The R2 device has to be fixed, the data has to be restored from the BCV device to the new R2 device, before the package can start.

Only Synchronous Mode is supported; Adaptive Copy must be disabled.
Domino Mode is recommended to ensure data currency on all Symmetrix frames and that there is no possibility of inconsistent data at the R2 side in case of SRDF links failure.
If Domino Mode is not enabled and if all SRDF links fail, and the application continues to modify the data on the R1, but the new data is not replicated to the R2 side. The R2 only contains a copy of the data up to the point of CA links failure. If additional failure occurs, such as a system failure before the SRDF link is fixed, this can cause the application to fail over to the R2 side, and the application will have to deal with non-current data.
If Domino Mode is not enabled, in the case of a rolling disaster, the data may be inconsistent. Additional failures taking place before the system has completely recovered from a previous failure. The inconsistent and therefore unusable data will result from the following sequence of circumstances:
- Domino Mode is not enabled
- the SRDF links fail
- the application continues to modify data
- The link is restored
- Resynchronization from R1 to R2 starts, but does not finish
- The R1 side fails
Although the risk of this occurrence is extremely low, if your business cannot afford even this quite small risk, then you must enable Domino Mode to ensure that the data at the R2 side are always consistent. The disadvantage of enabling Domino Mode is that when the SRDF link fails, all I/Os will be refused (to those devices) until the SRDF link is restored, or manual intervention is undertaken to disable Domino Mode. Applications may fail or may continuously retry the I/Os (depending on the application) if Domino Mode is enabled and the SRDF link fails.
To minimize contention, each package should be assigned its own, unique gatekeeper device. See “Configuring Gatekeeper Devices”.

Setting up Hardware for M by N Configurations

You can configure up to four Symmetrix disk arrays in the following combinations:

An array in Data Center A connected to one array in Data Center B.
An array in Data Center A connected to two arrays in Data Center B.
Two arrays in Data Center A connected to an array in Data Center B.
Two arrays in Data Center A connected to two arrays in Data Center B.




	WARNING! M by N configurations cannot be used with R1/R2 swapping.

Figure 3-5 “2 by 1 Node and Data Center Configuration” shows a 2 by 1 configuration with BCVs.

Figure 3-5 2 by 1 Node and Data Center Configuration

The figure indicates R1 volumes at Data Center A and R2 volumes and BCVs at Data Center B for pkg A and pkg B.

Figure 3-6 “2 by 2 Node and Data Center Configuration” shows a 2 by 2 configuration with R1 volumes for pkg A and pkg B on the Symmetrix frames located in Data Center A and R2 volumes and BCVs at Data Center B. Many of the examples given later in this chapter are based on this configuration.

Figure 3-6 2 by 2 Node and Data Center Configuration

Figure 3-7 “Bidirectional 2 by 2 Configuration” below shows a bidirectional 2 by 2 configuration with additional packages on node3 and node4, and R1 and R2 volumes at both data centers. In this configuration, R1 volumes for pkg A and pkg B are at Data Center A, and R2 volumes are at Data Center B. R1 volumes for pkg C and pkg D are at Data Center B, and R2 volumes are at Data Center A.

Figure 3-7 Bidirectional 2 by 2 Configuration

Additional EMC SRDF Configurations

Technical documentation

» Table of Contents

» Glossary

» Index

Setting up Hardware for 1 by 1 Configurations

Setting up Hardware for M by N Configurations