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Dell SRDF Introduction

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Four-site mainframe configurations (SRDF/SQAR)

SRDF/SQAR (Symmetrix Quadrilateral Asynchronous Replication) is a four-site implementation of SRDF/S and SRDF/A that enables differential resynchronization between sites along the perimeter of a 'square' SRDF topology. Dell GDDR is required to be able to implement SRDF/SQAR.

The SRDF/SQAR configuration provides the ability to recover from a single or dual unplanned site outage in one region with SRDF/S protection established differentially between the recover sites in another region. This enables rapid resumption of a workload with SRDF/S and Autoswap protection in another region. In certain failure scenarios, it also provides zero data loss recovery across regions.

Figure 1. SRDF/SQAR with Autoswap environment
SRDF/SQAR with Autoswap environment

The diagram shows four Dell GDDR control systems with their independent heartbeat communication paths, separate from the production disk and computer facilities. Each of the managed z/OS systems has Dell Autoswap and Dell Consistency Groups (ConGroup) installed.

Each GDDR SRDF/SQAR environment manages two consistency groups (one active, one defined) and two Multi-Session Consistency MSC groups (both active). A consistency group is a named group of source (R1) volumes that are managed by the ConGroup application as a unit. An MSC group is a named group consisting of multiple SRDF groups operating in SRDF/A mode, managed by the Dell MSC control software feature as a single unit. The relationship between Site A (DC1) and Site B (DC2) is maintained through SRDF/S replication of primary disk images at DC1 to DC2, while SRDF/A replication maintains out of region, mirrored data at Site C (DC3) and Site 4 (DC4).

Requirements and restrictions

  • SRDF/SQAR is required to be configured with the MSC High Availability feature with a second SCF instance and MSC configured using weight factor of 2.
  • SRDF Host Component actions that change the devices that are defined to the SQAR MSC groups require the MSC tasks to be inactive at the time of the change.
  • Connectivity is provided only along the perimeter of the SQAR topology. Cross site connectivity (for example, Site A to Site D) is not supported. Therefore, a traditional three-site SRDF/Star as a recovery configuration is not available.
  • In the case of a single site failure, it is important to know which SRDF/A site is more current. The existing SRDF/A secondary time-of-day value is used to determine which site is ahead.
NOTE:SRDF/SQAR can support FBA volumes, it must be managed by GDDR which runs on z/OS.

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