This article provides the overview of Consistency Group about VMAX SRDF & TimeFinder and its operation guide.
Consistency Group in SRDF
SRDF Consistency Groups allow customers to define logical volume groups, which can be associated with given workloads. These groups of SRDF logical volumes are automatically suspended in case of SRDF communications failures. SRDF consistency preserves the dependent-write consistency of devices within a group by monitoring data propagation from source devices to their corresponding target devices. If a source R1 device in the consistency group cannot propagate data to its corresponding R2 device, SRDF consistency suspends data propagation from all the R1 devices in the group.
SRDF consistency group protection is available for SRDF/S and SRDF/A. An SRDF consistency group is a composite group comprised of SRDF devices with consistency enabled. The devices in the consistency group are configured to act in unison to maintain the integrity of a database when distributed across multiple arrays or across multiple devices within an array. SRDF composite groups are initially created using the symcg create command. Once they are created, they are populated with devices and device groups. In order to be enabled as an SRDF consistency group, the composite group must be:
Following figure provides an example of using an SRDF consistency group with a parallel database configuration.
In this example, the same SRDF consistency group definition exists on both hosts. Once consistency group created and enabled, Solutions Enabler’s daemon Group Name Services (GNS) automatically propagates a composite group definition to the arrays and to all locally-attached hosts running the GNS daemon. Although each production host can provide I/O to both R1 devices in the configuration, the DBMS has a distributed lock manager that ensures two hosts cannot write data to the same R1 device at the same time. The SRDF links to two remote arrays (B and D) enable the R2.
Consistency Group in TimeFinder
TimeFinder/Consistency Groups provides the option to ensure data consistency between volumes and across Symmetrix systems. It allowa TimeFinder copy sessions to be activated with a consistent, restartable copy of the data. Also, TimeFinder use the VMAX Enginuity Consistency Assist (ECA) feature to perform consistent splits between source and target device pairs across multiple, heterogeneous hosts. Consistent split (which is an implementation of instant split) helps to avoid inconsistencies and restart problems that can occur if you split database-related devices without first quiescing the I/Os.
Following figure provides the layout of TimeFinder/Clone with consistency group enabled.
In this example, a device group, composite group, or a device file must be created on the controlling host for the target database to be consistently activated. Device groups can be created to include all of the devices being accessed or defined by database host access. For example, if you define a device group that includes all of the devices being accessed by Hosts A, B, and C (see above Figure), then user can consistently activate all of the copy sessions related to those hosts with a single command. However, if you define a device group that includes only the devices accessed by Host A, then you can activate those copy sessions related to Host A without affecting the other hosts.
For detail operations about the creating Consistency Group of SRDF and TimeFinder, please refer the Solutions Enabler guide from EMC online support:
Author: Fenglin Li
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