More info on EMC's SRDF from the Recovery Blueprint version 1.0 recently released by EMC, Cisco and Oracle.
At the simplest conceptual level, SRDF is mirroring (RAID level 1) of one disk device (source) to a second device (target) in a physically separate Symmetrix system over ESCON, Fibre or other high speed communication links. The distance separating the two Symmetrix systems can vary from within the same room to thousands of miles. As with RAID 1, if either disk in the mirrored pair fails, the requested data is instantly available from its mirror copy. No disruption to normal operations occurs. Once the disk again becomes available (through repair, replacement, or operational procedure) the newly available member of the pair can be resynchronized with its mate.
SRDF requires a minimum of two Symmetrix systems - one source system and one target system. Additionally, there may be one server system sending information to multiple target systems and/or multiple sources sending information to one target. SRDF requires a minimum of two Remote Link Directors (RLDs) to be installed in the Symmetrix systems, both source and target. The Remote Link Director is a two-port channel adapter, microcode-configured as the link between the Symmetrix systems. Symmetrix supports up to eight RLDs depending on the Symmetrix model in use and the business continuity requirements.
Solutions
SRDF can be configured in two different solutions: Campus and Extended Distance.
Campus Solution
The Symmetrix systems are connected with ESCON up to 66 KM and Fibre up to 20 KM (using converters and repeaters when the sites are over 3 KM apart).
Extended Distance Solution
The Symmetrix systems are over 66 KM apart. In this solution the RLA's output is connected to a tele-communications protocol conversion unit, which is provided by an independent vendor. These network boxes convert the ESCON protocol to the appropriate telecommunications protocol, whether T1/T3, E1/E3, or ATM.
SRDF Modes
SRDF can be configured in four modes:
Synchronous Mode - Synchronous mode ensures that a second copy of the data, accurate up to the last transaction, is immediately available for use. In this mode SRDF writes data simultaneously to the cache of both source and target volumes, in real time, before the IO is complete.
Semi-Synchronous mode - Semi-synchronous mode is used to mask performance delays that could result from IO propagation delays. Here SRDF writes data to the source system, completes the IO and the re-synchronizes with the target system.
Adaptive Copy Mode - The Adaptive Copy Mode of operation is useful for data center migrations or for migrating large blocks of data from one place to another. When choosing this mode of synchronization, the user should be aware that data is not transferred in a serialized manner. Therefore, Adaptive Copy Mode is not a prime choice for realtime recovery.
In Adaptive Copy Mode, the user specifies that a source volume's primary mode of operation be suspended. Data is transferred from the source volumes to the remote volumes without waiting for an acknowledgment and allows a lag between source and target volume of a specified number of I/Os. This allows SRDF to manage the Adaptive Copy volumes at a lower priority, thus enabling applications using that volume not to encounter propagation delays while data is transferred to the remote site.
Domino Mode - By specifying the Domino option for the mirrored pair, any failure to complete write operations to both volumes in the relationship will cause the source volume to present an 'intervention required' status to the operating system. This in turn makes its way back to the driving application and alerts the user to the fact that the mirrored write did not complete and gives the option of preventing further activity.
SRDF users also have the option of using the Consistency Group utility to provide database coherency protection for mainframe volume's spread across multiple Symmetrix systems. The Consistency Group (CGroup) software utility, available with microcode version 5265, maintains database coherency across an SRDF configuration by monitoring data propagation from the source devices (R1) in a defined consistency group to their corresponding target (R2) devices.
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