Home > Storage > PowerMax and VMAX > Data Protection > Dell PowerMax and VMAX All Flash: SRDF/Metro Overview and Best Practices > SRDF/Metro overview
SRDF synchronous (SRDF/S) mode maintains a real-time copy at arrays generally located within 200 kilometers (dependent upon application workload, network latency, and block size). Writes from the production host are acknowledged from the local array when they are written to cache at the remote array creating a real-time mirror of the primary devices.
SRDF disaster recovery solutions, including SRDF synchronous, traditionally use active, remote mirroring and dependent-write logic to create consistent copies of data. Dependent-write consistency ensures transactional consistency when the applications are restarted at the remote location.
An SRDF device is a logical device paired with another logical device that resides in a second array. The arrays are connected by SRDF links. R1 devices are the member of the device pair at the primary (production) site. R1 devices are generally read/write accessible to the host. R2 devices are the members of the device pair at the secondary (remote) site. During normal operations, host I/O writes to the R1 device are mirrored over the SRDF links to the R2 device.
Traditionally, data on R2 devices are not available to the host while the SRDF relationship is active. In SRDF synchronous mode, an R2 device is typically in read-only mode (write disabled) that allows a remote host to read from the R2 devices. In a typical open systems host environment, the production host has read/write access to the R1 device. A host connected to the R2 device has read-only access to the R2 device. To access the R2 device of a traditional synchronous relationship, a manual failover or swap operation must be performed to write enable the R2 site to accept host writes.
With the introduction of HYPERMAX OS 5977.691.684 and Solutions Enabler/Unisphere for VMAX 8.1, we have introduced support for SRDF/Metro for VMAX3 and VMAX All Flash families of storage arrays. SRDF/Metro significantly changes the traditional behavior of SRDF Synchronous mode with respect to the secondary or remote device availability to better support host applications in high-availability environments. With SRDF/Metro, the SRDF R2 device is also read/write accessible to the host and takes on the external identity of the primary R1 device (geometry, device WWN). By providing this external identity on the R2 device, both R1 and R2 devices may then appear as a single virtual device across the two SRDF paired arrays for host presentation. With both the R1 and R2 devices being accessible, the host or hosts (in the case of a cluster) can read and write to both R1 and R2 devices with SRDF/Metro ensuring that each copy remains current, consistent, and addressing any write conflicts which may occur between the paired SRDF devices.
The left example depicts a SRDF/Metro configuration with a stand-alone host which has visibility to both VMAX3 or VMAX All Flash arrays (R1 and R2 devices) using host multipathing software such as PowerPath, to enable parallel reads and writes to each array. This is enabled by federating the personality of the R1 device to ensure that the paired R2 device appears to the host as a single virtualized device. See Host support matrix example and Host multipathing software for additional requirements in this area.
The right example depicts a clustered host environment where each cluster node has dedicated access to an individual VMAX array. In either case, writes to the R1 or R2 devices are synchronously copied to its SRDF paired device. Should a conflict occur between writes to paired SRDF/Metro devices, the conflicts will be internally resolved to ensure a consistent image between paired SRDF devices are maintained to the individual host or host cluster.
SRDF/Metro may be managed through Solutions Enabler SYMCLI or Unisphere for VMAX 8.1 or greater client software and requires a separate SRDF/Metro license to be installed on each VMAX3, VMAX All Flash, or PowerMax array to be managed.