SRDF Devices

R1 (R/W)
Primary device contains production data.

R11
Concurrent primary device typically used in three-site solutions.Concurrent R11 devices are configured with local mirrors and two unique R1 SRDF mirrors.
Note: The SRDF mode for SRDF devices with multiple SRDF mirrors is set at the SRDF mirror level. For example, one R1 SRDF mirror can operate in synchronous and another in asynchronous mode. Each SRDF mirror must be configured to a separate SRDF group.

R2 (RO / WD)
Maintains a restartable, mirrored copy of the production data at the remote site while the production host is running I/Os to the R1 device.

R21
With Enginuity version 5876.82.57 or higher, SRDF supports cascaded SRDF devices (R21). R21 devices are dual-role SRDF devices that are typically used in three-site solutions. An R21 device can simultaneously assume the roles of an R2 and an R1 device. R21 devices have an R1 SRDF and an R2 SRDF mirror.
Note: The SRDF mode for SRDF devices with multiple SRDF mirrors is set at the SRDF mirror level. For example, the R2 SRDF mirror can operate in synchronous mode and the R1 SRDF mirror in adaptive copy disk mode. Each SRDF mirror must be configured to a separate SRDF group.

As long as the R1->R21->R2 SRDF relationship is established, no host can have write access to the R21 device.The R2 SRDF mirror of the R21 device can operate in all SRDF mode, but the R1 SRDF mirror of the R21 device can operate in only adaptive copy disk or asynchronous mode.

Diskless R21 devices (DL R21)
Diskless R21 devices (DL R21) operate like R21 devices except that DL R21 devices do not store full copies of data and have no local mirrors (RAID groups) configured to them. DL R21 devices store only the differences that would be owed to Symmetrix C if Symmetrix A failed.

Prior to Enginuity version 5876 Q4 2012 SR, DL R21 devices are not supported in SRDF topologies. With Enginuity 5876 Q4 2012 SR, you can configure DL R21 devices in a three-site SRDF solution. However, the Symmetrix array containing the DL R21 devices cannot be a Symmetrix VMAX 10K or VMAXe array.

Unlike R21 devices that are Symmetrix logical devices, the DL R21 devices are special cache devices that cannot be assigned to the host. At any point in time, data resides in Symmetrix B cache memory and on local mirrors in Symmetrix A and Symmetrix C.

Like R21 devices, DL R21 devices have two SRDF mirrors, an R1 and an R2 SRDF mirror.

R22
With Enginuity version 5876.82.57 or higher, SRDF supports ordinary R2 devices and concurrent R22 devices configured with two R2 SRDF mirrors that receive data from two different R1 devices.

Only one of the R2 SRDF mirrors that belong to the R22 device can accept reads or writes from its R1 device at a time. The other SRDF mirror is marked as blocked to its R1 device. R22 devices are designed to benefit SRDF/Star solutions that operate by using only the active SRDF links, although a recovery path is in place to ensure that the user can fail over to another site in case of a disaster at the production site.

Dynamic SRDF devices
Dynamic SRDF devices are SRDF devices that allow flexible control over the SRDF topology. Dynamic SRDF device attributes are stored dynamically in the mirrored and protected Symmetrix cache memory. Dynamic SRDF devices are only enabled in the Symmetrix configuration file that enables you to configure and control SRDF devices by using EMC host-based SRDF software.

EMC host-based SRDF software can initiate the following actions to modify dynamic SRDF device attributes:

  • Create a new R1/R2 pair relationship from non-SRDF devices.
  • Terminate and establish an SRDF relationship with a new R2 device.
  • Swap personalities between R1 and R2 devices.
  • Move R1/R2 pairs between different SRDF groups.

Dynamic SRDF devices are supported in the following topologies:

  • Fibre Channel SRDF, point-to-point connection
  • Fibre Channel SRDF, switched Fibre Channel
  • GigE SRDF.