Dell SRDF Introduction

SRDF consistency

Many applications (in particular, database management systems) use dependent write logic to ensure data integrity if a failure occurs. A dependent write is a write operation that the application does not issue unless some prior I/O has completed. If the writes are out of order, and an event such as a failure occurs at that exact time, unrecoverable data loss may occur.

An SRDF consistency group (SRDF/CG) contains SRDF devices with consistency enabled.

An SRDF consistency group preserves the dependent-write consistency of devices within the group. Consistency is maintained by monitoring data propagation from source devices to their corresponding target devices. If consistency is enabled, and SRDF detects any write I/O to a R1 device that cannot communicate with its R2 device, SRDF:

1. Suspends remote mirroring for all devices in the consistency group
2. Completes the intercepted I/O to the R1 device
3. Returns control to the application

In this way, SRDF/CG prevents a dependent-write I/O from reaching the secondary site if the previous I/O only gets as far as the primary site.

SRDF consistency allows quick recover from certain types of failure or physical disasters by retaining a consistent, DBMS-restartable copy of the database.

SRDF consistency group protection is available for both SRDF/S and SRDF/A.