SRL Layout

This section explains how the SRL affects application performance and how a good SRL layout can improve performance.

How SRL Affects Performance

Incoming writes to a data volume on the Primary are written to the SRL, first, and then to the data volume. VVR manages writes in the same way, irrespective of the replication settings, including the mode of replication. Note that writes to different data volumes within the RVG are also written in the same SRL. Therefore, the SRL throughput may affect performance.

The following reasons could alleviate the degradation of performance:

  The writes to SRL are sequential, whereas, the writes to the data volumes are spatially random in most cases. Typically, sequential writes are processed faster than the random writes.

  The SRL is not used to process read operations performed by the application. If a large percentage of the operations are read operations, then the SRL is not busy at these times.

If the rate at which the application writes to the data volume is greater than the rate at which the SRL can process writes, then the application could become slow. The following sections explain how to lay out the SRL to improve performance.

Striping the SRL

Striping the SRL over several physical disks to increase the available bandwidth can improve performance.

Choosing Disks for the SRL

It is recommended that there be no overlap between the physical disks comprising the SRL and those comprising the data volumes, because all write requests to VVR result in a write to both the SRL and the requested data volume. Any such overlap is guaranteed to lead to major performance problems, as the disk head thrashes between the SRL and data sections of the disk. Slowdowns of over 100% can be expected.

Mirroring the SRL

It is recommended that the SRL be mirrored to improve its reliability. The loss of the SRL immediately stops replication. The only way to recover from this is to perform a full resynchronization, which is a time-consuming procedure to be avoided whenever possible. Under certain circumstances, the loss of the SRL may even cause loss of the data volumes. The risk of this failure can be minimized by mirroring the SRL.