Dell SRDF Introduction

Group pacing

SRDF/A group pacing adjusts the pace of host writes to match the SRDF/A session’s link transfer rate. When host I/O rates spike, or slowdowns make transmit or apply cycle times longer, group pacing extends the host write I/O response time to match slower SRDF/A service rates.

When DSE is activated for an SRDF/A session, host-issued write I/Os are paced so their rate does not exceed the rate at which DSE can offload the SRDF/A session’s cycle data to the DSE Storage Resource Pool.

Group pacing behavior varies depending on whether the maximum pacing delay is specified:

• If the maximum write pacing delay is not specified, SRDF adds up to 50 ms to the host write I/O response time to match the speed of either the SRDF links or the apply operation on the R2 side, whichever is slower.
• If the maximum write pacing delay is specified, SRDF adds up to the user-specified maximum write pacing delay to keep the SRDF/A session running.

Group pacing balances the incoming host I/O rates with the SRDF link bandwidth and throughput capabilities when:

• The host I/O rate exceeds the SRDF link throughput.
• Some SRDF links that belong to the SRDF/A group are lost.
• Reduced throughput on the SRDF links.
• Enginuity 5876 only: The write-pending level on an R2 device in an active SRDF/A session reaches the device write-pending limit.
• Enginuity 5876 only:The apply cycle time on the R2 side is longer than 30 s and the R1 capture cycle time (or in MSC, the capture cycle target).

Group pacing can be activated by configurations or activities that result in slow R2 operations, such as:

• Slow R2 physical drives resulting in longer apply cycle times.
• Director sparing operations that slow restore operations.
• I/O to the R2 array that slows restore operations.