Dell EMC Solutions Enabler 9.2 SRDF Family CLI User Guide

SRDF/A provides an R2 copy that is slightly behind its associated R1. Host writes are collected for a configurable interval (specified by the -cycle_time option) into delta sets. Delta sets are transferred to the remote array in predefined timed cycles.

Control of SRDF/A cycles varies depending on whether the array is running in legacy mode (Enginuity 5876) or multi-cycle mode (HYPERMAX OS):

Enginuity 5876

If either array in the solution is running Enginuity 5876, there are 2 cycles on the R1 side, and 2 cycles on the R2 side.

Each cycle switch moves the delta set to the next cycle in the process. This mode is referred to as "legacy mode".

A new capture cycle cannot start until the transmit cycle completes its commit of data from the R1 side to the R2 side, and the R2 apply cycle is empty.

The basic steps in the life of a delta set in legacy mode include:

1. On the R1 side, host writes collect in the Capture cycle's delta set for a specified number of seconds.

   The length of the cycle is specified using the -cycle_time option.

   If a given track is overwritten multiple times, only the last write is preserved.

2. Once the cycle timer expires, and both the R1's Transmit cycle and the R2's Apply cycle are empty:
   • The delta set in the R2's Receive cycle is moved to the R2's Apply cycle, from which it is transferred to disk.
   • The delta set in the R1's Capture cycle is moved to the R1's Transmit cycle, from which it begins transferring to the R2's Receive cycle.
   • A new delta set is created as the R1 Capture cycle, to collect host writes. The delta set is received on the R2 side.

Subsequent host writes are collected into the next delta set.

Mixed configurations

When one array in an SRDF configuration is running HYPERMAX OS, and one or more other arrays are running Enginuity 5876:

• SRDF/A single sessions (SSC) have only two cycles on the R1 side (legacy mode)
• SRDF/A multi-session consistency sessions (MSC) operate in legacy mode.

When a delta set is applied to the R2 target device, the R1 and R2 are in the consistent pair state. The R2 side is consistently 2 cycles behind the R1 site.

In the event of a failure at the R1 site or of the SRDF links, a partial delta set of data can be discarded, preserving consistency on the R2. The maximum data loss of for such failures is two SRDF/A cycles or less.

Multiple devices or device groups that require consistency can be grouped into consistency groups. Members of consistency groups cycle at the same time, to ensure consistency among the members, and if one member is interrupted, all other members suspend.

HYPERMAX OS

If both arrays in the solution are running HYPERMAX OS, both SSC and MSC operate in multi-cycle mode. There can be 2 or more cycles on the R1, but only 2 cycles on the R2 side. Cycle switches are decoupled from committing delta sets from the R1 to the R2.

When the preset Minimum Cycle Time is reached, the R1 data collected during the capture cycle is added to the transmit queue and a new R1 capture cycle is started. There is no wait for the commit on the R2 side before starting a new capture cycle.

The transmit queue holds cycles waiting to be transmitted to the R2 side. Data in the transmit queue is committed to the R2 receive cycle when the current transmit cycle and apply cycle are empty.

Queuing allows smaller cycles of data to be buffered on the R1 side and smaller delta sets to be transferred to the R2 side.

The SRDF/A session can adjust to accommodate changes in the solution. If the SRDF link speed decreases or the apply rate on the R2 side decreases, more SRDF/A capture cycles can be added to the R1 side.

Data on the R2 side can be more than 2 cycles behind the R1.

In event of R1 failure or link failure, a partial delta set of data can be discarded, preserving consistency on the R2. The maximum data loss of for such failures can be more than two SRDF/A cycles.

The EMC VMAX3 Family Product Guide for VMAX 100K, VMAX 200K, VMAX 400K with HYPERMAX OS and the Dell EMC VMAX All Flash Product Guide for VMAX 250F, 450F, 850F, 950F with HYPERMAX OS contain a detailed description of SRDF/A multi-cycle mode.