Features

The following sections describe the SRDF/A features that guarantee such high levels of data consistency, disaster-restart protection, and low host I/O response time.

A. Reserve Capacity
Enginuity versions 5772 and higher support the SRDF/A Reserve Capacity feature that keeps the SRDF/A session operational in the event of temporary network resource shortfalls that would have previously suspended the SRDF/A operations.

The two functions that implement SRDF/A Reserve Capacity are Transmit Idle and DSE.
Transmit Idle

  • Transmit Idle keeps the SRDF/A session active after all the SRDF network links have failed.
  • It allows SRDF/A to remain fully active during network outages that cause an All Links Lost condition. When the links return, SRDF/A resumes data transfer.

Delta Set Extension

  • SRDF/A DSE keeps the SRDF/A session active if the Symmetrix array approaches the SRDF/A maximum cache utilization limits.
  • DSE pages out (offloads) some or all of the delta set data into preconfigured storage pools known as DSE pools that are associated with SRDF/A groups.
  • EMC recommends that you configure DSE pools and enable DSE on the primary and on the secondary array. When TimeFinder/Snap sessions are used to replicate either R1 or R2 devices, you must create two separate preconfigured storage pools: DSE and Snap pools.

B.Write folding

  • Write folding improves the efficiency of the SRDF network links. If multiple updates to the same location arrive in the same cycle, the SRDF emulations send the most current data across the SRDF links.
  • This feature makes SRDF/A superior to competitive asynchronous mirroring solutions as it decreases network bandwidth consumption and the number of I/Os processed by the SRDF emulations.


C. Write pacing
Write pacing is an SRDF/A feature that balances cache utilization by extending the host write I/O response time to prevent SRDF/A operational interruptions. The write pacing feature provides the following options:

  • The group-level pacing option. This option requires Enginuity version 5874 or higher on both Symmetrix arrays. It is enabled for the entire SRDF/A group when slowdowns in host I/O rates, transmit cycle rates, or apply cycle rates occur.
  • The device-level pacing option. This option is for SRDF/A solutions in which the SRDF/A R2 devices participate in TimeFinder copy sessions. This option requires Enginuity version 5875.135.91 or higher on both Symmetrix arrays. You can enable or disable each write pacing option. All write pacing options are compatible with each other and with other SRDF/A features including tunable cache utilization, Reserve Capacity, and MSC.

The key benefit of the SRDF/A write pacing is its dynamic, self-paced mechanism. Once enabled, write pacing is employed only when required and applies the appropriate delay to the host write I/O response time to keep the SRDF/A session active. When write pacing is enabled, but pacing is not required to keep the SRDF/A session running, host write I/Os are not paced. The delay applied to the overall host write I/O response time is known as the write pacing delay.

With Enginuity version 5875.135.91 or higher, the write pacing delay can by default extend the host write I/O response time up to 50 milliseconds. You can overwrite the default value with a user-specified maximum write pacing delay that can extend the host I/O response time up to one second. The maximum write pacing delay applies to all write pacing options

D. Tolerance mode
Tolerance mode is an SRDF/A feature that allows you to balance the performance and data consistency requirements. Setting Tolerance mode ON allows one or more devices to be Not Ready on the link at the R1 side and the SRDF/A session remain active. R2 consistency is not maintained, but this may be acceptable in certain controlled service activities. If all links are lost, Tolerance mode does not keep SRDF/A active. You can use the Tolerance mode option to complete hardware service procedures like device replacement without disabling or dropping SRDF/A.

By default, EMC host-based SRDF control software does not allow an SRDF/A session participating in MSC operations to have tolerance mode set.

Note: When tolerance mode is set, dependent-write consistency is not guaranteed.