Let us review the legacy single cycle srdfa

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Let us review the legacy (single cycle) SRDF/A architecture The legacy SRDF/A architecture applies to VMAX family arrays running Enginuity 5876 and lower. SRDF/A uses Delta Sets to maintain a group of writes over a short period of time. Delta Sets are discrete buckets of data that reside in different sections of the VMAX cache. There are four types of Delta Sets to manage the data flow process. The Capture Delta Set in the source array (numbered n in this example), captures (in cache) all incoming writes to the source volumes in the SRDF/A group. The Transmit Delta Set in the source array (numbered n-1 in this example), contains data from the immediately preceding Delta Set. This data is being transferred to the remote array. The Receive Delta Set in the target system is in the process of receiving data from the transmit Delta Set n-1. The target array contains an older Delta Set, numbered N-2, called the Apply Delta Set. Data from the Apply Delta set is being assigned to the appropriate cache slots ready for de-staging to disk. The data in the Apply Delta set is guaranteed to be consistent and re-startable should there be a failure of the source Symmetrix. The factors that govern a cycle switch in legacy mode were: the minimum cycle time has expired, the transmit delta set has been completely transferred and the apply delta set was completely applied. The creation of a new capture cycle was dependent on the transmit cycle completing its commit of data from the R1 side to the R2 side. If a cycle switch could not occur, then the capture delta set would start accumulating writes. Longer times between cycle switching would cause large amounts of data to be buffered in the capture cycle on the R1 side. This in turn could cause large amounts of data to be transferred across the SRDF link and large amounts of data to be de-staged on the R2 apply cycle. SRDF/Asynchronous Operations
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4 Copyright 2015 EMC Corporation. All rights reserved. SRDF/A Multi-Cycle Mode (MCM) allows more than two delta sets on the R1 side. When the minimum_cycle_time has elapsed the data from the current capture cycle is added to a transmit queue and a new capture cycle is started. There is no wait for the commit to the R2 side before starting a new capture cycle. The transmit queue is a feature of SRDF/A. It provides a location for R1 captured cycle data to be placed so a new capture cycle can occur. The capture cycle will occur even if no data is transmitted across the link. If no data is transmitted across the link the capture cycle data will again be added to the transmit queue. Data in the transmit queue is committed to the R2 receive cycle when the current transmit cycle and apply cycle are empty. The transmit cycle will transfer the data in the oldest capture cycle to the R2 first and then repeat the process.
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