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OceanStor 18500, 18800, and 18800F Storage System V100R001C30 Remote Replication Feature Guide 09

This document describes the remote replication feature, in terms of the implementation principle, application scenario, configuration process, and reference information.
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Data Replication

Data Replication

Data replication refers to the process of writing service data generated by hosts to the secondary LUN in the secondary storage system for backup and disaster recovery. The writing process varies depending on the remote replication mode. This section describes how a synchronous remote replication session and an asynchronous remote session process host writes.

Writing Process in Synchronous Remote Replication

A synchronous remote replication session replicates data in real time from the primary storage system to the secondary storage system. The characteristics of synchronous remote replication are as follows:
  • After receiving a write I/O request from a host, the primary storage system sends the request to the primary and secondary LUNs.
  • The data write result is returned to the host only after the data is written to both primary and secondary LUNs. The data write result is determined by whether the data is successfully written to the primary LUN and is irrelevant to the secondary LUN. However, if data fails to be written to the secondary LUN, the secondary LUN returns a message indicating data write failure to the primary LUN. The controller changes the dual-write mode to the single-write mode at the same time. The remote replication task enters the abnormal state.
After a synchronous remote replication pair relationship is set up between the primary LUN and the secondary LUN, a manually triggered synchronization needs to be performed so that the two LUNs have consistent data. Every time a host writes data to the storage system after the synchronization, the data is copied from the primary LUN to the secondary LUN in real time. Figure 1-2 shows how a synchronous replication processes a write (the data block being written is N).
Figure 1-2  Writing process in synchronous remote replication mode

RM: Remote replication management module.

LOG: Records data write logs.

DCL: Data change log.

  1. The host sends a write I/O request to the RM.
  2. The RM records the write I/O request in the LOG.
  3. The RM concurrently writes the write I/O request to both the primary cache and secondary cache.
  4. The primary cache and secondary cache return the write I/O result to the RM.
  5. A storage array returns the write I/O result to the host after receiving the response from the primary cache and secondary cache. If data fails to be written to the primary cache or secondary cache, the primary cache or secondary cache returns a write I/O failure to the RM. Then, the RM changes the mode from dual-write to single-write, and the remote replication works incorrectly.
  6. The storage array determines whether dual-write succeeds.
    • The LOG is deleted if data has been successfully written to both the primary cache and secondary cache.
    • If data fails to be written to the primary cache or secondary cache, the LOG is stored in the DCL.

Writing Process in Asynchronous Remote Replication

An asynchronous remote replication session periodically replicates data from the primary storage system to the secondary storage system. The characteristics of asynchronous remote replication are as follows:
  • Asynchronous remote replication relies on the snapshot technology. A snapshot is a point-in-time copy of source data.
  • When a host writes data to a primary LUN, the primary storage system returns a response indicating a successful write to the host, as soon as the primary LUN returns a response indicating a successful write.
  • Data synchronization is triggered by a user manually or by the system periodically to keep data consistent between the primary LUN and the secondary LUN.
After an asynchronous remote replication relationship is set up between a primary LUN and a secondary LUN, initial synchronization is performed to copy all of the data from the primary LUN to the secondary LUN so that the two LUNs have consistent data. After the initial synchronization is complete, the storage system processes host writes as follows:
When receiving a host write, the primary storage system sends the data to the primary LUN. As soon as the primary LUN returns a response indicating a successful write, the primary storage system returns a response indicating a successful write to the host. At the scheduled synchronization time, new data on the primary LUN is copied to the secondary LUN. Figure 1-3 shows how an asynchronous replication processes a write in a synchronization period (the data block being written is N).
Figure 1-3  Writing process in asynchronous remote replication mode

NOTE:
Snapshots for the primary and secondary LUNs are generated and activated when synchronization starts for the first time. The functions of the snapshots are as follows:
  • The snapshot generated for the primary LUN ensures that data read from the primary LUN during data synchronization is always consistent and allows concurrent implementation of data synchronization and data write to the primary LUN.
  • The snapshot for the secondary LUN stores backup data for the data on the secondary LUN before synchronization so that data on the secondary LUN can still be usable even when an exception occurs during synchronization.
The reason for stopping the snapshots: The snapshot function is used only during a data synchronization. After the data synchronization is complete, the snapshot function is stopped to reduce the system overhead and improve performance.
  1. The host sends a write I/O request to the RM, and the RM directly writes data to the primary cache.
  2. The primary cache returns the write I/O result to the RM, and the RM further returns the result to the host.
  3. After data in the primary cache is written to the LUN, the RM records data differences between the primary LUN and the secondary LUN in the DCL.
  4. When a replication period starts, the snapshots of the primary and secondary LUNs are activated.
  5. Data is replicated from the primary cache and LUN to the secondary cache.
    Asynchronous remote replication employs multi-time-point caching technology to replicate data from cache to the DR site. The working principle is as follows:
    1. When a replication period starts, new time segments (T2 and P2) are respectively generated in the caches of primary and secondary LUNs.
    2. New data from the host is written into time segment T2 in the cache of the primary LUN.
    3. The primary LUN returns a message indicating that the write I/O operation is complete.
    4. Data in time segment T1 is replicated to time segment P2.
    5. The secondary LUN writes the data to disks.
  6. The RM delete differences recorded in the DCL after replicating data.
  7. After the remote replication, the snapshot for the primary and secondary LUNs is stopped.
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Updated: 2019-01-28

Document ID: EDOC1000077758

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