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Remote Replication Feature Guide for File 04

OceanStor 5300 V3, 5500 V3, 5600 V3, 5800 V3, and 6800 V3 Storage System V300R002

"This document describes the file system remote replication feature, in terms of the implementation principle,application scenario, and explains how to configure and manage this feature."
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Data Replication

Data Replication

Data replication refers to the process of writing service data generated by hosts to the secondary file system 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.

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. For more detail refer to OceanStor 5300 V3&5500 V3&5600 V3&5800 V3&6800 V3 Storage System V300R002 Snapshot Feature Guide for File.
  • When a host writes data to a primary file system, the primary storage system returns a response indicating a successful write to the host, as soon as the primary file system 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 file system and the secondary file system.
After the file system remote replication relationship is set up between the primary file system and secondary file system, initial synchronization will be started to copy all data from the primary file system to the secondary file system, ensuring data consistency at a specific point in time (when the snapshot is generated.)
When receiving a host write, the primary storage system sends the data to the primary file system. As soon as the primary file system 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 file system is copied to the secondary file system. Figure 1-2 shows how an asynchronous replication processes a write in a synchronization period (the data block being written is N).
Figure 1-2  Writing process in asynchronous remote replication mode

DCL: Data change log.

NOTE:

The DCL is stored on all the disks in the disk domain and four-disk RAID 1 is used for data protection.

  1. The host sends a write I/O request to the primary cache.
  2. The primary cache returns the write result to the host.
  3. After data in the primary cache is written to the primary file system, data differences between the primary file system and the secondary file system are recorded in the DCL.
  4. When a replication period starts, the system writes data in the primary cache to the primary file system and creates a snapshot.
  5. Data is replicated from the primary file system to the secondary cache based on the differences recorded in the DCL.
  6. After the data is synchronized:
    • A snapshot is created for the secondary file system.
    • The snapshot created when the last synchronization was completed is deleted from the secondary file system.
    • The snapshot created when the last synchronization was started is deleted from the primary file system.
    • The data changes recorded in the DCL are deleted.
NOTE:
  • The snapshot generated for the primary file system ensures that data read from the primary file system during data synchronization is always consistent and allows concurrent implementation of data synchronization and data write to the primary file system.
  • The snapshot generated for the secondary file system stores backup data for the data on the secondary file system before synchronization so that data on the secondary file system can still be usable even when an exception occurs during synchronization.
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Updated: 2019-01-02

Document ID: EDOC1000069276

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