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HyperReplication Feature Guide for Block

OceanStor V5 Series V500R007

This document is applicable to OceanStor 5110 V5, 5110F V5, 5300 V5, 5300F V5, 5500 V5, 5500F V5, 5600 V5, 5600F V5, 5800 V5, 5800F V5, 6800 V5, 6800F V5, 18500 V5, 18500F V5, 18800 V5, and 18800F V5. It describes the working principle and application scenarios of the HyperReplication feature (for block). Also, it explains how to configure and manage the feature.
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Data Replication

Data Replication

Data replication is a process of writing service data generated by hosts to the secondary LUN in the secondary storage system for backup and DR. The writing process varies depending on the remote replication mode. This section describes data replication performed in synchronous and asynchronous remote replication modes.

Writing Process in Synchronous Remote Replication

Synchronous remote replication 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. If data fails to be written to the primary LUN or secondary LUN, the primary LUN or secondary LUN returns a write I/O failure to the remote replication management module. Then, the remote replication management module changes the mode from dual-write to single-write, and the remote replication is interrupted.

After a synchronous remote replication pair is created between the primary LUN and the secondary LUN, you need to manually perform synchronization so that data on the two LUNs is consistent. Every time a host writes data to the primary storage system after synchronization, the data is copied from the primary LUN to the secondary LUN of the secondary storage system in real time. Figure 1-2 shows how synchronous remote replication processes a write request (N: data block being written).

Figure 1-2 Writing process in synchronous remote replication mode

RM: remote replication management module

LOG: data write log

DCL: data change log

NOTE:

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

  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. After receiving the response from the primary cache and secondary cache, the storage system returns the write I/O result to the host. 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. The RM then changes the mode from dual-write to single-write and the remote replication is interrupted.
  6. The storage system determines whether dual-write succeeds.
    • If data has been successfully written to both the primary cache and secondary cache, the LOG is deleted.
    • 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

Asynchronous remote replication 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 successfully writes data to a primary LUN, the primary storage system returns a response to the host declaring the successful write.
  • Data synchronization is triggered manually or automatically at preset intervals to ensure data consistency between the primary and secondary LUNs.

When an asynchronous remote replication relationship is set up between primary and secondary LUNs, the initial synchronization begins. The initial synchronization copies all data from the primary LUN to the secondary LUN to ensure data consistency.

When a host sends a write I/O to the primary storage system, the primary storage system sends the write I/O to the primary LUN. If the write is successful, a response message will be sent from the primary LUN to the primary storage system and then to the host. At preset synchronization intervals, new data is copied from the primary LUN to the secondary LUN. Figure 1-3 shows the writing process in asynchronous remote replication mode (N: data block being written).

Figure 1-3 Writing process in asynchronous remote replication mode

RM: remote replication management module

DCL: data change log

NOTE:

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

When synchronization is started (synchronization is manually started or automatically triggered when the synchronization period reaches), snapshots of the primary and secondary LUNs are generated and activated. The functions of snapshots are as follows:

  • Primary LUN snapshot

    Ensures that data read from the primary LUN during data synchronization is always consistent and allows simultaneous implementation of data synchronization and data write to the primary LUN.

  • Secondary LUN snapshot

    Stores backup data for the data on the secondary LUN before synchronization so that data on the secondary LUN is still usable even if a failure occurs during synchronization.

    The snapshot technology is used only during data synchronization. After data synchronization is complete, the snapshot function stops to reduce 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 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 based on the differences recorded in the DCL.

    Asynchronous remote replication employs multi-time-point caching technology to replicate data from cache to the DR site. The working principle is as follows:

    NOTE:

    Time segment indicates a logical space in cache that manages new data received during a specific period.

    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 to 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. Data in the caches of the primary and secondary LUNs is written to LUNs.
  6. The RM deletes 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: 2019-07-11

Document ID: EDOC1000181492

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