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HyperClone 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. This document describes the implementation principles and application scenarios of the clone feature. Also, it explains how to configure and manage clones.

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Working Principle

Working Principle

This section describes the working principles of LUN clone such as synchronization, split, and reverse synchronization.

Concepts

Concept

Description

Primary LUN and secondary LUN

In a LUN clone, the LUN that stores production data is called the primary LUN, and the LUNs that each store a data copy of the primary LUN are called secondary LUNs.

Pair

In a LUN clone, the primary LUN and each of the secondary LUNs form a pair. A pair indicates a mirror relationship between the primary LUN and a secondary LUN.

Each clone has only one primary LUN but can have multiple secondary LUNs. Each time a secondary LUN is added to a clone, the secondary LUN and the primary LUN form a new pair.

NOTE:

For details about the maximum number of secondary LUNs supported by a clone, see the Product Description.

Synchronization

Synchronization is the process of copying data from the primary LUN to a secondary LUN.

Dual write

Dual write is the process of writing host I/Os to both the primary LUN and a secondary LUN.

Split

For a non-immediately available clone, split is an operation that stops the dual write status of a pair to make the secondary LUN store an available point-in-time data copy of the primary LUN.

For an immediately available clone, data synchronization from the primary LUN to the secondary LUN is suspended to reduce the impact on host performance.

Reverse synchronization

Reverse synchronization is the process of copying data from a secondary LUN to the primary LUN.

Incremental copy

Incremental copy is a mechanism that only copies new data or changed data during a synchronization or reverse synchronization.

Copy-on-write (COW)

Core technology for snapshot implementation, which is used to store the changed data of the primary LUN. In the event that a snapshot is created and activated, if an application server attempts to write data to the primary LUN, the storage system copies the to-be-replaced original data (COW data) to the COW space, changes the mapping relationship of the COW data, records the new location of the COW data in the COW space, and then writes the new data to the primary LUN.

Restore on write

During rollback using the snapshot, if the host writes data to the primary LUN/secondary LUN, the data block in the snapshot is copied to the primary LUN/secondary LUN, and then the host continues to write. If no host reads or writes the LUN, data of the snapshot is rolled back to the primary LUN/secondary LUN in sequence.

Read direct-on-time

When the host delivers a read request to the secondary LUN, data on the primary LUN has not been synchronized to the secondary LUN. In this case, the host reads data from the instant snapshot.

In V500R007C60SPC200 and the later vision, the immediately available clone function is added and is different from HyperClone in earlier versions (referred to as the non-immediately available clone). After being created, the immediately available clone can immediately read data from and write data to the secondary LUN without waiting for the completion of the copy. The following describes the principles of the non-immediately available clone and immediately available clone.

Non-immediately Available Clone

Brief Working Principle

The LUN clone feature is typically used to back up data on a primary LUN for possible data restoration in the future and to store a point-in-time data copy of a primary LUN for independent data access. To serve the two major purposes, LUN clone implementation is divided into three phases: synchronization, splitting, and reverse synchronization, as shown in Figure 1-1.

Figure 1-1 LUN clone implementation phases
  1. Synchronization: Data is copied from the primary LUN to a secondary LUN. Then dual write is performed to the primary LUN and secondary LUN.
  2. Splitting: After a synchronization is complete, you can split the pair at a certain point in time. Then, the secondary LUN becomes an available copy of the primary LUN, preserving all the data on the primary LUN at the time when the pair was split.

    After a pair is split, the secondary LUN is accessible to hosts, allowing hosts to access data identical to that stored on the primary LUN at the splitting time point without affecting the performance of the primary LUN. After a pair is split, you can also perform a synchronization or reverse synchronization again between the primary LUN and the secondary LUN.

  3. Reverse synchronization: If you want to restore data on the primary LUN, you can perform a reverse synchronization to copy data from the secondary LUN to the primary LUN. After the reverse synchronization is complete, the pair is automatically split.

During a synchronization or reverse synchronization, hosts are still allowed to access the primary LUN, ensuring service continuity.

Synchronization

Figure 1-2 illustrates the synchronization principle.

Figure 1-2 Synchronization principle

Split

Figure 1-3 illustrates the split principle.

Figure 1-3 Split principle

Reverse Synchronization

Figure 1-4 illustrates the reverse synchronization principle.

Figure 1-4 Reverse synchronization principle

Immediately Available Clone

HyperClone provides a full copy of the primary LUN's data at the synchronization start time. Hosts can read and write the secondary LUN immediately, without waiting for the copy process to complete. The primary and secondary LUNs are physically isolated, and operations on them will not affect each other. Writes to the primary and secondary LUNs are recorded in the differential bitmap, which will be used for incremental synchronization.

Synchronization

When a HyperClone pair starts synchronization, the storage system generates an instant snapshot for the primary LUN, and then synchronizes the snapshot data to the secondary LUN. Any subsequent write operations are recorded in a differential bitmap. When synchronization is performed again, the system checks the differential bitmap and only synchronizes the differential data to the secondary LUN.

The data written to the secondary LUN between two synchronizations will be overwritten. To retain the existing data on the secondary LUN, you can create a snapshot for it before synchronization.

The following figure shows the synchronization principle.

Figure 1-5 Synchronization principle

Split

After the synchronization for the immediately available clone starts, the host can read and write the secondary LUN immediately. If the amount of data to be synchronized is large, the copy will last for a period of time. To reduce the impact of the background copy on the read and write performance of the host, you can split the clone pair and suspend data synchronization. The following figure shows the split principle.

Figure 1-6 Split principle

Read/write principle

The HyperClone running status can be Normal, Synchronizing, Split, Stop, or Interrupted. Read and write I/Os of the primary LUN and secondary LUN are processed differently in different states.

  • In the Normal or Stop status, data of each HyperClone member LUN is independent. The host can directly read, write, and modify the primary or secondary LUN. Differences are recorded for subsequent incremental synchronization.

    The following figure shows the read/write principle when the HyperClone is in the Normal or Stop status.

    Figure 1-7 Read/write principle (Normal or Stop status)
  • Read/write principle when the HyperClone running status is Synchronizing, Split, or Interrupted:
    • Read and write operations on the primary LUN
      • Read: Reads data from the primary LUN directly.
      • Write: The COW technology is used. Before the primary LUN is written, data is copied to the COW space. Then, the data is directly written to the primary LUN and differences are recorded in the differential bitmap.
    • Read and write operations on the secondary LUN
      • Read: The read direct-on-time technology is used. If the requested data is found on the secondary LUN (the data has been synchronized) or the data does not need to be synchronized, the host reads the data from the secondary LUN. If the requested data is not found on the secondary LUN (the data has not been synchronized), the host reads the data from the snapshot of the primary LUN.
      • Write: The restore on write technology is used. If data has been synchronized, the data will be overwritten. If data has not been synchronized, the snapshot data to be written is synchronized to the secondary LUN first. After the synchronization, the snapshot data will be overwritten.
    The following figure shows the read/write principle when the HyperClone running status is Synchronizing, Split, or Interrupted.
    Figure 1-8 Read/write principle (Synchronizing, Split, or Interrupted status)
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Updated: 2020-01-17

Document ID: EDOC1000181501

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