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OceanStor 9000 V300R006C00 File System Feature Guide 12

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What Are the Ways to Implement Snapshot? What Are Differences Between Them? How Does the OceanStor 9000 Implement Snapshot?


What are the ways to implement snapshot? What are differences between them? How does the OceanStor 9000 implement snapshot?


Copy On Write (COW) and Redirect On Write (ROW) are the two ways to implement snapshot.

COW copies data objects protected by a snapshot to other locations or objects before replacing these data objects with new data, so that the original data objects serve as snapshot data.

ROW writes new data to new locations or objects, instead of replacing the original data protected by a snapshot.

The OceanStor 9000 implements the snapshot function based on metadata COW and data ROW.

What Are the Ways to Create a Snapshot?


What are the ways to create a snapshot?


A snapshot can be created in either of the following ways:
  • Manually create a snapshot.
  • Create a timing snapshot policy so that snapshots are automatically created.

Is the Snapshot Function Oriented to Files or Directories? What Are the Restrictions on Using It?


Is the snapshot function oriented to files or directories? What are the restrictions on using it?


The OceanStor 9000 provides a directory-level snapshot function.

The restrictions on using this function are as follows:
  • Snapshot nesting is not supported. If snapshots have been created for a directory, no snapshots can be created for the parent directory and subdirectories of the directory.
  • To delete a snapshot root directory, you need to delete the snapshot of the directory first.
  • Hard links cannot be created for files in snapshots.
  • Hard links are not protected by snapshots. Source files of hard links are protected by snapshots. The hard link in a snapshot root directory cannot be deleted, moved, or renamed. Source files that are protected by snapshots and have hard links cannot be deleted.
  • InfoMigrator cannot synchronize the snapshot directory (.snapshot) and snapshot files in the directory.
  • Creating a snapshot for the shared root directory is not supported.

What Is the Maximum Number of Snapshots That Can Be Created for a Single Directory? What Is the Maximum Number of Snapshots Supported by the Entire System?


What is the maximum number of snapshots that can be created for a single directory? What is the maximum number of snapshots supported by the entire system?


The OceanStor 9000 allows a maximum of 2048 snapshots to be created for one directory, including those that a user has run a deletion command to delete but are not deleted. The OceanStor 9000 supports a maximum of 8192 snapshots. However, you are advised to create 4096 snapshots at most on the OceanStor 9000.

Excessive snapshots occupy large storage space and reduce system response speed. To save storage space and optimize system performance, you are advised to periodically check the snapshot count and delete useless snapshots.

You can run the CLI command show snapshot count to query the snapshot count.

What Are the Types of Timing Snapshot Policies?


What are the types of timing snapshot policies?


Timing snapshot policies include schedule and snapshot expiration time.
  • Based on the specified timing snapshot policies, the system automatically creates snapshots. A time policy can be configured by month, week, or day.
  • You can specify a specific snapshot expiration time or select Never Expire. If a snapshot expiration time is selected, snapshots are automatically deleted at the specified snapshot expiration time.

How Do I Delete Snapshots and Set a Snapshot Expiration Time?


How do I delete snapshots and set a snapshot expiration time?


In DeviceManager, choose Data Protection > Snapshot > Snapshot. Then, you can delete snapshots. If a snapshot expiration time has been set for a snapshot, the system automatically deletes it at the specified time and reclaims the storage space occupied by the snapshot data.

When manually creating a snapshot or timing snapshot policy, you can set a snapshot expiration time or select Never Expire. You can also change the snapshot expiration time of an existing snapshot. After you change the snapshot expiration time in a timing snapshot policy, snapshots generated subsequently will expire at the new expiration time.

How Do I Query the Storage Space Occupied by Snapshots?


How do I query the storage space occupied by snapshots?


In DeviceManager, choose Data Protection > Snapshot. On the Snapshot page, view Snapshot Capacity of existing snapshots.

Is There an Upper Limit on the Storage Space Occupied by Snapshots?


Is there an upper limit on the storage space occupied by snapshots?


The OceanStor 9000 adopts a single file system. Snapshots can use all the storage space of the system. Snapshot space is not restricted. Users cannot set an upper limit.

How Do I Use a Snapshot to Roll Back Data?


How do I use a snapshot to roll back data?


DeviceManager does not provide a snapshot rollback function.

If you want to use a snapshot to roll back data, you can use the Volume Shadow Copy Service (VSS) or Shadow Copy function provided by Windows, as shown in Figure 5-13. Alternatively, manually copy snapshot data to recover source data.

Figure 5-13  Rolling back data using VSS

Updated: 2019-06-27

Document ID: EDOC1000122519

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