Snapshot Storage Usage Calculation

The total snapshot storage used by an EVS disk is calculated by snapshot chain. A snapshot chain collects the storage space used by data blocks of all the snapshots of a disk.

• Snapshot chain's storage usage calculation after snapshots are added
  Figure 1 Snapshot chain with snapshots added
  

  Take the scenario in Figure 1 as an example. Assume that the size of a snapshot's data block is fixed at 2 MiB. The snapshot chain's storage usage is calculated as follows:

  • After snapshot 1 is created, the snapshot chain of the disk contains only one snapshot. Snapshot chain's storage usage = Snapshot 1's storage usage = Size of data block A + Size of data block B + Size of data block C = 6 MiB
  • After snapshot 2 is created, the snapshot chain of the disk contains two snapshots: snapshot 1 and snapshot 2. Snapshot chain's storage usage = Snapshot 1's storage usage + Snapshot 2' storage usage = 6 MiB + (Size of data block A1 + Size of data block B1 + Size of data block D) = 12 MiB
  • After snapshot 3 is created, the snapshot chain of the disk contains three snapshots: snapshot 1, snapshot 2, and snapshot 3. Snapshot chain's storage usage = Snapshot 1's storage usage + Snapshot 2' storage usage + Snapshot 3's storage usage = 6 MiB + 6 MiB + (Size of data block A2 + Size of data block C1 + Size of data block E) = 18 MiB
• Snapshot chain's storage usage calculation after snapshots are deleted

  When a snapshot is deleted, all data block information in this snapshot's metadata file is traversed, and the following deletion rules are applied:

  • If a data block is inherited by the next snapshot, it will not be deleted.
  • If a data block is not inherited by the next snapshot:
    • For an inherited data block, if the previous snapshot that the data block is inherited from is not deleted, the data block will not be deleted. Otherwise, it will be deleted.
    • For a modified data block, it will be deleted.
    • For a new data block, it will be deleted.
  Figure 2 Snapshot data block deletion rules
  

  The following example describes how to calculate a snapshot chain's storage usage after snapshots are deleted.

  Figure 3 Snapshot chain with snapshots deleted
  

  Take the scenario in Figure 3 as an example. Assume that snapshot 2 is deleted at 14:00 and snapshot 3 is deleted at 15:00. The snapshot chain's storage usage is calculated as follows:

  • Before any snapshot is deleted, the snapshot chain's storage usage is 18 MiB (Snapshot 1's storage usage + Snapshot 2's storage usage + Snapshot 3's storage usage).
  • When snapshot 2 is deleted at 14:00, information about all data blocks in the metadata file of snapshot 2 is traversed.
    • Data block A1: It is not inherited by snapshot 3 and is modified from data block A of snapshot 1. So, data block A1 will be deleted.
    • Data block B1: It is inherited by snapshot 3, so it will not be deleted.
    • Data block C: It is not inherited by snapshot 3, but is inherited from snapshot 1 and snapshot 1 is not deleted. So, data block C will not be deleted.
    • Data block D: It is inherited by snapshot 3. So, it will not be deleted.

    After snapshot 2 is deleted, the snapshot chain's storage usage is 16 MiB (18 MiB – Size of data block A1).

  • When snapshot 3 is deleted at 15:00, information about all data blocks in the metadata file of snapshot 3 is traversed.
    • Data block A2: It is not inherited by the next snapshot and is modified from data block A1 of snapshot 2. So, data block A2 will be deleted.
    • Data block B1: It is not inherited by the next snapshot, but is inherited from snapshot 2 and snapshot 2 has been deleted. So, data block B1 will be deleted.
    • Data block C1: It is not inherited by the next snapshot and is modified from data block C of snapshot 2. So, data block C1 will be deleted.
    • Data block D: It is not inherited by the next snapshot, but is inherited from snapshot 2 and snapshot 2 has been deleted. So, data block D will be deleted.
    • Data block E: It is not inherited by the next snapshot and is newly added in snapshot 3. So, data block E will be deleted.

    After snapshot 3 is deleted, the snapshot chain's storage usage is 6 MiB (16 MiB – Size of data block A2 – Size of data block B1 – Size of data block C1 – Size of data block D – Size of data block E).

The total snapshot storage used by an EVS disk is calculated by snapshot chain. A snapshot chain collects the storage space used by data blocks of all the snapshots of a disk.

1. Log in to the consoleconsole.
2. Click in the upper left corner and choose Storage > Elastic Volume Service.

   The Elastic Volume Service page is displayed.

3. Locate the disk that you want to check their total snapshot usage and click to copy the disk ID.
4. In the navigation pane on the left, choose Elastic Volume Service > Snapshots.

   The Snapshots page is displayed.

5. Click the Snapshot Chains tab.
6. In the search box above the list, select Disk ID, paste the copied disk ID, and click .
7. View the capacity displayed in the Snapshot Space Usage column.
8. (Optional) Click the number displayed in the Snapshots column to view all the snapshots in the snapshot chain.

Perform the following operations to view the total snapshot storage usage in a specified period in the current region.

1. Log in to the consoleconsole.
2. Click in the upper left corner and choose Storage > Elastic Volume Service.
3. In the navigation pane on the left, choose Elastic Volume Service > Snapshots.

   The Snapshots page is displayed.

4. Click the Snapshot Space Usages tab.
5. View the snapshot storage usage and snapshot quantity on the top of the page.
6. Specify a time range for the query (minimum interval: 1 hour). You can also query the snapshot storage usage by the last 1 day, last 7 days, last 15 days, or last 30 days.
   Figure 4 Querying the total snapshot storage usage in a specified period