How snapshots work

Last updated:2020-06-23 11:32:29

The snapshot feature of EBS 3.0 allows you to protect data by creating snapshots based on EBS volumes. You can create multiple snapshots for an EBS volume at different time points. This helps you ensure service continuity.

A snapshot is a data replica created based on a data source at a specific time point. Snapshots are one of the various methods for data backup. Creating a snapshot is like taking a photo. Both behaviors capture the state of an object in the current moment. Snapshots can be created instantaneously, allowing you to back up data in real time and quickly restore data. This meets corporate requirements for business continuity and data reliability. In addition, you can use one snapshot to create multiple EBS volumes at a time. This helps you quickly build an application environment and improves business deployment efficiency.

Snapshot technology

EBS adopts the Redirect-On-Write (ROW) snapshot technology, which is significantly improved compared with the conventional Copy-On-Write (COW) snapshot technology. The biggest disadvantage of the COW technology is that write operations are poorly supported. When data is written on a source volume for the first time, the data block to be modified must be copied first. This involves a whole process of data block migration that includes data reading and data writing. Consequently, the write operation will only be performed after the preceding process is completed. However, data on the source volume is not changed, which means read operations are not affected. With the ROW technology, all write operations on an EBS volume will be redirected to a new volume, whereas the source snapshot data is reserved on the read-only source volume. In this way, the process of updating data on the source volume only includes data writing. EBS takes full advantage of the ROW technology and bypasses its disadvantages to improve the read and write performance and ensure business continuity.

Snapshot mechanism

EBS snapshots are incremental backups. A snapshot only records data that has been changed since the last time a snapshot was created for the EBS volume.

Snapshot.jpg

  • For example, Snapshot 1, Snapshot 2, and Snapshot 3 are three snapshots created based on the same EBS volume.
  • Snapshot 1 is the first snapshot created based on the EBS volume. It is a full snapshot that copies all the data on the EBS volume.
  • Snapshot 2 is the second snapshot created based on the EBS volume. It is an incremental snapshot. Snapshot 2 copies only blocks B1 and C1, in which data has been changed since Snapshot 1 was created, from the EBS volume and references blocks A and D, in which data has not been changed, from Snapshot 1.
  • Snapshot 3 is the third snapshot created base on the EBS volume. It is also an incremental snapshot. Snapshot 3 copies only block B2, in which data has been changed since Snapshot 2 was created, and references other blocks from Snapshot 1 and Snapshot 2.
  • To restore data on the EBS volume to the time point when Snapshot 2 was created, the system copies blocks A, B1, C1, and D in Snapshot 2 to the EBS volume.
  • If Snapshot 2 is deleted, Snapshot 1 and Snapshot 3 are not be affected. You can still restore data on the EBS volume to the time point when Snapshot 1 or Snapshot 3 was created.

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