Alibaba Cloud ECS offers the snapshot service to create snapshots for disks and Shared Block Storage (hereinafter referred to as disks) as scheduled operations. Such operations allow you to retain disk data for one or more specific points in time. Snapshots can guarantee service security while also improving your application deployment efficiency.
- Snapshot 1 copies all data on the disk for the specific point in time that the snapshot is created.
- Snapshot 2 copies only data blocks B1 and C1 that have been modified since snapshot 1. Data blocks A and D are referenced from snapshot 1.
- Snapshot 3 copies only data block B2 that has been modified since snapshot 2. Data blocks A and D are referenced from snapshot 1, and data block C1 is referenced from snapshot 2.
- If you roll back the disk to the state of snapshot 3, the Roll Back Disk feature copies data blocks A, B2, C1, and D to the disk to replicate snapshot 3.
- If you delete snapshot 2, data block B1 is deleted, but data block C1 is retained because it is being referenced in other snapshots. If you roll back a disk to snapshot 3, data block C1 will still be recovered.
- Snapshot capacity: the storage space occupied by each snapshot in the snapshot chain.
Note The snapshot service is billed by snapshot capacity. You can use the snapshot chain to check the snapshot capacity of each disk.
- Snapshot quota: Each disk can have up to 256 manual snapshots and 256 automatic snapshots.
For more information, see Limits.
Note If the snapshot quota has been reached, but more automatic snapshots are required, the earliest automatic snapshots are deleted first to make room for new automatic snapshots. If you want to create manual snapshots, then you must first delete unneeded snapshots. For more information, see Apply or disable an automatic snapshot policy and Delete a snapshot.
- Snapshot node: Each node in the snapshot chain represents a snapshot of the corresponding disk. Each snapshot chain can have up to 512 snapshot nodes made up of 256 manual snapshot nodes and 256 automatic snapshot nodes.
- Alibaba Cloud ECS analyzes all of the data blocks in snapshot S1 to be deleted, and then deletes the data blocks that are not referenced by other snapshots in the chain.
- Alibaba Cloud ECS adds the dirty data blocks of snapshot S1 to snapshot S2. Other
snapshots record the information of 10 data blocks altogether:
- Six data blocks from snapshot S0
- Two dirty data blocks from snapshot S1
- Two data blocks from snapshot S2