A snapshot is a fully usable copy of a specified data set, which contains the image of the corresponding data at a specific point in time (the time when the copy starts). A snapshot can be either a duplicate or a replica of the data it represents.

In Sangfor's snapshot function, either Disk Based Snapshot or Storage Based Snapshot (Recommended) can be used:

Disk Based Snapshot

QCOW2 snapshots adopt Copy-on-Write (CoW) technology, with a relatively simple implementation principle. During snapshot creation, a copy of the index of the original disk is generated, and the actual data it points to is all from the original disk. Only when the data of the original image changes will the data of the changed areas (before the changes occur) be copied to the corresponding location of the snapshot. Therefore, the snapshot size is small when initially created and will gradually increase over time.

Storage Based Snapshot (Recommended)

Storage Based Snapshots use Redirect-on-Write (RoW). When creating a snapshot for a virtual machine (VM), the system sets all virtual disks to read-only and generates a corresponding number of new disk files (i.e., snapshot space). After the snapshot is created, all new data of the VM and modifications to the source data will be written to the newly generated snapshot space, and the corresponding relationship between the logical addresses of the source virtual disks and the snapshot space will be recorded in the mapping table.

Consistency Group Snapshot

VMs of Oracle-RAC or multiple VMs of a single service can be added to a consistency group to implement consistent snapshots for the entire group of VMs. This ensures that the snapshot data of multiple VMs is generated at the same point in time, guaranteeing normal business availability after recovery using the snapshots. Multiple VMs can be added to a consistency group; when a snapshot operation is performed on the consistency group, the data of all VMs in the group is captured at the same time point, ensuring business availability during subsequent snapshot rollback and snapshot cloning.

Snapshot Policy

Scheduled snapshot policies can be created for business VMs to automatically generate snapshots (e.g., snapshots for VMs at frequencies such as hourly, daily, or weekly). Additionally, snapshot retention policies are supported: snapshots exceeding the retention period will be automatically deleted to free up storage space. By configuring snapshot policies, regular protection of VMs can be achieved. In the event of logical errors in VMs (e.g., virus infections), snapshots can be used for rollback to restore the VMs and minimize losses.