Split Mirror SnapshotĪ split-mirror snapshot creates a full copy of the original storage volume instead of only snapshotting the modified blocks. If the snapshot is deleted, the coordination between multiple new blocks and the original block becomes complicated.
The old data remains in place as a point-in-time reference to the original block.Īdvantages: In contrast to copy-on-write, read-on-write snapshots consume fewer performance resources because each modified block generates a single write IO.ĭisadvantage: redirect-on-write snapshots rely on the original block. The snapshot utility redirects writes to the new block and updates the relevant pointers. The system executes write commands to make changes to snapshot-protected blocks. Here's how a read-write snapshot is created: Redirect-on-write snapshots use pointers to reference snapshot-protected blocks. The new data overwrites the original data.Īdvantages: Since copy-on-write snapshots do not create a copy of the metadata, they are faster and almost instant.ĭisadvantages: However, they are performance intensive as each snapshot requires one read and two writes. Create/write snapshots of original blocks on reserved snapshot storage. 
Snapshot utilities read raw blocks before writing. 
When the system executes a write command to a protected block, three IOs are triggered:. Before creating a snapshot, the system stores the metadata of the original block. Here's how a copy-on-write snapshot is created: The implementation of storage snapshot technology varies by vendor. Snapshots can also be used to restore the system to a previous state if the wrong patch is installed. Storage snapshots can be used for disaster recovery (DR) when information is lost due to human error. Snapshots are usually created for data protection, but they can also be used for testing application software and data mining. Each snapshot taken creates another branch of the tree. Snapshots have a parent-child relationship and form a tree. The file system continues to behave as if it were on a physical machine. The filesystem is completely unaware of the existence of diff disks. All future writes are directed to the differencing disk, leaving the original virtual hard disk unchanged. 
When an administrator creates a storage snapshot, the underlying system creates a differencing disk bound to the original virtual hard disk. A differencing disk is a special type of virtual hard disk that is linked to a parent virtual hard disk. Storage snapshots are usually based on the use of differencing disk. Now, you can continue to read this post from MiniTool to get more information about the snapshot. A snapshot is like a detailed catalog, providing users with an accessible copy of the data they can roll back to. A storage snapshot is a set of reference markers for data at a specific point in time.
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