Snapshot products

In these pages, Snapshot refers to either general hardware products which can take an 'instantaneous' copy of a disk, filesystem or file or a copy of source data. 'Source' is the original data. 'T0' or time zero refers to the time that a snapshot is taken, and then exists as an independent entity. The snapshot could be fixed at that point, or could be updated independently of the source data. Why would you need a snapshot? Some reasons are -

What is snapshot?

When storage subsystems started to use virtualisation, then they were able to take 'instant' snapshot copies of disks, or even files. Exactly how this works depends on the way the virtualisation is implemented.

A data file (or a NAS share or a Container) is split into blocks, which can be scattered over several physical disks, and the disk subsystem maintains an index of pointers to each block. When you make a snapshot copy of a file, the subsystem simply creates another pointer index, which points to the same file blocks. At this stage, which ever way you read the data, you might use uses different pointers but you get to the same underlying data blocks. The file blocks themselves are not copied so this is a very fast operation.
Initially, the original index and the copied index will be pointing to the same file blocks. What happens next depends on the way the virtualisation is implemented. Some different implementations are :

snapshot copy illustrated

Many variants of instant copy and snapshots exist, ranging from Windows ShadowImage and VMware snapshot to snapshot embedded in storage hardware. Selected hardware snapshot products include -

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Snapshot Backups

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