Choosing the right data snapshot approach

Are storage array-based snapshots preferable to other types of data snapshots (file system, hypervisor, etc.)? Why or why not?

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As with many things in IT, there are both advantages and disadvantages to creating storage-level data snapshots. The main advantage is that storage-level snapshots do not tend to incur the performance impact of software snapshots. Software-based data snapshots are often (but not always) based on the use of differencing disks. This approach can have a significant impact on read performance as multiple snapshots are accumulated. Hardware-level data snapshots on the other hand, are oftentimes based on the use of pointers, which offer better performance.

The main advantage offered by software-level snapshots is consistency. File-system-level data snapshots are almost always file-system consistent. This means that a snapshot will never contain a partially written file. In contrast, storage-level snapshots usually occur at the block level, and it is sometimes possible to take a storage-level snapshot while a file update is in progress, thereby resulting in a corrupt file (if the snapshot is ever rolled back).

Similarly, hypervisors try to provide application awareness. This means that when a hypervisor-level data snapshot occurs, the hypervisor attempts to create the snapshot in a way that places any applications running on virtual machines into a consistent state before the snapshot is created. Storage-level snapshots are generally not application-aware, so snapshots will likely be created with applications in an inconsistent state.

This was first published in January 2014

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