Backup in a snap: A guide to snapshot technologies


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Snapshots with server virtualization hypervisors

The ascendancy of server virtualization has made hypervisor-based snapshot technology progressively more popular. This technology is available with virtualization software such as Citrix Systems Inc.'s XenServer, Microsoft's Hyper-V, Sun's xVM Ops Center, and VMware's ESX and vSphere4.

The advantages of using hypervisor-based snapshots are straightforward. The technology comes bundled with the hypervisor; it provides the same snapshot methodology for all virtual machines (VMs); it's integrated with Microsoft's VSS; and it's easy to implement, use and manage.

What's not to like about this approach? Snapshots must be managed separately for each hypervisor, and when snapshots are used for any OS other than Windows, only the entire VM will be imaged. That means restores are coarse grain and time consuming, and the snapshots aren't structured-data-aware outside of Windows and may produce non-consistent images.

Snapshots with SQL databases

In SQL databases, snapshotting is called "snapshot isolation." Snapshot isolation is required for databases such as Oracle and PostgreSQL to guarantee that all transactions are serializable and appear to be isolated and serially executed. Other SQL databases also support snapshot isolation but don't require it for serialization. In general, the SQL databases backup features take advantage of snapshot isolation to provide crash consistent dumps

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of tables.

The main advantage of using SQL database snapshot technology is that snapshots of the database, and any applications based on the database, will be crash consistent.

But there are some significant disadvantages. The snapshot technology is very limited and it only works with that particular database and the apps tied to it. It doesn't work with the file system, any other application on the server, or with other databases or servers. So you'll need other snapshot technologies or data protection, thus complicating operation and management.

This was first published in October 2009

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