Introduction to Snapshot Virtual Disks

Caution: Risk of application errors - You cannot create a snapshot for a containing unreadable sectors.

A snapshot virtual disk is a point-in-time image of a virtual disk. It is the logical equivalent of a complete physical copy, but you create it much more quickly than a physical copy, and it requires less disk space. In this release of the storage management software, the virtual disk from which you are basing the snapshot, called the , must be a in your . Typically, you create a snapshot so that an application; for example, a backup application, can access the snapshot and read the data while the source virtual disk remains online and is user accessible. When the backup completes, the snapshot virtual disk is no longer needed.

You can also create several snapshots of a source virtual disk and write data to the snapshot virtual disks to perform testing and analysis. Before upgrading your database management system, for example, you can use snapshot virtual disks to test different configurations. Then you can use the performance data provided by the storage management software to help you decide how to configure your live database system.

When you take a snapshot, the suspends I/O to the source virtual disk for a few seconds while it creates a physical virtual disk called the to store snapshot metadata and data. When the RAID controller module is finished creating the snapshot repository virtual disk, I/O write requests to the source virtual disk can continue. However, before a data block on the source virtual disk is modified, a copy-on-write occurs, copying the contents of blocks that are to be modified into the snapshot repository virtual disk for safekeeping. Because the snapshot repository virtual disk stores copies of the original data in those data blocks, further changes to those data blocks write directly to the source virtual disk without another copy-on-write. And, because the only data blocks that are physically stored in the snapshot repository virtual disk are those that have changed since the time of the snapshot, the snapshot technology uses less disk space than a full physical copy.

When you create a snapshot virtual disk, you specify where to create the snapshot repository virtual disk, its capacity, and other parameters. You can disable the snapshot when you are finished with it; for example, after a backup completes. Then, you can re-create the snapshot the next time you perform a backup and reuse the same snapshot repository virtual disk. Selecting Disable Snapshot >> Re-create Snapshots provides a shortcut to creating a new snapshot virtual disk of a particular source virtual disk because you do not need to create a new snapshot repository virtual disk. You can also delete a snapshot virtual disk, which also deletes the associated snapshot repository virtual disk.

The storage management software provides a warning message when your snapshot repository virtual disk nears a user-specified threshold (a percentage of its full capacity, the default is 50 percent). When this condition occurs, you can use the storage management software to expand the capacity of your snapshot repository virtual disk from free capacity on the . If you are out of free capacity on the disk group, you can even add unconfigured capacity to the disk group to expand the snapshot repository virtual disk.

Introduction to Snapshot Virtual Disks

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