Introduction to FlashCopy Logical Drives

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A FlashCopy logical drive is a point-in-time image of a logical drive. 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 logical drive from which you are basing the FlashCopy, called the base logical drive , must be a standard logical drive in your storage subsystem. Typically, you create a FlashCopy so that an application, for example a backup application, can access the FlashCopy and read the data while the base logical drive remains online and user-accessible. When the backup completes, the FlashCopy logical drive is no longer needed.

You can also create several FlashCopys of a base logical drive and write data to the FlashCopy logical drives in order to perform testing and analysis. Before upgrading your database management system, for example, you can use FlashCopy logical drives 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 FlashCopy, the controller suspends I/O to the base logical drive for a few seconds while it creates a physical logical drive called the FlashCopy repository logical drive to store FlashCopy metadata and copy-on-write data. When the controller is finished creating the FlashCopy repository logical drive, I/O write requests to the base logical drive can continue. However, before a data block on the base logical drive is modified, a copy-on-write occurs, copying the contents of blocks that are to be modified into the FlashCopy repository logical drive for safekeeping. Since the FlashCopy repository logical drive stores copies of the original data in those data blocks, further changes to those data blocks write directly to the base logical drive without another copy-on-write. And, since the only data blocks that are physically stored in the FlashCopy repository logical drive are those that have changed since the time of the FlashCopy, the FlashCopy technology uses less disk space than a full physical copy.

When you create a FlashCopy logical drive, you specify where to create the FlashCopy repository logical drive, its capacity, and other parameters. You can disable the FlashCopy when you are finished with it, for example after a backup completes. Then, you can re-create the FlashCopy the next time you do a backup and reuse the same FlashCopy repository logical drive. Using the Disable FlashCopy and Re-create FlashCopy pull-down menu options provides a shortcut to creating a new FlashCopy logical drive of a particular base logical drive because you do not need to create a new FlashCopy repository logical drive. You can also delete a FlashCopy logical drive, which also deletes the associated FlashCopy repository logical drive.

The storage management software provides a warning message when your FlashCopy repository logical drive nears a user-specified threshold (a percentage of its full capacity, the default is 50%). When this condition occurs, you can use the storage management software to expand the capacity of your FlashCopy repository logical drive from free capacity on the array. If you are out of free capacity on the array, you can even add unconfigured capacity to the array in order to expand the FlashCopy repository logical drive.

Related Topics

Creating a FlashCopy Logical Drive Using the FlashCopy Wizard

Disabling a FlashCopy Logical Drive

Re-creating a FlashCopy Logical Drive

Learn About Disabling and Re-creating a FlashCopy Logical Drive

Learn About FlashCopy Logical Drive Maintenance

Increasing the Capacity of a FlashCopy Repository Logical Drive