FlashCopy function

The FlashCopy function creates a point-in-time copy of data that is stored on a source volume to a target volume. In its basic mode, the FlashCopy function creates copies of content on a source volume to a target volume in a mapping. The function associates a source volume and a target volume in a mapping. If data exists on the target volume, that data is replaced by the copied data.

A FlashCopy mapping defines the relationship that copies data between a source volume and a target volume. A mapping can be created between any two volumes in a system. The volumes do not have to be in the same I/O group or pool. When a FlashCopy operation starts, a checkpoint is made of the source volume. No data is copied at the time a start operation occurs. Instead, the checkpoint creates a bitmap that indicates that no part of the source volume has been copied. Each bit in the bitmap represents one region of the source volume. Each region is called a grain. During a read operation to the target volume, the bitmap is used to determine whether the grain was copied. If the grain was copied, the data is read from the target volume. If the grain was not copied, the data is read from the source volume.

After a FlashCopy operation starts, read and write operations to the source volume continue to occur. If new data is written to the source or target volume, the existing data on the source is copied to the target volume before the new data is written to the source or target volume. The bitmap is updated to mark that the grain of the source volume has been copied so that later write operations to the same grain do not recopy the data.

After the copy operation has completed, the target volumes contain the contents of the source volumes as they existed at a single point in time, unless target writes have been processed. FlashCopy is sometimes described as an instance of a time-zero copy (T 0) or point-in-time copy technology. Although the copy operation takes some time to complete, the resulting data on the target volume is presented so that the copy appears to have occurred immediately, and all data is available immediately. (If needed, data that is still in the process of being copied can be accessed from the source.) The system can support up to 2048 volumes, but the maximum number of target volumes for the system is 64.

Although it is difficult to make a consistent copy of a data set that is constantly updated, point-in-time copy techniques help solve this problem. If a copy of a data set is created using a technology that does not provide point-in-time techniques and the data set changes during the copy operation, the resulting copy might contain data that is not consistent. For example, if a reference to an object is copied earlier than the object itself and the object is moved before it is copied, the copy contains the referenced object at its new location, but the copied reference still points to the previous location. You can also assign background copy and cleaning rates to a FlashCopy mapping to control the rate at which updates are propagated to the remote system. FlashCopy mapping copy rate values can be from 128 KBps to 2 GBps and can be changed when the FlashCopy mapping is in any state.

More advanced functions allow operations to occur on multiple source and target volumes. Management operations are coordinated to provide a common, single point-in-time for copying target volumes from their respective source volumes. This creates a consistent copy of data that spans multiple volumes. The function also supports multiple target volumes to be copied from each source volume. This can be used to create images from different points in time for each source volume.