1.  Technical Field
This application generally relates to a data storage system, and more particularly to techniques used with copying data. 
2.  Description of Related Art
Computer systems may include different resources used by one or more host processors.  Resources and host processors in a computer system may be interconnected by one or more communication connections.  These resources may include, for example, data storage devices such as those included in the data storage systems manufactured by EMC Corporation.  These data storage systems may be coupled to one or more host processors and provide storage services to each host processor.  Multiple data storage systems from one or more different vendors may be connected and may provide common data storage for one or more host processors in a computer system. 
A host processor may perform a variety of data processing tasks and operations using the data storage system.  For example, a host processor may perform basic system I/O operations in connection with data requests, such as data read and write operations. 
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units.  Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat.  No.  5,206,939 to Yanai et al.,  U.S. Pat.  No.  5,778,394 to Galtzur et al.,  U.S. Pat.  No.  5,845,147 to Vishlitzky et al.,  and U.S. Pat.  No.  5,857,208 to Ofek.  The host systems access the storage device through a plurality of channels provided therewith.  Host systems provide data and access control information through the channels to the storage device and storage device provides data to the host systems also through the channels.  The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units.  The logical disk units may or may not correspond to the actual disk drives.  Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein. 
Different tasks may be performed in connection with data stored on one or more data storage systems.  A copy of data from a data storage device to another device may be made, for example, as part of a data backup procedure.  Data may also be restored from a first device to a second device in which the first device includes a previously made point in time copy of data from the second device.  Such data restoration may be performed, for example, if the data on the first device has become corrupted.  Making a copy of data from a device, or restoring data from one device to another may be performed offline (without host access to the device).  However, it may be desirable to perform such operations while one or more of the associated devices to which data is being restored, or from which data is being copied, are online (with host access to the device) and available for use by one or more host systems connected thereto.