Described are techniques for use with online data copying or migration.
Data is pushed from a source to a target device while one or both of the
source and target devices are available for use by one or more host
systems. The host systems may perform I/O operations to the source and/or
target devices while the data migration is ongoing if a copy session is
defined on the data storage system(s) of the source and/or target
devices. A copying operation performs data migration from the source to
the target device. A migration vector is used to indicate which portions
of the source have been migrated. A change vector may be associated with
each of the source and/or target devices to indicate portions that have
been modified in order to perform a differential copy operation.