Patent ID: 7188222

Claim:
A method for forming a consistency group of data, comprising: receiving a command to form a consistency group with respect to data received at a first storage site that is mirrored to a second storage site; providing a first data structure indicating updates to the first storage site not included in the consistency group that are received after the command; providing a second data structure indicating updates to the first storage site in the consistency group to be formed; transmitting a command to cause data copied to the second storage site that is part of the consistency group to be copied to a third storage site; receiving indication when the data in the second storage site that is part of the consistency group is copied to the third storage site; maintaining a flag indicating which of the first and second data structures to use to indicate updates received after the command to form the consistency group; and toggling the flag to indicate the first or second data structure not currently indicated by the flag, wherein the flag identifies the first or second data structure that indicates updates to the first storage site not included in the consistency group.