Patent ID: 7200603

Claim:
A method of operating a data storage system configured to perform data mirroring, the method comprising: receiving a set of data, the set of data having a first number of subsets; defining a compression group corresponding to the set of data, the compression group having a plurality of entries, each entry containing a pointer to a corresponding one of the subsets; compressing the set of data so that the set of data occupies a smaller number of the subsets than the first number; for each of the subsets which does not contain compressed data after said compressing, storing a predetermined value in the corresponding entry of the compression group, the predetermined value being indicative that corresponding data is compressed; and during a logical mirroring operation, sending the set of data in its entirety from said data storage system to a remote data storage system at a mirror site, for use in a mirror copy of the set of data, if the set of data has been compressed and any one or more of the subsets of the set of data has been modified since a prior logical mirroring operation, to enable the remote data storage system to update the mirror copy of the set of data.