A data storage system adapted to maintain redundant data storage sets at a destination
location(s) is disclosed. The data storage system establishes a copy set comprising
a source volume and a destination volume. Data written to a source volume is automatically
copied to the destination volume. The data storage system maintains a data log
that may be activated when the destination volume is inaccessible due to, for example,
a malfunction in the destination storage system or in the communication link between
the source system and the destination system. I/O commands and the data associated
with those commands are written to the data log, and after a destination system
becomes available the information in the data log is merged into the destination
volume to conform the data in the destination volume to the data in the source
volume. The data log competes for disk capacity with other volumes on the system,
and log memory is allocated as needed. Command entries and data entries are stored
in distinct areas of the data log, and the command entries include information
identifying the memory offset of the associated data entries.
