the present invention as the write log 26 may grow large it is preferably offloaded to secondary storage devices such as tape drives to free up disk space to log more changes to the data set 24 as such the disk array 17 fig3 c is used as a write log cache in a three step process 1 when the host needs to write data to a disk rather than writing to the final destination in a disk drive that data is first written to the log 26 satisfying the host 2 then when the disk drive is not busy that data from the log 26 is transferred to the final destination data set on the disk drive transparent to the host and 3 the log data is backed up to e g tape to free up storage space to log new data from the host the log and the final destination data are maintained in a hybrid raid configuration as described referring to the example steps in the flowchart of fig4 b upon receiving a host read request step 120 a determination is made if the requested data is in the write log 26 maintained as a cache in the mirror area 20 i e cache hit step 122 and if so the requested data is transferred to the host 20 from the log 26 step 124 statistically since recently written data is more likely to be read back than previously written data there is a tradeoff such that the larger the log area the higher the probability that the requested data is in the log 26 in the mirror area 20 when reading multiple blocks from the mirror area 20 different blocks can be read from different disk drives.