Impact of Extra Space on Mirroring

Figure 1(b) shows how the components of the average read cost respond to different degrees of replication ($D_m$) in a mirrored system under a random read workload. (In this case, $D_d = D_s = 1$.) The read overhead is lower than the write overhead, because it takes longer for the disk head to settle when servicing a write request. Note that mirroring reduces both the seek and rotational delays of read requests. These components, however, remain significant if mirroring is the only technique employed.