Table 6-9 shows the relationship between the  rate value  and the  data copied per second . Rate attribute value:  The rate attribute is configured on each volume that you want to  mirror. The default value of a new volume mirror is 50%. In large, IBM FS9100 system configurations, the settings of the copy rate can considerably  affect the performance in scenarios where a back-end storage failure occurs. For instance,  consider a scenario where a failure of a back-end storage controller is affecting one copy of  300 mirrored volumes. The host continues the operations by using the remaining copy.  When the failed controller comes back online, the resynchronization process for all the 300  mirrored volumes starts at the same time. With a copy rate of 100 for each volume, this  process would add a theoretical workload of 18.75 GB/s, which will considerably overload  the system.  The general suggestion for the copy rate settings is then to evaluate the impact of massive  resynchronization and set the parameter accordingly. Consider setting the copy rate to high  values for initial synchronization only, and with a limited number of volumes at a time.  Alternatively, consider defining a volume provisioning process that allows the safe creation of  already synchronized mirrored volumes, as described in 6.5.4, “Volume Mirroring  synchronization options” on page 236. A mirrored volume has pointers to the two copies of data, usually in different storage pools,  and each write completes on both copies before the host receives I/O completion status. For  a synchronized mirrored volume, if a write I/O to a copy has failed or a long timeout has  expired, then system has completed all available controller level Error Recovery Procedures  (ERPs). In this case, that copy is taken offline and goes out of sync. The volume remains  online and continues to service I/O requests from the remaining copy. Volume mirroring I/O Time-out configuration Table 6-9 Relationship between the rate value and the data copied per second User-specified rate attribute value per volume Data copied/sec 00 Synchronization is disabled 01 - 10 128 KB 11 - 20 256 KB 21 - 30 512 KB 31 - 40 001 MB 41 - 50 002 MB  ** 50% is the default value 51 - 60 004 MB 61 - 70 008 MB 71 - 80 016 MB 81 - 90 032 MB 91 - 100 064 MB Chapter 6. Copy services  237