The process of resynchronization depends on the type of volume. RAID-5 volumes that contain RAID-5 logs can "replay" those logs. If no logs are available, the volume is placed in reconstruct-recovery mode and all parity is regenerated. For mirrored volumes, resynchronization is done by placing the volume in recovery mode (also called read-writeback recovery mode). Resynchronization of data in the volume is done in the background. This allows the volume to be available for use while recovery is taking place.
The process of resynchronization can impact system performance. The recovery process reduces some of this impact by spreading the recoveries to avoid stressing a specific disk or controller.
For large volumes or for a large number of volumes, the resynchronization process can take time. These effects can be addressed by using dirty region logging (DRL) and FastResync (fast mirror resynchronization) for mirrored volumes, or by ensuring that RAID-5 volumes have valid RAID-5 logs.
See "Dirty region logging" on page 58.
See "FastResync" on page 65.
For raw volumes used by database applications, the SmartSync feature can be used if this is supported by the database vendor.
See "SmartSync recovery accelerator" on page 60.