Costs of mirroring

Disk-space costs and performance costs are associated with mirroring. The disk-space cost is due to the additional space required for storing the mirror data. The performance cost results from performing writes to both the primary and mirror chunks. The use of multiple virtual processors for disk writes reduces this performance cost. The use of split reads, whereby the database server reads data from either the primary chunk or the mirror chunk, depending on the location of the data within the chunk, actually causes performance to improve for read-only data. For more information about how the database server performs reads and writes for mirror chunks, see Actions during processing.