I may be wrong here so forgive me if I misunderstood you… Mirroring is RAID… the common name for RAID-1, which is one of several RAID schemes.
Do you mean replication?
RAID has a number of schemes that offer various levels of protection in different configurations. The number of schemes has increased over time as each offers different pros and cons.
RAID-1, or mirroring, doesnt suffer from the write penalty that RAID-5 does (in calculating parity data), and RAID-1 can tolerate a single physical disk failure without service degradation. However, RAID-1 requires that you have double the usable capacity you require, as each physical disk requires another physical disk to mirror data to. So, if you need 35GB of usable capacity, you will need to have 70GB of physcial disk, also known as RAW capacity.
Other RAID schemes such as RAID-5 can reduce the amount of RAW disk required. RAID-5 uses an extra disk to store parity data that is used to ‘rebuild’ the data should a failure occur. So, if you set up a 4+1 RAID-5 scheme (4 data disks + 1 parity disk) you only lose 20% of RAW capacity to the protection scheme (parity data). e.g. you could get 400GB usable out of 500GB RAW.
Hope that helps – let me know if u need anymore info.