If there is a failure, the system was not reliable. Availability is how well a system can work in times of a failure. If a system is able to work even in the presence of a failure of one or more system components, the system is said to be available. Redundancy improves the availability of a system, but cannot improve the reliability. Reliability can only be increased by improving manufacturing technologies or using lesser individual components in a system.
Every time there is a write operation, there is a change of data.
This change also, has to be reflected in the disks storing redundant information. This worsens the performance of writes in redundant disk arrays significantly compared to the performance of writes in non redundant disk arrays.