d resources (e.g. 99.9% uptime) Durability Probability of CSC data loss Reliability Ability of CSP to complete all required functions for a given period of time Power Total watt usage by the CSP system Density Refers to number of workload instances that can be run on the cloud system simultaneously before the QoS degradation occurs Variability This is the standard deviation of the other QoS metrics. Latency Round trip delay of packets between CSC and CSP 6.8 Cloud Computing Services Important QoS metrics for cloud services are availability and reliability, which are related. While availability is the percentage of time when the cloud services are available to CSCs, reliability refers to the ability of the cloud system to perform any required functions under the stated conditions for a specified period of time. For example, QoS metric reliability can be measured by the mean time between failures. If this is denoted with T between_failures' the number of disk drives is Disk_drives and T hours is num- ber of hours run, then the reliability can be calculated by using the following equation: Reliability Tbetween_failures NDisk_drives Thours (6.4) Density as a QoS metric is dependent upon the cloud service category. For example, when the cloud service is IaaS, density may refer to the number of virtual machines running. For PaaS, the density may be measured as the number of application servers that are running on the cloud system. For SaaS, the density refers to the number of users that the cloud system can service simultaneously. Finally, latency (i.e. delay) is important for cloud services because most of the cloud services are interactive by nature (e.g. similar to Web services, which also can be based on cloud computing, for storage of user data for instance). Latency directly influences the amount of data that the CSC can back up, access, or restore [37]. Also, latency can limit the maximum amount of data that can be transmitted over the IP network (either public Internet or managed IP network) because cloud servi