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| Understanding application characteristics | ||
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Before you configure an RDS, you must know the data throughput that must be supported, that is, the rate at which the application can be expected to write data. Only write operations are of concern; read operations do not affect replication. To perform the analyses described in later sections, a profile of application write rate is required. For an application with relatively constant write rate, the profile could take the form of certain values, such as:
Average application write rate
Peak application write rate
Period of peak application write rate
For a more volatile application, a table of measured usages over specified intervals may be needed. Because matching application write rate to disk capacity is not an issue unique to replication, it is not discussed here. It is assumed that an application is already running, and that Veritas Volume Manager (VxVM) has been used to configure data volumes to support the write rate needs of the application. In this case, the application write rate characteristics may already have been measured.
If the application characteristics are not known, they can be measured by running the application and using a tool to measure data written to all the volumes to be replicated. If the application is writing to a file system rather than a raw data volume, be careful to include in the measurement all the metadata written by the file system as well. This can add a substantial amount to the total amount of replicated data. For example, if a database is using a file system mounted on a replicated volume, a tool such as vxstat (see vxstat(1M)) correctly measures the total data written to the volume, while a tool that monitors the database and measures its requests fails to include those made by the underlying file system.
It is also important to consider both peak and average write rates of the application. These numbers can be used to determine the type of network connection needed. For Secondary hosts replicating in synchronous mode, the network must support the peak application write rate. For Secondary hosts replicating in asynchronous mode that are not required to keep pace with the Primary, the network only needs to support the average application write rate.
Finally, once the measurements are made, the numbers calculated as the peak and average write rates should be close to the largest obtained over the measurement period, not the averages or medians. For example, assume that measurements are made over a 30-day period, yielding 30 daily peaks and 30 daily averages, and then the average of each of these is chosen as the application peak and average respectively. If the network is sized based on these values, then for half the time there will be insufficient network capacity to keep up with the application. Instead, the numbers chosen should be close to the highest obtained over the period, unless there is reason to doubt that they are valid or typical.