180 IBM System Storage N series Hardware Guide Number of storage groupsBecause a storage group cannot span N series storage systems, the number of storagegroups affects sizing. There is no recommendation on number of storage groups per IBMSystem Storage N series storage system. However, the number and type of users perstorage group helps determine the number of storage groups per storage system. Volume typeAre FlexVols or traditional volumes used? The type of volume used affects bothperformance and capacity. Drive typeEarlier, this chapter addressed the storage capacity effect of drive type. For MicrosoftExchange, the drive type and performance characteristics are also significant, especiallywith a highly used Exchange server. In an active environment, use smaller drives andhigher performance characteristics such as RPM and Fibre Channel versus SATA. Read-to-write ratioThe typical read-to-write ratio is 70% to 30%. Growth rateIndustry estimates place data storage growth rates at 50% or higher. Size for at least twoyears into the future. Deleted mailbox cache spaceThis is a feature of Microsoft Exchange that must also be sized for storage usage on theN series. Microsoft allows for a time-specified retention of documents even after deletionof a mailbox. You also must size the storage effect of this feature.14.2.8 Backup serversProtecting and archiving critical corporate data is increasingly important. Deploying serversfor this purpose is becoming more common, and these configurations call for their ownplanning guidelines.A backup server generally is not designed to deliver high transactional performance. Datacenter managers rely on the backup server being available to receive the backup streamswhen they are sent. Often the backup server is an intermediate repository for data before itgoes to backup tape and ultimately offsite. But frequently the backup server takes the place ofbackup tapes.The write throughput of a backup server is frequently the most important factor to consider inplanning. Another important factor is the number of simultaneous backup streams that asingle server can handle. The more effective the write throughput and the greater the numberof simultaneous threads, the more rapidly backup processes complete. The faster theprocesses complete, the sooner that production servers are taken out of backup mode andreturned to full performance.Each IBM System Storage N series platform has different capabilities in each of these areas.The planning process must take these characteristics into account to ensure that the backupserver is capable of the workload expected.14.2.9 Backup and recoveryIn addition to backup servers, all storage systems must be backed up. Generally, the goal is tohave the backup process occur at a time and in a way that minimizes the effect on overallproduction. Therefore, many backup processes are scheduled to run during off-hours.
