Ensuring that you have all the files needed to restore your system is a complex task. Typically, it is not practical – from either a time or a media perspective – to create a full backup every day. The solution involves running different types of jobs (full, incremental, differential or copy) on predefined schedule intervals using predefined numbers of media sets that get reused over time. The process of reusing media is referred to as media rotation. The media rotation type determines how and when each media set is used, how long it is retained once it contains data, and the granularity of your backup history.
Yosemite Server Backup organizes media into sets based on the rotation type and schedule interval. Whether the job requires several or only one physical media to complete, they are identified in the Yosemite Server Backup catalog as a set. When more than one physical media is required for a job, Yosemite Server Backup will create a unique name for each media in the set.
When planning scheduled backup jobs, it is important to know whether one or several physical media will be required to complete a backup job. This can usually be estimated by comparing the size of the backup selection to the capacity of the selected media. If you do not want Yosemite Server Backup to use more than one media for a backup job, then you must select fewer files to back up.
Job schedules are defined using the Intervals Daily, Weekly, Monthly, and Yearly . Intervals are used to defined which days a job will run, what type of backup (full, incremental, differential, or copy) will be done, and how many sets of media are dedicated to the interval. The size of an interval refers to the amount of time between runs of that interval.
When the Run repeatedly schedule type is chosen the job Configuration page will show an additional section, Interval settings, that control the schedule parameters. Each interval type is listed along with a textual description of its current setting. To customize the settings for an interval, click on one of the interval buttons. Most schedules are defined in terms of the following intervals:
Daily – run on sequential weekdays.
Weekly – run once per week on the day specified by the user, for example, Friday.
Monthly – run once per month on a day specified by the user such as the first day, the last day, the first Monday, and others. You can also specify how many months should elapse between monthlies. Setting the monthly interval to every 3 months will create a backup every quarter.
Yearly – run once per year on a specified day of the year. By increasing the interval you can also schedule a job to run once every so many years.
Implications of Intervals for Restoring Data
Intervals also define the granularity of the data you can restore. Rotations are set up to capture more granularity in the recent past and less granularity as data gets older. Larger intervals, like Yearly and Monthly, produce lower granularity data history. Smaller intervals, like Daily, produce higher granularity history. Take, for example, a rotation with three full monthly backup sets on the last day of each month, four full weekly backup sets created on each Friday, and four incremental daily backup sets created Monday through Thursday. Now suppose you have a critical file that changes daily. On Wednesday, you are asked to retrieve the file as of a specific date. With this rotation you can roll back to the Monday and Tuesday versions of the file in the current week and the Wednesday, Thursday, and Friday versions of the file in the previous week. Beyond that, you will only have the versions of the file as they existed on Friday for the previous four weeks previous to the current week. And beyond that you will only have the versions of the file that existed on the last day of the month for the previous three months.
The catalog keeps track of the files and versions that have been backed up so you don't have to remember what media they are on. This knowledge makes the restoration process very simple. You only need to specify the files you want restored and Yosemite Server Backup will prompt you for the media it needs restore the files. Full reconstruction of data may require multiple media sets. For example, to reconstruct the data for a Wednesday from a GFS 20 set rotation type, you will require the full backup media set from the previous end of week and all of the incremental media sets from that week (that is, Mondays, Tuesdays and Wednesdays). In some circumstances, the preceding full backup media set will be a monthly or yearly job and not a weekly job. As long as none of these media sets has been overwritten, full data recovery is possible.