The size of the snap reserve is a function of how quickly the data changes in the primary file system and the number of snapshots to be retained. The rate of change is a function of what the primary file system is used for. For NAS, Windows home directories and public shares have a data change rate that's typically 3% to 5% of the volume size per day. For SANs, the change rate is dependent on the application using the storage.
Techniques for creating snapshots
FULL COPY. All full-copy snapshot techniques are essentially the same. In many ways, they're just another use of traditional volume mirroring. The first step is to initialize the relationship between the two data sets. Most systems allow modifications to be made to the primary data set while the initialization occurs. When the initialization is complete, the two data sets are kept in sync using synchronous or asynchronous updates. When it's time to use the snapshot, the application accessing the primary data set is quiescent. The method used to keep the app quiescent is application dependent. If the updates are done asynchronously, a final update to the snapshot data set is done. Finally, the relationship is broken and the destination becomes writable so that another host can access the data. Many products allow access to the destination in a read-only mode without breaking the relationship with the primary data set (see "Full-copy snapshots,").
DIFFERENTIAL: COPY-ON-WRITE. The most popular technique for creating a differential snapshot is the copy-on-write method. The first step is to specify a snap reserve area, which is usually on a different set of spindles than the primary data set.
The second step is to initialize or enable the snapshot service. This notifies the storage subsystem to track changes to the primary data set. As changes are made to the primary data set, the blocks of data affected by those changes are copied to the snap reserve location. There are two things to consider when using copy-on-write snapshots. The first involves performance. A file delete will cause all the blocks associated with that file to be read off the primary data spindles and then written to the snap reserve area. An overwrite of a block of data in the primary data set will result in two additional I/Os: a read of the old data block and a write of the old data block to the snap reserve location. The second involves the size of the snap reserve. Most storage systems will proactively delete the oldest snapshot to free up space when the snap reserve nears full capacity (see "Differential snapshots: Copy-on-write," above).
