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The following guidelines can be used to help calculate snapshot usage overhead:
Overhead is the product of multiplying the rate of change (1) and the number of snapshots to retain (2). For example, consider the following scenario:
In this case, overhead is 5% x 3 (copies) = 15% for that directory only.
Within OneFS, file system snapshot overhead is very low and, in most cases, marginal. It can be complex to calculate, and changes based on the number of files and file sizes in the given dataset. However, even in the most extreme cases (such as many small files) the file system snapshot overhead is typically only 1-3%. For example, the file system snapshot overhead for a dataset with 1 million files of 1 K size, 5% daily change rate, and three snapshots retained, is 1%.
Sometimes, however, a cluster has many old snapshots that take up a lot of space. Reasons for this may include: