Files are migrated to lower levels of the storage hierarchy in family groups, that means files belonging to two different families are not stored on the same media. Purging is the removal of data files from the disk cache level of the storage hierarchy. It is interlocked with the migration process, insofar as only files which have been migrated can be purged. The purging algorithm within UniTree is extremely flexible, allowing the System Administrator to implement an appropriate strategy according the requirements of the system users. There are two parameters which control when purging should begin, these are expressed in terms of the percentage filled of the header and data space available on the disk cache.
Data space refers to space taken up on the disk cache by the actual data within user files.
Purging ends when both the header and data stop purge criteria are met. These are also expressed in terms of percentages of available space on the disk cache. The selection algorithm for deciding which files to purge comprises several steps. Firstly, only those files which have been migrated become candidates. Next, a weighting factor is calculated for each file.