Use cases

There are currently three use cases which concern the FST garbage collector.

Manage a disk cache for tape - Delete old files when space is running out

When a disk area of an EOSCTA instance is to be used as a cache, the FST garbage collector should only consider deleting old files when space is running out. This could be viewed as "gentle euthanasia". An example of this use case is the "spinner" EOS space of the ALICE instance.

Manage a disk buffer - Delete old files as soone as they're old

When the disk area of an EOSCTA instance is to be used a a strict buffer there should not be any files hanging around on disk. In this case the garbage collector should delete a file as soon as it is older because it should never have had the chance to get old. This could be viewed as "strict euthanasia"

Provide a clear signal to monitoring about the health of a disk buffer

This use case has the same implementation as for managing a disk buffer. The difference is the motivation behind it. The EOSCTA monitoring system cannot tell if a file on disk should stick around for a little while longer. This is because the monitoring system does not know which files are currently scheduled for retrieval to another disk system but just haven't been processed yet. By deleting files that are older than a fixed and well known amount of time, the monitoring system knows the system is healthly if no files older than that amount of time exist on disk.