Ideally, this cache should hold the working set of MSS bitfiles. Bitfiles flow into the cache from the clients as they are written, and from the offline archive as they are staged. Bitfiles flow out of the cache to the offline archive when they become inactive, making room for newly written bitfiles and newly activated bitfiles. The flow of bitfiles into the cache is dynamic. Access patterns govern when a bitfile is staged from offline to online storage.
End users control when bitfiles are written.
There are two types of inactive bitfile migration out of the cache, dynamic and deterministic. When a given online partition becomes full, the MSS automatically starts a demand scrub process which moves bitfiles out of the online cache to the offline archive, a dynamic migration. In addition, a daily scrub process is executed each day on each online partition, a deterministic migration. The daily scrub removes bitfiles from the online cache until a free space threshold is reached. The intent of the daily scrub is to minimize the number of demand scrubs by creating sufficient free, or unused, space in the online partitions to hold newly written bitfiles and staged bitfiles for the next 24 hours.