What is /tigress-hsm
/tigress-hsm is a hierarchical storage system based on a tiered GPFS filesystem. It consists of 3 different tiers and is presented as what looks like a single filespace.
Tier 1 is comprised of the fastest disks and consists of a number of arrays of disks consolidated together. The files on this tier of 90TB of disk are there because 1) they are new files, created over the past 90 days, or 2) they have been promoted from one of the lower tiers because they have been accessed over the past 90 days. These are the files which are being actively used.
As files age, they move from tier1 to tier2 storage. This too is disk storage. There is 200TB of spinning disk there also assembled from a number of disk arrays. The disks are slower than tier1 but there are a lot more of them. Files land here typically by age only. After 90 days of inactivity, tier1 files move to tier2.
As files continue to age, they move from this tier2 storage to tier3 which is tape storage. This provides an almost infinite amount of storage but since tapes need to be loaded for every access, this is the slowest tier. What GPFS does is create a "stub" file for the system which simply tells it that it resides only on tape.
Because tier3 already provides a tape library and the necessary software, /tigress-hsm has been backed up on a best effort basis in order to protect against catastrophic loss.