Enhancement: If occupancy is 99% for a tape RSE, set dm_weight to 0

[haozturk]

Enhancement Description

Rucio doesn't ensure that the quota will not be exceeded for an RSE. Even distribution of data for tapes relies on the dm_weight attribute. This attribute is set based on amount of free space, the relative free space and the pledge of tapes. It's a number between 1 and 100 for each RSE. As long as it's non-zero, there's a chance WM will choose this tape as destination based on dm_weight. I'm proposing to set this attribute to 0 if the relative free space in a tape is below 1%.

Use Case

IN2P3 has reached to 99.2% occupancy but since dm_weight was still non-zero, it kept receiving data. We had to switch it off by disabling the wmcore_output_tape attribute.

Possible Solution

If occupancy is 99% for a tape RSE, set dm_weight to 0.

Related Issues

Open questions:

• What if all tapes have less than 1% free space?
• Should the threshold be in terms of TBs?
• Before creating rules, should WM check if there's enough space at the RSE?
• Should rucio check if there's enough space at RSE before submitting requests to FTS? [1]

@ericvaandering @Panos512 @amaltaro thoughts?

[1] rucio/rucio#7371