TPA-RFC-84: design and implement backup strategy for MinIO buckets or the entire server

We're considering using MinIO for more and more things, mainly GitLab (artifacts storage in #41403 and gitaly backups in #40518) but possibly other (e.g. metrics storage in tpo/network-health/metrics/collector#40023 (closed)).

Right now, we don't have any backups of that server, which is probably fine: we only store container images there, which can be regenerated in case of a catastrophe. But if we start storing gitaly backups and gitlab artifacts, it needs to be permanent now.

Research how backups can be performed, develop a policy and implement it.

Next steps:

• research articles anarcat found on the topic (see wallabag)
• discuss the idea in the network
• decide if we want this per bucket or per site
• write up a proposal in particular (in progress, see https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-84-minio-backups-and-scaling)
  • backup/restore recovery
  • impact on other teams
  • timeline
  • estimates
  • review this issue
• implement proposal
  • minio-fsn-02 setup (4TiB), consider splitting in chunks? (#42136 (closed)) see https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-84-minio-backups-and-scaling#warm-hard-disk-storage
  • implement quotas (#42155 (closed)) (should resolve #42077 (closed)), see https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-84-minio-backups-and-scaling#quotas
    • add monitoring for bucket quota usage
  • sync up minio-fsn-02 and minio-01 with hot/cold storage (#42156 (closed))
    • setup tiered storage
    • join minio-fsn-02 to cluster
    • test assigning a bucket to a specific tier. tie the network-health bucket to the "warm" tier
  • add more storage capacity to the warm tier cluster (#42237 (closed))
  • implement storage backups for both clusters (minio-01 and minio-fsn-02), see https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-84-minio-backups-and-scaling#minio-native-backups-with-possible-exceptions
  • consider setting up a new minio-dal-03 server
• document and test backup/restore procedures