Design + master mirror of max volume id outside raft (#11763) (#11818)

* design: durable volume-id allocation boundary (#11763)

* master: mirror max volume id to a file outside raft (#11763)

The allocation bound lived only in raft state, so a master that lost it
rebuilt the bound from whatever volume servers were connected and could
reissue ids held by offline servers.

NextVolumeId now persists the bound to <mdir>/max_volume_id before the
raft apply, and both raft implementations persist on apply and recovery
so follower mirrors stay current. The file also carries the topology id,
which survives raft state cleanup the same way the snapshot recovery did.

* master: serialize and fully sync the max volume id mirror (#11763)

* master: rewrite the mirror when the topology id changes at the same bound

A same-bound raft apply carrying a new topology id was skipped by the
lastPersistedMaxVolumeId check, leaving the mirrored topology id empty
until the next allocation. Track the mirrored topology id too and
rewrite when either value differs.

* drop the design doc from the PR stack

weed/server/raft_hashicorp.go

@@ -149,6 +149,7 @@ func NewHashicorpRaftServer(option *RaftServerOption) (*RaftServer, error) {
 	if err := os.MkdirAll(path.Join(s.dataDir, "snapshots"), os.ModePerm); err != nil {
 		return nil, err
 	}
+	option.Topo.SetMaxVolumeIdFile(filepath.Join(s.dataDir, "max_volume_id"))
 	baseDir := s.dataDir
 
 	ldb, err := boltdb.NewBoltStore(filepath.Join(baseDir, ldbFile))

weed/server/raft_server.go

@@ -76,6 +76,9 @@ func (s StateMachine) Recovery(data []byte) error {
 		s.topo.SetTopologyId(state.TopologyId)
 		glog.V(0).Infof("Recovered TopologyId: %s", state.TopologyId)
 	}
+	if err := s.topo.PersistMaxVolumeId(state.MaxVolumeId); err != nil {
+		glog.Warningf("persist max volume id %d: %v", state.MaxVolumeId, err)
+	}
 	return nil
 }
 
@@ -95,6 +98,9 @@ func (s *StateMachine) Apply(l *hashicorpRaft.Log) interface{} {
 			glog.V(0).Infof("Set TopologyId from raft log: %s", state.TopologyId)
 		}
 	}
+	if err := s.topo.PersistMaxVolumeId(state.MaxVolumeId); err != nil {
+		glog.Warningf("persist max volume id %d: %v", state.MaxVolumeId, err)
+	}
 
 	glog.V(1).Infoln("max volume id", before, "==>", s.topo.GetMaxVolumeId())
 	return nil
@@ -163,6 +169,7 @@ func NewRaftServer(option *RaftServerOption) (*RaftServer, error) {
 	if err := os.MkdirAll(path.Join(s.dataDir, "snapshot"), os.ModePerm); err != nil {
 		return nil, err
 	}
+	option.Topo.SetMaxVolumeIdFile(path.Join(s.dataDir, "max_volume_id"))
 
 	stateMachine := StateMachine{topo: option.Topo}
 	s.raftServer, err = raft.NewServer(string(s.serverAddr), s.dataDir, transporter, stateMachine, option.Topo, s.serverAddr.ToGrpcAddress())