DAOS-7485 control: Implement dmg system drain to act on all hosts

docs/admin/pool_operations.md

@@ -591,8 +591,8 @@ with the following information for each pool:
 - The imbalance percentage indicating whether data distribution across
   the difference storage targets is well balanced. 0% means that there is
   no imbalance and 100% means that out-of-space errors might be returned
-  by some storage targets while space is still available on others. Again
-  for the NVMe or DATA tier.
+  by some storage targets while space is still available on others. Applies
+  only for the NVMe or DATA tier.
 - The number of disabled targets (0 here) and the number of targets that
   the pool was originally configured with (total).
 
@@ -1260,6 +1260,23 @@ The pool target drain command accepts 2 parameters:
 * The engine rank of the target(s) to be drained.
 * The target indices of the targets to be drained from that engine rank (optional).
 
+#### System Drain
+
+To drain ranks or hosts from all pools that they belong to, the 'dmg system drain'
+command can be used. The command takes either a host-set or rank-set:
+
+To drain a set of hosts from all pools (drains all ranks on selected hosts):
+
+```Bash
+$ dmg system drain --rank-hosts foo-[001-100]
+```
+
+To drain a set of ranks from all pools:
+
+```Bash
+$ dmg system drain --ranks 1-100
+```
+
 ### Reintegration
 
 After an engine failure and exclusion, an operator can fix the underlying issue

src/control/cmd/dmg/command_test.go

@@ -134,6 +134,8 @@ func (bci *bridgeConnInvoker) InvokeUnaryRPC(ctx context.Context, uReq control.U
 		resp = control.MockMSResponse("", nil, &mgmtpb.SystemStartResp{})
 	case *control.SystemExcludeReq:
 		resp = control.MockMSResponse("", nil, &mgmtpb.SystemExcludeResp{})
+	case *control.SystemDrainReq:
+		resp = control.MockMSResponse("", nil, &mgmtpb.SystemDrainResp{})
 	case *control.SystemQueryReq:
 		if req.FailOnUnavailable {
 			resp = control.MockMSResponse("", system.ErrRaftUnavail, nil)

src/control/cmd/dmg/json_test.go

@@ -113,9 +113,7 @@ func TestDmg_JsonOutput(t *testing.T) {
 				testArgs = append(testArgs, "foo:bar")
 			case "system del-attr":
 				testArgs = append(testArgs, "foo")
-			case "system exclude":
-				testArgs = append(testArgs, "--ranks", "0")
-			case "system clear-exclude":
+			case "system exclude", "system clear-exclude", "system drain":
 				testArgs = append(testArgs, "--ranks", "0")
 			}