rrd: Fix absolute rate calculations (#6646)

Absolute metric should work as follows:

    Timestamp    = 300, 600, 900, 1200
    Step         = 300 seconds
    ABSOLUTE DS  =    1,  2,   3,    4

But they do not seem to have ever worked correctly - they were
previously divided by the interval but incorrectly handled NaNs,
resulting in wrong behaviour. Then the refactoring (including 73ca3cc
("CA-404597: rrd - Pass Gauge and Absolute data source values as-is"))
broke them further.

Divide absolute metrics by the interval in process_ds_value. Fix the
unit test to expect the right behaviour - it passes with this fix.

---

This means absolute metrics are now correctly calculated as unit/s,
changing the behaviour in
#6640

I am not 100% sure this fix goes all the way either - unit tests should
cover expected values in RRAs as well, and the logic is still really
confusing to me.

Author: robhoes

ocaml/libs/xapi-rrd/lib/rrd.ml

@@ -342,9 +342,9 @@ let rra_update rrd proc_pdp_st elapsed_pdp_st pdps =
 
 (* We assume that the data being given is of the form of a rate; that is,
    it's dependent on the time interval between updates.
-   Gauge and Absolute data sources are simply kept as is without any
-   time-based calculations, while Derive data sources will be changed according
-   to the time passed since the last measurement. (see CA-404597) *)
+   Gauge data sources are simply kept as is without any time-based
+   calculations, while Absolute and Derive data sources will be changed
+   according to the time passed since the last measurement. (see CA-404597) *)
 let process_ds_value ds value interval new_rrd =
   if interval > ds.ds_mrhb then
     nan
@@ -361,8 +361,10 @@ let process_ds_value ds value interval new_rrd =
 
     let rate =
       match (ds.ds_ty, new_rrd) with
-      | Absolute, _ | Derive, true | Gauge, _ ->
+      | Derive, true | Gauge, _ ->
           value_raw
+      | Absolute, _ ->
+          value_raw /. interval
       | Derive, false -> (
         match (ds.ds_last, value) with
         | VT_Int64 x, VT_Int64 y ->

ocaml/libs/xapi-rrd/lib_test/unit_tests.ml

@@ -147,46 +147,22 @@ let absolute_rrd =
 
 let absolute_rrd_CA_404597 () =
   let rra = rra_create CF_Average 100 1 0.5 in
-  let rra2 = rra_create CF_Average 100 10 0.5 in
-  let rra3 = rra_create CF_Average 100 100 0.5 in
-  let rra4 = rra_create CF_Average 100 1000 0.5 in
-  let ts = 1000000000.0 in
+  let ts = 0.0 in
   let ds =
-    ds_create "foo" Absolute ~mrhb:10.0 ~min:0. ~max:infinity (VT_Float 0.0)
+    ds_create "foo" Absolute ~mrhb:1000.0 ~min:0. ~max:infinity (VT_Float 0.0)
   in
-  let ds2 =
-    ds_create "bar" Absolute ~mrhb:10.0 ~min:0. ~max:infinity (VT_Float 0.0)
-  in
-  let ds3 =
-    ds_create "baz" Absolute ~mrhb:10.0 ~min:0. ~max:infinity (VT_Float 0.0)
-  in
-  let ds4 =
-    ds_create "boo" Absolute ~mrhb:10.0 ~min:0. ~max:infinity (VT_Float 0.0)
-  in
-  let rrd = rrd_create [|ds; ds2; ds3; ds4|] [|rra; rra2; rra3; rra4|] 1L ts in
+  let rrd = rrd_create [|ds|] [|rra|] 1L ts in
   let id = Identity in
   for i = 1 to 100000 do
-    let t = 1000000.0 +. (0.7 *. float_of_int i) in
+    let t = 300. *. float_of_int i in
     let ((_, val1) as v1) =
-      (0, {value= VT_Float (0.5 +. (0.5 *. sin (t /. 10.0))); transform= id})
+      (0, {value= VT_Float (300. *. float_of_int i); transform= id})
     in
-    let ((_, val2) as v2) =
-      (1, {value= VT_Float (1.5 +. (0.5 *. cos (t /. 80.0))); transform= id})
-    in
-    let ((_, val3) as v3) =
-      (2, {value= VT_Float (3.5 +. (0.5 *. sin (t /. 700.0))); transform= id})
-    in
-    let ((_, val4) as v4) =
-      (3, {value= VT_Float (6.5 +. (0.5 *. cos (t /. 5000.0))); transform= id})
-    in
-    ds_update rrd t [|v1; v2; v3; v4|] false ;
+    ds_update rrd t [|v1|] false ;
 
-    Array.iter2
-      (fun ds value ->
-        compare_float __LOC__ ds.ds_value
-          (float_of_string (ds_value_to_string value.value))
-      )
-      rrd.rrd_dss [|val1; val2; val3; val4|]
+    compare_float __LOC__
+      (float_of_string (ds_value_to_string val1.value) /. 300.)
+      rrd.rrd_dss.(0).ds_value
   done
 
 (** Verify that Gauge data soruce values are correctly handled by the RRD lib