reject speedtest when there isn't enough disk space available (#14402)

small setups do not return appropriate errors when speedtest
cannot run on small tiny setups, allow the tests to fail
appropriately more pro-actively.

many users bring toy setups, this PR simply returns an error
in such situations.

cmd/admin-handlers.go

@@ -984,6 +984,31 @@ func (a adminAPIHandlers) ObjectSpeedtestHandler(w http.ResponseWriter, r *http.
 		duration = time.Second * 10
 	}
 
+	// ignores any errors here.
+	storageInfo, _ := objectAPI.StorageInfo(ctx)
+	capacityNeeded := uint64(concurrent * size)
+	capacity := uint64(GetTotalUsableCapacityFree(storageInfo.Disks, storageInfo))
+
+	if capacity < capacityNeeded {
+		writeErrorResponseJSON(ctx, w, toAdminAPIErr(ctx, AdminError{
+			Code: "XMinioSpeedtestInsufficientCapacity",
+			Message: fmt.Sprintf("not enough usable space available to perform speedtest - expected %s, got %s",
+				humanize.IBytes(capacityNeeded), humanize.IBytes(capacity)),
+			StatusCode: http.StatusInsufficientStorage,
+		}), r.URL)
+		return
+	}
+
+	// Verify if we can employ autotune without running out of capacity,
+	// if we do run out of capacity, make sure to turn-off autotuning
+	// in such situations.
+	newConcurrent := concurrent + (concurrent+1)/2
+	autoTunedCapacityNeeded := uint64(newConcurrent * size)
+	if autotune && capacity < autoTunedCapacityNeeded {
+		// Turn-off auto-tuning if next possible concurrency would reach beyond disk capacity.
+		autotune = false
+	}
+
 	deleteBucket := func() {
 		objectAPI.DeleteBucket(context.Background(), pathJoin(minioMetaBucket, "speedtest"), DeleteBucketOptions{
 			Force:      true,