allow disabling O_DIRECT in certain environments for reads (#14115)

repeated reads on single large objects in HPC like
workloads, need the following option to disable
O_DIRECT for a more effective usage of the kernel
page-cache.

However this optional should be used in very specific
situations only, and shouldn't be enabled on all
servers.

NVMe servers benefit always from keeping O_DIRECT on.

cmd/handler-api.go

@@ -49,6 +49,7 @@ type apiConfig struct {
 	staleUploadsExpiry          time.Duration
 	staleUploadsCleanupInterval time.Duration
 	deleteCleanupInterval       time.Duration
+	disableODirect              bool
 }
 
 const cgroupLimitFile = "/sys/fs/cgroup/memory/memory.limit_in_bytes"
@@ -150,6 +151,14 @@ func (t *apiConfig) init(cfg api.Config, setDriveCounts []int) {
 	t.staleUploadsExpiry = cfg.StaleUploadsExpiry
 	t.staleUploadsCleanupInterval = cfg.StaleUploadsCleanupInterval
 	t.deleteCleanupInterval = cfg.DeleteCleanupInterval
+	t.disableODirect = cfg.DisableODirect
+}
+
+func (t *apiConfig) isDisableODirect() bool {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+
+	return t.disableODirect
 }
 
 func (t *apiConfig) getListQuorum() int {

cmd/xl-storage.go

@@ -1652,7 +1652,7 @@ func (s *xlStorage) ReadFileStream(ctx context.Context, volume, path string, off
 	}
 
 	alignment := offset%xioutil.DirectioAlignSize == 0
-	if !alignment {
+	if !alignment || globalAPIConfig.isDisableODirect() {
 		if err = disk.DisableDirectIO(file); err != nil {
 			file.Close()
 			return nil, err