vfs: Add exponential backoff during ENOSPC retries

Add an exponentially increasing delay during retries up ENOSPC error
to avoid exhausting the 10 retries too soon when the cache space
recovery from item resets is not available from the file system yet
or consumed by other large cache writes.

vfs/vfscache/cache.go

@@ -609,7 +609,6 @@ func (c *Cache) clean(removeCleanFiles bool) {
 	if os.IsNotExist(err) {
 		return
 	}
-
 	c.mu.Lock()
 	oldItems, oldUsed := len(c.item), fs.SizeSuffix(c.used)
 	c.mu.Unlock()

vfs/vfscache/item.go

@@ -1182,6 +1182,7 @@ func (item *Item) setModTime(modTime time.Time) {
 // ReadAt bytes from the file at off
 func (item *Item) ReadAt(b []byte, off int64) (n int, err error) {
 	n = 0
+	var expBackOff int
 	for retries := 0; retries < fs.Config.LowLevelRetries; retries++ {
 		item.preAccess()
 		n, err = item.readAt(b, off)
@@ -1195,6 +1196,12 @@ func (item *Item) ReadAt(b []byte, off int64) (n int, err error) {
 			break
 		}
 		item.c.KickCleaner()
+		expBackOff = 2 << uint(retries)
+		time.Sleep(time.Duration(expBackOff) * time.Millisecond) // Exponential back-off the retries
+	}
+
+	if fserrors.IsErrNoSpace(err) {
+		fs.Errorf(item.name, "vfs cache: failed to _ensure cache after retries %v", err)
 	}
 
 	return n, err