enable object reader prefetch for block uploader

Signed-off-by: Lyndon-Li <lyonghui@vmware.com>

pkg/uploader/block/uploader.go

@@ -176,7 +176,10 @@ func (blkup *blockUploader) Restore(snapshot udmrepo.Snapshot, dest destInfo, bi
 		return 0, errors.Errorf("dest dev(%s) size is too small (%v vs. %v)", dest.path, dest.size, sourceSize)
 	}
 
-	reader, err := blkup.repoWriter.OpenObject(blkup.ctx, meta.SubObjects[0].ID, udmrepo.ObjectReadOptions{})
+	reader, err := blkup.repoWriter.OpenObject(blkup.ctx, meta.SubObjects[0].ID, udmrepo.ObjectReadOptions{
+		Prefetch:         true,
+		PrefetchBudgetMB: 256,
+	})
 	if err != nil {
 		return 0, errors.Wrapf(err, "error opening bdev object %v", meta.SubObjects[0].Name)
 	}