diff --git a/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt b/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt
index 46643ef2..33c07f66 100644
--- a/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt
+++ b/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt
@@ -1,9 +1,13 @@
 package hep.dataforge.io
 
 import hep.dataforge.context.Global
+import java.nio.ByteBuffer
 import java.nio.file.Files
+import java.util.*
 import kotlin.test.Test
+import kotlin.test.assertEquals
 import kotlin.test.assertTrue
+import kotlin.test.fail
 
 
 class FileEnvelopeTest {
@@ -61,4 +65,36 @@ class FileEnvelopeTest {
         assertTrue { envelope.contentEquals(restored) }
     }
 
+    @Test
+    fun testDataSize() {
+        val tmpPath = Files.createTempFile("dataforge_test", ".df")
+        Global.io.writeEnvelopeFile(tmpPath, envelope)
+        println(tmpPath.toUri())
+        val scan = Scanner(tmpPath.toFile().inputStream()).useDelimiter("\n").nextLine()
+        println(scan)
+        val format = scan.slice(2..5)
+        when (format) {
+            "DF03" -> {
+                val buff = ByteBuffer.allocate(4)
+                buff.put(scan.slice(12..19).toByteArray())
+                buff.flip()
+                val size = buff.long
+                println(size)
+                assertEquals(8, size)
+            }
+            "DF02" -> {
+                val buff = ByteBuffer.allocate(4)
+                buff.put(scan.slice(12..15).toByteArray())
+                buff.flip()
+                val size = buff.int
+                println(size)
+                assertEquals(8, size)
+            }
+            else -> {
+                fail("Format $format don't have test")
+            }
+        }
+    }
+
+
 }
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