diff --git a/lib/bytesutil/bytebuffer.go b/lib/bytesutil/bytebuffer.go
index 7a1351fb36..1fccdb034d 100644
--- a/lib/bytesutil/bytebuffer.go
+++ b/lib/bytesutil/bytebuffer.go
@@ -1,6 +1,7 @@
 package bytesutil
 
 import (
+	"fmt"
 	"io"
 	"sync"
 
@@ -25,6 +26,11 @@ type ByteBuffer struct {
 	B []byte
 }
 
+// Path returns an unique id for bb.
+func (bb *ByteBuffer) Path() string {
+	return fmt.Sprintf("ByteBuffer(%p)", bb)
+}
+
 // Reset resets bb.
 func (bb *ByteBuffer) Reset() {
 	bb.B = bb.B[:0]
@@ -92,6 +98,11 @@ type reader struct {
 	readOffset int
 }
 
+// Path returns an unique id for the underlying ByteBuffer.
+func (r *reader) Path() string {
+	return r.bb.Path()
+}
+
 // Read reads up to len(p) bytes from bb.
 func (r *reader) Read(p []byte) (int, error) {
 	var err error
diff --git a/lib/filestream/filestream.go b/lib/filestream/filestream.go
index 2f0e4e5953..6b5b3d2332 100644
--- a/lib/filestream/filestream.go
+++ b/lib/filestream/filestream.go
@@ -17,12 +17,14 @@ const dontNeedBlockSize = 16 * 1024 * 1024
 
 // ReadCloser is a standard interface for filestream Reader.
 type ReadCloser interface {
+	Path() string
 	Read(p []byte) (int, error)
 	MustClose()
 }
 
 // WriteCloser is a standard interface for filestream Writer.
 type WriteCloser interface {
+	Path() string
 	Write(p []byte) (int, error)
 	MustClose()
 }
@@ -53,6 +55,11 @@ type Reader struct {
 	st streamTracker
 }
 
+// Path returns the path to r
+func (r *Reader) Path() string {
+	return r.f.Name()
+}
+
 // OpenReaderAt opens the file at the given path in nocache mode at the given offset.
 //
 // If nocache is set, then the reader doesn't pollute OS page cache.
@@ -171,6 +178,11 @@ type Writer struct {
 	st streamTracker
 }
 
+// Path returns the path to r
+func (w *Writer) Path() string {
+	return w.f.Name()
+}
+
 // OpenWriterAt opens the file at path in nocache mode for writing at the given offset.
 //
 // The file at path is created if it is missing.
diff --git a/lib/fs/fs.go b/lib/fs/fs.go
index 11e46d80f6..e7fe8450a6 100644
--- a/lib/fs/fs.go
+++ b/lib/fs/fs.go
@@ -353,16 +353,16 @@ func ReadFullData(r io.Reader, data []byte) error {
 }
 
 // MustWriteData writes data to w.
-func MustWriteData(w io.Writer, data []byte) {
+func MustWriteData(w filestream.WriteCloser, data []byte) {
 	if len(data) == 0 {
 		return
 	}
 	n, err := w.Write(data)
 	if err != nil {
-		logger.Panicf("FATAL: cannot write %d bytes: %s", len(data), err)
+		logger.Panicf("FATAL: cannot write %d bytes to %s: %s", len(data), w.Path(), err)
 	}
 	if n != len(data) {
-		logger.Panicf("BUG: writer wrote %d bytes instead of %d bytes", n, len(data))
+		logger.Panicf("BUG: writer wrote %d bytes instead of %d bytes to %s", n, len(data), w.Path())
 	}
 }
 
diff --git a/lib/storage/block_stream_writer.go b/lib/storage/block_stream_writer.go
index b60662c6e9..a43725f3cb 100644
--- a/lib/storage/block_stream_writer.go
+++ b/lib/storage/block_stream_writer.go
@@ -3,7 +3,6 @@ package storage
 import (
 	"bytes"
 	"fmt"
-	"io"
 	"path/filepath"
 	"sync"
 	"sync/atomic"
@@ -18,14 +17,10 @@ import (
 type blockStreamWriter struct {
 	compressLevel int
 
-	// Use io.Writer type for timestampsWriter and valuesWriter
-	// in order to remove I2I conversion in WriteExternalBlock
-	// when passing them to fs.MustWriteData
-	timestampsWriter io.Writer
-	valuesWriter     io.Writer
-
-	indexWriter     filestream.WriteCloser
-	metaindexWriter filestream.WriteCloser
+	timestampsWriter filestream.WriteCloser
+	valuesWriter     filestream.WriteCloser
+	indexWriter      filestream.WriteCloser
+	metaindexWriter  filestream.WriteCloser
 
 	mr metaindexRow
 
@@ -47,11 +42,6 @@ type blockStreamWriter struct {
 	prevTimestampsBlockOffset uint64
 }
 
-func (bsw *blockStreamWriter) assertWriteClosers() {
-	_ = bsw.timestampsWriter.(filestream.WriteCloser)
-	_ = bsw.valuesWriter.(filestream.WriteCloser)
-}
-
 // Init initializes bsw with the given writers.
 func (bsw *blockStreamWriter) reset() {
 	bsw.compressLevel = 0
@@ -86,8 +76,6 @@ func (bsw *blockStreamWriter) InitFromInmemoryPart(mp *inmemoryPart, compressLev
 	bsw.valuesWriter = &mp.valuesData
 	bsw.indexWriter = &mp.indexData
 	bsw.metaindexWriter = &mp.metaindexData
-
-	bsw.assertWriteClosers()
 }
 
 // InitFromFilePart initializes bsw from a file-based part on the given path.
@@ -144,8 +132,6 @@ func (bsw *blockStreamWriter) InitFromFilePart(path string, nocache bool, compre
 	bsw.indexWriter = indexFile
 	bsw.metaindexWriter = metaindexFile
 
-	bsw.assertWriteClosers()
-
 	return nil
 }
 
@@ -161,8 +147,8 @@ func (bsw *blockStreamWriter) MustClose() {
 	fs.MustWriteData(bsw.metaindexWriter, bsw.compressedMetaindexData)
 
 	// Close writers.
-	bsw.timestampsWriter.(filestream.WriteCloser).MustClose()
-	bsw.valuesWriter.(filestream.WriteCloser).MustClose()
+	bsw.timestampsWriter.MustClose()
+	bsw.valuesWriter.MustClose()
 	bsw.indexWriter.MustClose()
 	bsw.metaindexWriter.MustClose()