diff --git a/src/backup.rs b/src/backup.rs
index ab7e400a..28054bc1 100644
--- a/src/backup.rs
+++ b/src/backup.rs
@@ -2,6 +2,6 @@
 
 pub mod chunker;
 pub mod chunk_store;
-pub mod image_index;
+pub mod fixed_index;
 pub mod archive_index;
 pub mod datastore;
diff --git a/src/backup/archive_index.rs b/src/backup/archive_index.rs
index e72580de..c991d1a2 100644
--- a/src/backup/archive_index.rs
+++ b/src/backup/archive_index.rs
@@ -13,6 +13,7 @@ use uuid::Uuid;
 //use chrono::{Local, TimeZone};
 
 #[repr(C)]
+//pub struct DynamicIndexHeader {
 pub struct ArchiveIndexHeader {
     pub magic: [u8; 12],
     pub version: u32,
diff --git a/src/backup/datastore.rs b/src/backup/datastore.rs
index df5686fe..0206b7ec 100644
--- a/src/backup/datastore.rs
+++ b/src/backup/datastore.rs
@@ -10,7 +10,7 @@ use std::sync::{Mutex, Arc};
 use crate::tools;
 use crate::config::datastore;
 use super::chunk_store::*;
-use super::image_index::*;
+use super::fixed_index::*;
 use super::archive_index::*;
 
 use chrono::{Utc, TimeZone};
@@ -75,16 +75,16 @@ impl DataStore {
         })
     }
 
-    pub fn create_image_writer<P: AsRef<Path>>(&self, filename: P, size: usize, chunk_size: usize) -> Result<ImageIndexWriter, Error> {
+    pub fn create_fixed_writer<P: AsRef<Path>>(&self, filename: P, size: usize, chunk_size: usize) -> Result<FixedIndexWriter, Error> {
 
-        let index = ImageIndexWriter::create(self.chunk_store.clone(), filename.as_ref(), size, chunk_size)?;
+        let index = FixedIndexWriter::create(self.chunk_store.clone(), filename.as_ref(), size, chunk_size)?;
 
         Ok(index)
     }
 
-    pub fn open_image_reader<P: AsRef<Path>>(&self, filename: P) -> Result<ImageIndexReader, Error> {
+    pub fn open_fixed_reader<P: AsRef<Path>>(&self, filename: P) -> Result<FixedIndexReader, Error> {
 
-        let index = ImageIndexReader::open(self.chunk_store.clone(), filename.as_ref())?;
+        let index = FixedIndexReader::open(self.chunk_store.clone(), filename.as_ref())?;
 
         Ok(index)
     }
@@ -214,7 +214,7 @@ impl DataStore {
         for entry in walker.filter_entry(|e| !is_hidden(e)) {
             let path = entry?.into_path();
             if let Some(ext) = path.extension() {
-                if ext == "iidx" {
+                if ext == "fidx" {
                     list.push(path);
                 } else if ext == "aidx" {
                     list.push(path);
@@ -231,8 +231,8 @@ impl DataStore {
 
         for path in image_list {
             if let Some(ext) = path.extension() {
-                if ext == "iidx" {
-                    let index = self.open_image_reader(&path)?;
+                if ext == "fidx" {
+                    let index = self.open_fixed_reader(&path)?;
                     index.mark_used_chunks(status)?;
                 } else if ext == "aidx" {
                     let index = self.open_archive_reader(&path)?;
diff --git a/src/backup/image_index.rs b/src/backup/fixed_index.rs
similarity index 93%
rename from src/backup/image_index.rs
rename to src/backup/fixed_index.rs
index d15cb06f..9beefea0 100644
--- a/src/backup/image_index.rs
+++ b/src/backup/fixed_index.rs
@@ -11,7 +11,7 @@ use uuid::Uuid;
 use chrono::{Local, TimeZone};
 
 #[repr(C)]
-pub struct ImageIndexHeader {
+pub struct FixedIndexHeader {
     pub magic: [u8; 12],
     pub version: u32,
     pub uuid: [u8; 16],
@@ -23,7 +23,7 @@ pub struct ImageIndexHeader {
 
 // split image into fixed size chunks
 
-pub struct ImageIndexReader {
+pub struct FixedIndexReader {
     store: Arc<ChunkStore>,
     filename: PathBuf,
     chunk_size: usize,
@@ -33,7 +33,7 @@ pub struct ImageIndexReader {
     pub ctime: u64,
 }
 
-impl Drop for ImageIndexReader {
+impl Drop for FixedIndexReader {
 
     fn drop(&mut self) {
         if let Err(err) = self.unmap() {
@@ -42,7 +42,7 @@ impl Drop for ImageIndexReader {
     }
 }
 
-impl ImageIndexReader {
+impl FixedIndexReader {
 
     pub fn open(store: Arc<ChunkStore>, path: &Path) -> Result<Self, Error> {
 
@@ -50,7 +50,7 @@ impl ImageIndexReader {
 
         let mut file = std::fs::File::open(&full_path)?;
 
-        let header_size = std::mem::size_of::<ImageIndexHeader>();
+        let header_size = std::mem::size_of::<FixedIndexHeader>();
 
         // todo: use static assertion when available in rust
         if header_size != 4096 { bail!("got unexpected header size for {:?}", path); }
@@ -58,9 +58,9 @@ impl ImageIndexReader {
         let mut buffer = vec![0u8; header_size];
         file.read_exact(&mut buffer)?;
 
-        let header = unsafe { &mut * (buffer.as_ptr() as *mut ImageIndexHeader) };
+        let header = unsafe { &mut * (buffer.as_ptr() as *mut FixedIndexHeader) };
 
-        if header.magic != *b"PROXMOX-IIDX" {
+        if header.magic != *b"PROXMOX-FIDX" {
             bail!("got unknown magic number for {:?}", path);
         }
 
@@ -152,7 +152,7 @@ impl ImageIndexReader {
     }
 }
 
-pub struct ImageIndexWriter {
+pub struct FixedIndexWriter {
     store: Arc<ChunkStore>,
     filename: PathBuf,
     tmp_filename: PathBuf,
@@ -164,7 +164,7 @@ pub struct ImageIndexWriter {
     pub ctime: u64,
 }
 
-impl Drop for ImageIndexWriter {
+impl Drop for FixedIndexWriter {
 
     fn drop(&mut self) {
         let _ = std::fs::remove_file(&self.tmp_filename); // ignore errors
@@ -174,13 +174,13 @@ impl Drop for ImageIndexWriter {
     }
 }
 
-impl ImageIndexWriter {
+impl FixedIndexWriter {
 
     pub fn create(store: Arc<ChunkStore>, path: &Path, size: usize, chunk_size: usize) -> Result<Self, Error> {
 
         let full_path = store.relative_path(path);
         let mut tmp_path = full_path.clone();
-        tmp_path.set_extension("tmp_iidx");
+        tmp_path.set_extension("tmp_fidx");
 
         let mut file = std::fs::OpenOptions::new()
             .create(true).truncate(true)
@@ -188,7 +188,7 @@ impl ImageIndexWriter {
             .write(true)
             .open(&tmp_path)?;
 
-        let header_size = std::mem::size_of::<ImageIndexHeader>();
+        let header_size = std::mem::size_of::<FixedIndexHeader>();
 
         // todo: use static assertion when available in rust
         if header_size != 4096 { panic!("got unexpected header size"); }
@@ -199,9 +199,9 @@ impl ImageIndexWriter {
         let uuid = Uuid::new_v4();
 
         let buffer = vec![0u8; header_size];
-        let header = unsafe { &mut * (buffer.as_ptr() as *mut ImageIndexHeader) };
+        let header = unsafe { &mut * (buffer.as_ptr() as *mut FixedIndexHeader) };
 
-        header.magic = *b"PROXMOX-IIDX";
+        header.magic = *b"PROXMOX-FIDX";
         header.version = u32::to_le(1);
         header.ctime = u64::to_le(ctime);
         header.size = u64::to_le(size as u64);
diff --git a/src/bin/proxmox-backup-client.rs b/src/bin/proxmox-backup-client.rs
index 12e0cdd1..cbd6953f 100644
--- a/src/bin/proxmox-backup-client.rs
+++ b/src/bin/proxmox-backup-client.rs
@@ -45,11 +45,11 @@ fn backup_image(datastore: &DataStore, file: &std::fs::File, size: usize, target
     let mut target = PathBuf::from(target);
 
     if let Some(ext) = target.extension() {
-        if ext != "iidx" {
-            bail!("got wrong file extension - expected '.iidx'");
+        if ext != "fidx" {
+            bail!("got wrong file extension - expected '.fidx'");
         }
     } else {
-        target.set_extension("iidx");
+        target.set_extension("fidx");
     }
 
     let mut index = datastore.create_image_writer(&target, size, chunk_size)?;