diff --git a/src/runtime-rs/crates/resource/src/coco_data/initdata_block.rs b/src/runtime-rs/crates/resource/src/coco_data/initdata_block.rs
index 0cc27e4d4b..129e301c12 100644
--- a/src/runtime-rs/crates/resource/src/coco_data/initdata_block.rs
+++ b/src/runtime-rs/crates/resource/src/coco_data/initdata_block.rs
@@ -6,7 +6,7 @@
 use flate2::{Compression, GzBuilder};
 use std::{
     fmt, fs,
-    io::{self, BufWriter, Seek, Write},
+    io::{self, BufWriter, Write},
     path::{Path, PathBuf},
 };
 use tempfile::NamedTempFile;
@@ -159,7 +159,7 @@ fn create_compressed_block(
     writer.write_all(MAGIC_HEADER)?;
     info!(sl!(), "Magic header written: {:?}", MAGIC_HEADER);
 
-    // 7. Configure compression level and initialize GZ writer
+    // 7. First compress data to get the actual compressed size
     let compression =
         compression_level.map_or(Compression::best(), |lvl| Compression::new(lvl.min(9)));
 
@@ -169,22 +169,24 @@ fn create_compressed_block(
         compression.level()
     );
 
-    let mut gz = GzBuilder::new()
-        .filename("initdata.toml") // Embed original filename metadata
-        .comment("Generated by Confidential Containers")
-        .write(writer, compression);
+    // Compress data to a temporary buffer first to get the compressed size
+    let mut compressed_data = Vec::new();
+    {
+        let mut gz = GzBuilder::new()
+            .filename("initdata.toml") // Embed original filename metadata
+            .comment("Generated by Confidential Containers")
+            .write(&mut compressed_data, compression);
 
-    // 8. Write data in chunks to avoid large memory allocation
-    let mut bytes_written = 0;
-    for chunk in initdata.as_bytes().chunks(buffer_size) {
-        bytes_written += gz.write(chunk)?;
+        // Write data in chunks to avoid large memory allocation
+        for chunk in initdata.as_bytes().chunks(buffer_size) {
+            gz.write_all(chunk)?;
+        }
+
+        // Finalize compression
+        gz.finish()?;
     }
-    info!(sl!(), "written {} bytes", bytes_written);
-
-    // 9. Finalize compression and retrieve writer
-    let mut writer = gz.finish()?;
-    let compressed_size = writer.stream_position()?;
 
+    let compressed_size = compressed_data.len() as u64;
     info!(
         sl!(),
         "Data compressed: {} -> {} bytes (ratio: {:.2}%)",
@@ -193,8 +195,19 @@ fn create_compressed_block(
         (compressed_size as f64 / initdata_size as f64) * 100.0
     );
 
+    // 8. Write compressed data length (8 bytes, little-endian)
+    writer.write_all(&compressed_size.to_le_bytes())?;
+    info!(
+        sl!(),
+        "Compressed data length written: {} bytes", compressed_size
+    );
+
+    // 9. Write compressed data
+    writer.write_all(&compressed_data)?;
+    info!(sl!(), "Compressed data written");
+
     // 10. Calculate padding for sector alignment
-    let current_pos = compressed_size;
+    let current_pos = MAGIC_HEADER.len() as u64 + 8 + compressed_size; // magic + length + data
     let padding = (SECTOR_SIZE - (current_pos % SECTOR_SIZE)) % SECTOR_SIZE;
 
     // 11. Zero-byte padding using small blocks