diff --git a/ocaml/test/OMakefile b/ocaml/test/OMakefile
index 777c996755d..53623d3afa2 100644
--- a/ocaml/test/OMakefile
+++ b/ocaml/test/OMakefile
@@ -1,6 +1,6 @@
 OCAMLPACKS = oUnit sexpr xcp xmlm stunnel xml-light2 http-svr uuid	\
              netdev tapctl xenctrl xenctrlext xenstore-compat	\
-             pciutil oclock gzip sha1 xcp.network xcp.rrd xcp.storage	\
+             pciutil oclock gzip sha1 sha.sha1 xcp.network xcp.rrd xcp.storage	\
              xcp.xen xcp.memory tar tar.unix oPasswd xcp-inventory \
              rrdd-plugin pci xapi-test-utils
 
diff --git a/ocaml/xapi/OMakefile b/ocaml/xapi/OMakefile
index 9e4101b0c2d..46951a3ea4c 100644
--- a/ocaml/xapi/OMakefile
+++ b/ocaml/xapi/OMakefile
@@ -1,6 +1,6 @@
 OCAMLPACKS = oclock xml-light2 cdrom pciutil sexpr xcp stunnel		\
              http-svr netdev tapctl rpclib xenstore-compat	\
-             uuid gzip sha1 xcp.network xcp.rrd xcp.storage	\
+             uuid gzip sha1 sha.sha1 xcp.network xcp.rrd xcp.storage	\
              xcp.xen xcp.memory tar tar.unix oPasswd xcp-inventory \
              rrdd-plugin pci
 
diff --git a/ocaml/xapi/stream_vdi.ml b/ocaml/xapi/stream_vdi.ml
index 151110e56ee..f5bd95d7706 100644
--- a/ocaml/xapi/stream_vdi.ml
+++ b/ocaml/xapi/stream_vdi.ml
@@ -94,11 +94,8 @@ let write_block ~__context filename buffer ofd len =
   let hdr = Tar_unix.Header.make filename (Int64.of_int len) in
 
   try
-	let csum = Sha1sum.sha1sum
-	  (fun checksumfd ->
-		   Tar_unix.write_block hdr (fun ofd -> Tar_unix.Archive.multicast_n_string buffer 
-									[ ofd; checksumfd ] len) ofd
-	  ) in
+	let csum = Sha1.to_hex (Sha1.string buffer) in
+	Tar_unix.write_block hdr (fun ofd -> Tar_unix.Archive.multicast_n_string buffer [ ofd ] len ) ofd;
 	(* Write the checksum as a separate file *)
 	let hdr' = Tar_unix.Header.make (filename ^ checksum_extension) (Int64.of_int (String.length csum)) in
 	Tar_unix.write_block hdr' (fun ofd -> ignore(Unix.write ofd csum 0 (String.length csum))) ofd
@@ -126,6 +123,7 @@ let send_all refresh_session ofd ~__context rpc session_id (prefix_vdis: vdi lis
     with_open_vdi __context rpc session_id vdi_ref `RO [Unix.O_RDONLY] 0o644
       (fun ifd ->
 
+	 let reusable_buffer = String.make (Int64.to_int chunk_size) '\000' in
 
 	 (* NB. It used to be that chunks could be larger than a native int *)
 	 (* could handle, but this is no longer the case! Ensure all chunks *)
@@ -153,7 +151,10 @@ let send_all refresh_session ofd ~__context rpc session_id (prefix_vdis: vdi lis
 			 (* no progress has been made *)
 			 stream_from (chunk_no + 1) offset
 		   end else begin
-			 let buffer = String.make this_chunk '\000' in
+			 let buffer = if (Int64.of_int this_chunk) = chunk_size
+				then reusable_buffer
+				else String.make this_chunk '\000'
+			 in
 			 Unixext.really_read ifd buffer 0 this_chunk;
 			 if not (Zerocheck.is_all_zeros buffer this_chunk) || first_or_last then begin
 			   last_transmission_time := now;
@@ -214,6 +215,8 @@ let recv_all refresh_session ifd (__context:Context.t) rpc session_id vsn force
     
     with_open_vdi __context rpc session_id vdi_ref `RW [Unix.O_WRONLY] 0o644
       (fun ofd ->
+	let reusable_buffer = String.make (Int64.to_int chunk_size) '\000' in
+
 	let rec stream_from (last_suffix: string) (offset: int64) = 
 	  refresh_session ();
 
@@ -261,10 +264,14 @@ let recv_all refresh_session ifd (__context:Context.t) rpc session_id vsn force
 		  done
 	      end;
 		
-	    let csum = Sha1sum.sha1sum
-	      (fun checksumfd ->
-		 Tar_unix.Archive.multicast_n ifd [ ofd; checksumfd ] length) in
-	    
+	    let buffer = if length = chunk_size
+	      then reusable_buffer
+	      else String.make (Int64.to_int length) '\000'
+	    in
+	    Unixext.really_read ifd buffer 0 (Int64.to_int length);
+	    Tar_unix.Archive.multicast_n_string buffer [ ofd ] (Int64.to_int length);
+	    let csum = Sha1.to_hex (Sha1.string buffer) in
+	
 	    checksum_table := (file_name, csum) :: !checksum_table;
 
 	    Tar_unix.Archive.skip ifd (Tar_unix.Header.compute_zero_padding_length hdr);
diff --git a/opam b/opam
index ecb5a63ace8..cda68ac8169 100644
--- a/opam
+++ b/opam
@@ -32,6 +32,7 @@ depends: [
   "opasswd" {>= "0.9.3"}
   "xapi-rrdd-plugin"
   "pci" {>= "0.2.0"}
+  "sha"
 ]
 depexts: [
   [["centos"] ["pam-devel"]]