diff --git a/dune-project b/dune-project
index 9e7f1f9b299..8d329288de3 100644
--- a/dune-project
+++ b/dune-project
@@ -177,7 +177,9 @@
   (xapi-idl
    (= :version))
   (xapi-types
-   (= :version)))
+   (= :version))
+  (xapi-stdext-zerocheck
+    (= :version)))
  (synopsis "A CLI for xapi storage services")
  (description
   "The CLI allows you to directly manipulate virtual disk images, without them being attached to VMs."))
@@ -322,6 +324,7 @@
   xapi-types
   xapi-stdext-pervasives
   xapi-stdext-unix
+  xapi-stdext-zerocheck
   xen-api-client
   xen-api-client-lwt
   xenctrl
@@ -874,4 +877,5 @@
  (synopsis "Xapi's standard library extension, Zerocheck")
  (authors "Jonathan Ludlam")
  (depends
+  (alcotest :with-test)
   (odoc :with-doc)))
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.ml
new file mode 100644
index 00000000000..52897ee2a0c
--- /dev/null
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.ml
@@ -0,0 +1,130 @@
+open Bechamel
+open Toolkit
+module XString = Xapi_stdext_std.Xstringext.String
+
+(* Test data generators *)
+let make_string len = String.init len (fun i -> Char.chr (33 + (i mod 94)))
+
+let escape_rules =
+  [('a', "[A]"); ('e', "[E]"); ('i', "[I]"); ('o', "[O]"); ('u', "[U]")]
+
+(* Reference implementation from xstringext_test.ml *)
+let escaped_spec ?rules string =
+  match rules with
+  | None ->
+      String.escaped string
+  | Some rules ->
+      let apply_rules char =
+        match List.assoc_opt char rules with
+        | None ->
+            Seq.return char
+        | Some replacement ->
+            String.to_seq replacement
+      in
+      string |> String.to_seq |> Seq.concat_map apply_rules |> String.of_seq
+
+let escaped_benchmark n =
+  let s = make_string n in
+  Staged.stage @@ fun () -> ignore (XString.escaped ~rules:escape_rules s)
+
+let escaped_spec_benchmark n =
+  let s = make_string n in
+  Staged.stage @@ fun () -> ignore (escaped_spec ~rules:escape_rules s)
+
+let test_escaped =
+  Test.make_indexed ~name:"escaped" ~fmt:"%s %d" ~args:[100; 500; 1000]
+    escaped_benchmark
+
+let test_escaped_spec =
+  Test.make_indexed ~name:"escaped-spec" ~fmt:"%s %d" ~args:[100; 500; 1000]
+    escaped_spec_benchmark
+
+let benchmark () =
+  let ols =
+    Analyze.ols ~bootstrap:0 ~r_square:true ~predictors:Measure.[|run|]
+  in
+  let instances =
+    Instance.[minor_allocated; major_allocated; monotonic_clock]
+  in
+  let cfg =
+    Benchmark.cfg ~limit:2000 ~quota:(Time.second 0.5) ~kde:(Some 1000) ()
+  in
+  let test =
+    Test.make_grouped ~name:"escaped-comparison"
+      [test_escaped; test_escaped_spec]
+  in
+  let raw_results = Benchmark.all cfg instances test in
+  let results =
+    List.map (fun instance -> Analyze.all ols instance raw_results) instances
+  in
+  let results = Analyze.merge ols instances results in
+  (results, raw_results)
+
+let () =
+  let all_results = benchmark () in
+  let results, _ = all_results in
+
+  (* Extract timing data from the actual benchmark results *)
+  let result_groups =
+    Hashtbl.fold
+      (fun _ v a -> Hashtbl.fold (fun k v a -> (k, v) :: a) v [] :: a)
+      results []
+  in
+
+  (* Find the monotonic-clock result group (timing data) *)
+  let timing_group =
+    match result_groups with _ :: _ :: timing :: _ -> Some timing | _ -> None
+  in
+
+  let get_timing test_name =
+    match timing_group with
+    | None ->
+        None
+    | Some group -> (
+      match List.assoc_opt test_name group with
+      | Some estimator -> (
+          let estimates = Analyze.OLS.estimates estimator in
+          match estimates with Some (x :: _) -> Some x | _ -> None
+        )
+      | None ->
+          None
+    )
+  in
+
+  Printf.printf "\n=== Performance Comparison: Optimized vs Reference ===\n\n" ;
+
+  let sizes = ["100"; "500"; "1000"] in
+  List.iter
+    (fun size ->
+      Printf.printf "String size %s:\n" size ;
+      let opt_test = Printf.sprintf "escaped-comparison/escaped %s" size in
+      let ref_test = Printf.sprintf "escaped-comparison/escaped-spec %s" size in
+      match (get_timing opt_test, get_timing ref_test) with
+      | Some opt_time, Some ref_time ->
+          let improvement = (ref_time -. opt_time) /. ref_time *. 100.0 in
+          Printf.printf "  Optimized: %.3f μs\n" opt_time ;
+          Printf.printf "  Reference: %.3f μs\n" ref_time ;
+          Printf.printf "  Improvement: %.1f%% %s\n\n" improvement
+            (if improvement > 0.0 then "faster" else "slower")
+      | None, _ ->
+          Printf.printf "  Optimized implementation data missing\n\n"
+      | _, None ->
+          Printf.printf "  Reference implementation data missing\n\n"
+    )
+    sizes ;
+
+  Printf.printf "\n=== Detailed Results ===\n" ;
+  match result_groups with
+  | [results] ->
+      let print (k, ols) = Fmt.pr "%s: %a\n%!" k Analyze.OLS.pp ols in
+      List.iter print results
+  | results_list ->
+      Printf.printf "Results structure: %d result groups\n"
+        (List.length results_list) ;
+      List.iteri
+        (fun i results ->
+          Printf.printf "Result group %d:\n" i ;
+          let print (k, ols) = Fmt.pr "  %s: %a\n%!" k Analyze.OLS.pp ols in
+          List.iter print results
+        )
+        results_list
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.mli b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.mli
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/dune b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/dune
new file mode 100644
index 00000000000..27467a09029
--- /dev/null
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/dune
@@ -0,0 +1,6 @@
+(executable
+ (name bench_xstringext)
+ (modes exe)
+ (optional)
+ (libraries bechamel xapi-stdext-std bechamel-notty notty.unix fmt)
+)
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/dune b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/dune
index 67d48233bc4..d869973d411 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/dune
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/dune
@@ -7,5 +7,5 @@
   (names xstringext_test listext_test)
   (package xapi-stdext-std)
   (modules xstringext_test listext_test)
-  (libraries xapi_stdext_std fmt alcotest)
+  (libraries xapi_stdext_std fmt alcotest qcheck-core qcheck-alcotest)
 )
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml
index 16f60dedbae..4e5379d7b36 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml
@@ -42,21 +42,6 @@ module String = struct
   (** Returns true for whitespace characters, false otherwise *)
   let isspace = function ' ' | '\n' | '\r' | '\t' -> true | _ -> false
 
-  let escaped ?rules string =
-    match rules with
-    | None ->
-        String.escaped string
-    | Some rules ->
-        let aux h t =
-          ( if List.mem_assoc h rules then
-              List.assoc h rules
-            else
-              of_char h
-          )
-          :: t
-        in
-        concat "" (fold_right aux string [])
-
   let split_f p str =
     let split_one seq =
       let not_p c = not (p c) in
@@ -193,6 +178,13 @@ module String = struct
     ) else
       s
 
+  let escaped ?rules s =
+    match rules with
+    | None ->
+        String.escaped s
+    | Some rules ->
+        map_unlikely s (fun c -> List.assoc_opt c rules)
+
   let sub_to_end s start =
     let length = String.length s in
     String.sub s start (length - start)
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml
index 145ce632bbc..9b7eb2674a1 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml
@@ -147,6 +147,49 @@ let test_rtrim =
   in
   ("rtrim", List.map test spec)
 
+(** Simple implementation of escaped for testing against *)
+let escaped_spec ?rules string =
+  match rules with
+  | None ->
+      String.escaped string
+  | Some rules ->
+      let apply_rules char =
+        match List.assoc_opt char rules with
+        | None ->
+            Seq.return char
+        | Some replacement ->
+            String.to_seq replacement
+      in
+      string |> String.to_seq |> Seq.concat_map apply_rules |> String.of_seq
+
+let test_escaped =
+  let open QCheck2 in
+  (* Generator for escape rules: list of (char, string) mappings *)
+  let gen_rules =
+    let open Gen in
+    let gen_rule = pair char (string_size (int_range 0 5) ~gen:char) in
+    list gen_rule
+  in
+  (* Generator for test input: string and optional rules *)
+  let gen_input = Gen.pair Gen.string (Gen.opt gen_rules) in
+  let property (s, rules) =
+    let expected = escaped_spec ?rules s in
+    let actual = XString.escaped ?rules s in
+    String.equal expected actual
+  in
+  let test =
+    Test.make ~name:"escaped matches reference implementation" ~count:1000
+      gen_input property
+  in
+  ("escaped", [QCheck_alcotest.to_alcotest test])
+
 let () =
   Alcotest.run "Xstringext"
-    [test_rev_map; test_split; test_split_f; test_has_substr; test_rtrim]
+    [
+      test_rev_map
+    ; test_split
+    ; test_split_f
+    ; test_has_substr
+    ; test_rtrim
+    ; test_escaped
+    ]
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.mli b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.mli
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/ocaml/quicktest/quicktest_vdi_ops_data_integrity.ml b/ocaml/quicktest/quicktest_vdi_ops_data_integrity.ml
index 5b385d9b34e..01d830b8789 100644
--- a/ocaml/quicktest/quicktest_vdi_ops_data_integrity.ml
+++ b/ocaml/quicktest/quicktest_vdi_ops_data_integrity.ml
@@ -76,53 +76,107 @@ let check_vdi_unchanged rpc session_id ~vdi_size ~prepare_vdi ~vdi_op sr_info ()
       )
   )
 
+let check_vdi_delta rpc session_id ~vdi_size ~prepare_vdi ~prepare_vdi_base
+    ~vdi_op sr_info () =
+  let sR = sr_info.Qt.sr in
+  Qt.VDI.with_new ~virtual_size:vdi_size rpc session_id sR
+  @@ fun vdi_original ->
+  Qt.VDI.with_new ~virtual_size:vdi_size rpc session_id sR @@ fun base_vdi ->
+  prepare_vdi rpc session_id vdi_original ;
+  let checksum_original = checksum rpc session_id vdi_original in
+  prepare_vdi_base rpc session_id base_vdi ;
+
+  vdi_op rpc session_id ~vdi:vdi_original ~base_vdi ;
+  let checksum_copy = checksum rpc session_id base_vdi in
+  if checksum_copy <> checksum_original then
+    failwith
+      (Printf.sprintf
+         "New VDI (checksum: %s) has different data than original (checksum: \
+          %s)."
+         checksum_copy checksum_original
+      )
+
 let copy_vdi rpc session_id sr vdi =
   Client.Client.VDI.copy ~rpc ~session_id ~vdi ~base_vdi:API.Ref.null
     ~into_vdi:API.Ref.null ~sr
 
-let export_import_vdi rpc session_id ~exportformat sR vdi =
-  let vdi_uuid = Client.Client.VDI.get_uuid ~rpc ~session_id ~self:vdi in
+let export_vdi_to_file ~rpc ~session_id ~exportformat ?base_vdi ~vdi () =
+  let get_uuid vdi = Client.Client.VDI.get_uuid ~rpc ~session_id ~self:vdi in
+  let vdi_uuid = get_uuid vdi in
+  let base_vdi_uuid = Option.map get_uuid base_vdi in
   let file = "/tmp/quicktest_export_" ^ vdi_uuid in
+  Qt.cli_cmd
+    ([
+       "vdi-export"
+     ; "uuid=" ^ vdi_uuid
+     ; "filename=" ^ file
+     ; "format=" ^ exportformat
+     ]
+    @ match base_vdi_uuid with None -> [] | Some x -> ["base=" ^ x]
+    )
+  |> ignore ;
+  file
+
+let create_new_vdi ~rpc ~session_id ~sR ~vdi =
+  let virtual_size =
+    Client.Client.VDI.get_virtual_size ~rpc ~session_id ~self:vdi
+  in
+  let new_vdi =
+    Client.Client.VDI.create ~rpc ~session_id ~name_label:""
+      ~name_description:"" ~sR ~virtual_size ~_type:`user ~sharable:false
+      ~read_only:false ~other_config:[] ~xenstore_data:[] ~sm_config:[] ~tags:[]
+  in
+  let new_vdi_uuid =
+    Client.Client.VDI.get_uuid ~rpc ~session_id ~self:new_vdi
+  in
+  (new_vdi_uuid, new_vdi)
+
+let import_file_into_vdi ~file ~vdi_uuid ~exportformat =
   Qt.cli_cmd
     [
-      "vdi-export"
+      "vdi-import"
     ; "uuid=" ^ vdi_uuid
     ; "filename=" ^ file
     ; "format=" ^ exportformat
     ]
-  |> ignore ;
+  |> ignore
+
+let export_import_vdi rpc session_id ~exportformat sR vdi =
+  let file = export_vdi_to_file ~rpc ~session_id ~exportformat ~vdi () in
   Xapi_stdext_pervasives.Pervasiveext.finally
     (fun () ->
-      let virtual_size =
-        Client.Client.VDI.get_virtual_size ~rpc ~session_id ~self:vdi
-      in
-      let new_vdi =
-        Client.Client.VDI.create ~rpc ~session_id ~name_label:""
-          ~name_description:"" ~sR ~virtual_size ~_type:`user ~sharable:false
-          ~read_only:false ~other_config:[] ~xenstore_data:[] ~sm_config:[]
-          ~tags:[]
-      in
-      let new_vdi_uuid =
-        Client.Client.VDI.get_uuid ~rpc ~session_id ~self:new_vdi
-      in
-      Qt.cli_cmd
-        [
-          "vdi-import"
-        ; "uuid=" ^ new_vdi_uuid
-        ; "filename=" ^ file
-        ; "format=" ^ exportformat
-        ]
-      |> ignore ;
+      let new_vdi_uuid, new_vdi = create_new_vdi ~rpc ~session_id ~sR ~vdi in
+      import_file_into_vdi ~file ~vdi_uuid:new_vdi_uuid ~exportformat ;
       new_vdi
     )
     (fun () -> Sys.remove file)
 
+let export_delta_import_vdi rpc session_id ~exportformat ~vdi ~base_vdi =
+  let file =
+    export_vdi_to_file ~rpc ~session_id ~exportformat ~vdi ~base_vdi ()
+  in
+  Xapi_stdext_pervasives.Pervasiveext.finally
+    (fun () ->
+      (* Import delta on top of base_vdi *)
+      let base_uuid =
+        Client.Client.VDI.get_uuid ~rpc ~session_id ~self:base_vdi
+      in
+      import_file_into_vdi ~file ~vdi_uuid:base_uuid ~exportformat
+    )
+    (fun () -> Sys.remove file)
+
 let export_import_raw = export_import_vdi ~exportformat:"raw"
 
 let export_import_vhd = export_import_vdi ~exportformat:"vhd"
 
 let export_import_tar = export_import_vdi ~exportformat:"tar"
 
+let export_import_qcow = export_import_vdi ~exportformat:"qcow2"
+
+let delta_export_import_vhd = export_delta_import_vdi ~exportformat:"vhd"
+
+let delta_export_import_qcow = export_delta_import_vdi ~exportformat:"qcow2"
+
 let data_integrity_tests vdi_op op_name =
   [
     ( op_name ^ ": small empty VDI"
@@ -141,6 +195,47 @@ let data_integrity_tests vdi_op op_name =
     )
   ]
 
+let delta_data_integrity_tests vdi_op op_name =
+  [
+    ( op_name ^ ": delta between empty & empty VDI"
+    , `Slow
+    , check_vdi_delta
+        ~vdi_size:Sizes.(4L ** mib)
+        ~prepare_vdi:noop ~prepare_vdi_base:noop ~vdi_op
+    )
+  ; ( op_name ^ ": delta between random & empty VDI"
+    , `Slow
+    , check_vdi_delta
+        ~vdi_size:Sizes.(4L ** mib)
+        ~prepare_vdi:write_random_data ~prepare_vdi_base:noop ~vdi_op
+    )
+  ; ( op_name ^ ": delta between random & random VDI"
+    , `Slow
+    , check_vdi_delta
+        ~vdi_size:Sizes.(4L ** mib)
+        ~prepare_vdi:write_random_data ~prepare_vdi_base:write_random_data
+        ~vdi_op
+    )
+  ; ( op_name ^ ": delta between full and empty VDI"
+    , `Slow
+    , check_vdi_delta
+        ~vdi_size:Sizes.(4L ** mib)
+        ~prepare_vdi:fill ~prepare_vdi_base:noop ~vdi_op
+    )
+  ; ( op_name ^ ": delta between full and random VDI"
+    , `Slow
+    , check_vdi_delta
+        ~vdi_size:Sizes.(4L ** mib)
+        ~prepare_vdi:fill ~prepare_vdi_base:write_random_data ~vdi_op
+    )
+  ; ( op_name ^ ": delta between full and full VDI"
+    , `Slow
+    , check_vdi_delta
+        ~vdi_size:Sizes.(4L ** mib)
+        ~prepare_vdi:fill ~prepare_vdi_base:fill ~vdi_op
+    )
+  ]
+
 let large_data_integrity_tests vdi_op op_name =
   let b = Random.int64 16L in
   [
@@ -179,9 +274,21 @@ let tests () =
   @ (data_integrity_tests export_import_vhd "VDI export/import to/from VHD file"
     |> supported_srs
     )
+  @ (delta_data_integrity_tests delta_export_import_vhd
+       "VDI delta export/import to/from VHD file"
+    |> supported_srs
+    )
   @ (data_integrity_tests export_import_tar "VDI export/import to/from TAR file"
     |> supported_srs
     )
+  @ (data_integrity_tests export_import_qcow
+       "VDI export/import to/from QCOW file"
+    |> supported_srs
+    )
+  @ (delta_data_integrity_tests delta_export_import_qcow
+       "VDI delta export/import to/from QCOW file"
+    |> supported_srs
+    )
   @ (large_data_integrity_tests export_import_tar
        "VDI export/import to/from TAR file"
     |> supported_gfs2_srs
diff --git a/opam/xapi-debug.opam b/opam/xapi-debug.opam
index b5a017d9b8e..26871b97a7e 100644
--- a/opam/xapi-debug.opam
+++ b/opam/xapi-debug.opam
@@ -58,6 +58,7 @@ depends: [
   "xapi-types"
   "xapi-stdext-pervasives"
   "xapi-stdext-unix"
+  "xapi-stdext-zerocheck"
   "xen-api-client"
   "xen-api-client-lwt"
   "xenctrl"
diff --git a/opam/xapi-stdext-zerocheck.opam b/opam/xapi-stdext-zerocheck.opam
index c347d772146..2d856581f1c 100644
--- a/opam/xapi-stdext-zerocheck.opam
+++ b/opam/xapi-stdext-zerocheck.opam
@@ -8,6 +8,7 @@ homepage: "https://xapi-project.github.io/"
 bug-reports: "https://github.com/xapi-project/xen-api/issues"
 depends: [
   "dune" {>= "3.20"}
+  "alcotest" {with-test}
   "odoc" {with-doc}
 ]
 build: [
diff --git a/opam/xapi-storage-cli.opam b/opam/xapi-storage-cli.opam
index ee30d9c1c04..c91efa52615 100644
--- a/opam/xapi-storage-cli.opam
+++ b/opam/xapi-storage-cli.opam
@@ -17,6 +17,7 @@ depends: [
   "xapi-client" {= version}
   "xapi-idl" {= version}
   "xapi-types" {= version}
+  "xapi-stdext-zerocheck" {= version}
   "odoc" {with-doc}
 ]
 build: [
diff --git a/quality-gate.sh b/quality-gate.sh
index b2345f75ef7..6785610ff30 100755
--- a/quality-gate.sh
+++ b/quality-gate.sh
@@ -25,7 +25,7 @@ verify-cert () {
 }
 
 mli-files () {
-  N=461
+  N=460
   X="ocaml/tests"
   X+="|ocaml/quicktest"
   X+="|ocaml/message-switch/core_test"