diff --git a/bin/dune b/bin/dune
index 92a5c5a..f7f3805 100644
--- a/bin/dune
+++ b/bin/dune
@@ -1,5 +1,4 @@
 (executables
- (names main)
- (public_names fat)
- (libraries fat-filesystem mirage-block-unix cmdliner
-   io-page-unix))
+ (names main shell)
+ (public_names fat shell)
+ (libraries fat-filesystem mirage-block-unix cmdliner))
diff --git a/bin/impl.ml b/bin/impl.ml
index f28cfb7..04f362f 100644
--- a/bin/impl.ml
+++ b/bin/impl.ml
@@ -22,7 +22,7 @@ open Common
    have specific code to handle is to 'fail' the Lwt thread with
    the exception Failure "user-readable message". *)
 
-let fail fmt = Fmt.kstrf Lwt.fail_with fmt
+let fail fmt = Fmt.kstr Lwt.fail_with fmt
 
 let (>>*=) m f = m >>= function
   | Error e -> fail "%a" Filesystem.pp_write_error (e :> Filesystem.write_error)
@@ -180,7 +180,7 @@ let cat common filename path =
     let rec loop offset =
       Filesystem.read fs path offset 1024 >>*= fun bufs ->
       List.iter (fun x -> print_string (Cstruct.to_string x)) bufs;
-      let copied = List.fold_left (+) 0 (List.map Cstruct.len bufs) in
+      let copied = List.fold_left (+) 0 (List.map Cstruct.length bufs) in
       if copied < 1024
       then Lwt.return ()
       else loop (offset + copied)
diff --git a/bin/main.ml b/bin/main.ml
index 3bdfc93..365eed1 100644
--- a/bin/main.ml
+++ b/bin/main.ml
@@ -42,7 +42,7 @@ let common_options_t =
   let unbuffered =
     let doc = "Use unbuffered I/O (via O_DIRECT)." in
     Arg.(value & flag & info ["unbuffered"] ~docs ~doc) in
-  Term.(pure Common.make $ debug $ verb $ unbuffered)
+  Term.(const Common.make $ debug $ verb $ unbuffered)
 
 let filename =
   let doc = Printf.sprintf "Path to the FAT image file." in
@@ -65,8 +65,10 @@ let create_cmd =
   let size =
     let doc = "Size of the image" in
     Arg.(value & pos 1 string "16MiB" & info [] ~doc) in
-  Term.(ret(pure Impl.create $ common_options_t $ filename $ size)),
-  Term.info "create" ~sdocs:_common_options ~doc ~man
+  let term = Term.(ret(const Impl.create $ common_options_t $ filename $ size))
+  and info = Cmd.info "create" ~sdocs:_common_options ~doc ~man
+  in
+  Cmd.v info term
 
 let add_cmd =
   let doc = "add files to a FAT image" in
@@ -75,8 +77,10 @@ let add_cmd =
     `P "Add a set of files to a FAT filesystem image."
   ] @ help in
   let files = Arg.(non_empty & pos_all file [] & info [] ~docv:"FILE") in
-  Term.(ret(pure Impl.add $ common_options_t $ filename $ files)),
-  Term.info "add" ~sdocs:_common_options ~doc ~man
+  let term = Term.(ret(const Impl.add $ common_options_t $ filename $ files))
+  and info = Cmd.info "add" ~sdocs:_common_options ~doc ~man
+  in
+  Cmd.v info term
 
 let list_cmd =
   let doc = "list files in a FAT image" in
@@ -84,8 +88,10 @@ let list_cmd =
     `S "DESCRIPTION";
     `P "List the set of files contained within a FAT filesystem image."
   ] @ help in
-  Term.(ret(pure Impl.list $ common_options_t $ filename)),
-  Term.info "list" ~sdocs:_common_options ~doc ~man
+  let term = Term.(ret(const Impl.list $ common_options_t $ filename))
+  and info = Cmd.info "list" ~sdocs:_common_options ~doc ~man
+  in
+  Cmd.v info term
 
 let cat_cmd =
   let doc = "extract a single file from a FAT image" in
@@ -93,18 +99,21 @@ let cat_cmd =
     `S "DESCRIPTION";
     `P "extract a single file from a FAT filesystem image."
   ] @ help in
-  Term.(ret(pure Impl.cat $ common_options_t $ filename $ path)),
-  Term.info "cat" ~sdocs:_common_options ~doc ~man
+  let term = Term.(ret(const Impl.cat $ common_options_t $ filename $ path))
+  and info = Cmd.info "cat" ~sdocs:_common_options ~doc ~man
+  in
+  Cmd.v info term
 
 let default_cmd =
-  let doc = "manipulate FAT filesystem images" in
-  let man = help in
-  Term.(ret (pure (fun _ -> `Help (`Pager, None)) $ common_options_t)),
-  Term.info "fat" ~version:"1.0.0" ~sdocs:_common_options ~doc ~man
+  Term.(ret (const (fun _ -> `Help (`Pager, None)) $ common_options_t))
 
 let cmds = [create_cmd; add_cmd; list_cmd; cat_cmd]
 
-let _ =
-  match Term.eval_choice default_cmd cmds with
-  | `Error _ -> exit 1
-  | _ -> exit 0
+let () =
+  let doc = "manipulate FAT filesystem images" in
+  let man = help in
+  let info =
+    Cmd.info "fat" ~version:"%%VERSION%%" ~sdocs:_common_options ~doc ~man
+  in
+  let group = Cmd.group ~default:default_cmd info cmds in
+  exit (Cmd.eval group)
diff --git a/bin/shell.ml b/bin/shell.ml
index 58ac1d3..5be4cd1 100644
--- a/bin/shell.ml
+++ b/bin/shell.ml
@@ -1,8 +1,7 @@
 (* This is a toplevel-like test program *)
 open Lwt.Infix
-open Mirage_fs
 
-module Test = Fat.FS(Block)
+module Test = Fat.Make(Block)
 
 let with_file flags filename f =
   Lwt_unix.openfile filename flags 0o0 >>= fun file ->
@@ -14,7 +13,7 @@ let with_file flags filename f =
       Lwt_unix.close file >>= fun () ->
       Lwt.fail x)
 
-let fail fmt = Fmt.kstrf Lwt.fail_with fmt
+let fail fmt = Fmt.kstr Lwt.fail_with fmt
 
 let (>>|=) m f = m >>= function
   | Error e -> fail "%a" Test.pp_error e
@@ -58,7 +57,7 @@ let main filename _create_size =
       let n = ref 0 in
       List.iter (fun buf ->
           Printf.printf "%s" (Cstruct.to_string buf);
-          n := !n + (Cstruct.len buf)
+          n := !n + (Cstruct.length buf)
         ) datas;
       Printf.printf "\n%!";
       if !n <> file_size
@@ -160,10 +159,10 @@ let main filename _create_size =
     inner (snd(parse_path x))
   in
 
-  let space = Re_str.regexp_string " " in
+  let space = Re.Str.regexp_string " " in
   let rec loop () =
     Printf.printf "A:%s> %!" (Path.to_string !cwd);
-    match Re_str.split space (input_line stdin) with
+    match Re.Str.split space (input_line stdin) with
     | [ "dir" ] -> do_dir "" >>= loop
     | [ "dir"; path ] -> do_dir path >>= loop
     | [ "cd"; path ] -> do_cd path >>= loop
diff --git a/fat-filesystem.opam b/fat-filesystem.opam
index ee1c03d..618acb1 100644
--- a/fat-filesystem.opam
+++ b/fat-filesystem.opam
@@ -8,23 +8,23 @@ bug-reports: "https://github.com/mirage/ocaml-fat/issues"
 depends: [
   "ocaml" {>= "4.06.0"}
   "dune" {>= "1.0"}
-  "cstruct" {>= "3.0.0"}
+  "cstruct" {>= "6.0.0"}
   "ppx_cstruct"
   "rresult"
   "lwt" {>= "2.4.3"}
-  "mirage-block" {>= "2.0.0"}
-  "mirage-block-unix" {>= "2.5.0"}
+  "mirage-block" {>= "3.0.0"}
+  "mirage-block-unix" {>= "2.13.0"}
   "mirage-kv"
   "mirage-block-combinators" {with-test}
-  "io-page-unix" {>= "2.0.0"}
-  "io-page" {>= "2.0.0"}
+  "io-page" {>= "2.4.0"}
   "re" {>= "1.7.2"}
-  "cmdliner"
+  "cmdliner" {>= "1.1.0"}
   "astring"
+  "fmt" {>= "0.8.7"}
   "alcotest" {with-test}
 ]
 build: [
-  ["dune" "subst"] {pinned}
+  ["dune" "subst"] {dev}
   ["dune" "build" "-p" name "-j" jobs]
   ["dune" "runtest" "-p" name] {with-test}
 ]
diff --git a/src/fat.ml b/src/fat.ml
index 24f8d41..ad2b368 100644
--- a/src/fat.ml
+++ b/src/fat.ml
@@ -89,8 +89,8 @@ module Make (B: Mirage_block.S) = struct
   let read_sectors bps device xs =
     let buf = alloc (List.length xs * bps) in
     let rec split buf =
-      if Cstruct.len buf = 0 then []
-      else if Cstruct.len buf <= bps then [ buf ]
+      if Cstruct.length buf = 0 then []
+      else if Cstruct.length buf <= bps then [ buf ]
       else Cstruct.sub buf 0 bps :: (split (Cstruct.shift buf bps))
     in
     let page = alloc bps in
@@ -146,7 +146,7 @@ module Make (B: Mirage_block.S) = struct
     let fat = Fat_entry.make boot format in
     let root_sectors = Fat_boot_sector.sectors_of_root_dir boot in
     let root = alloc (List.length root_sectors * 512) in
-    for i = 0 to Cstruct.len root - 1 do Cstruct.set_uint8 root i 0 done;
+    for i = 0 to Cstruct.length root - 1 do Cstruct.set_uint8 root i 0 done;
     let fs = { boot = boot; format = format; fat = fat; root = root } in
     Ok fs
 
@@ -175,12 +175,12 @@ module Make (B: Mirage_block.S) = struct
     let get_fs sector =
       match Fat_boot_sector.unmarshal sector with
       | Error reason ->
-        Fmt.kstrf Lwt.fail_with
+        Fmt.kstr Lwt.fail_with
           "error unmarshalling first sector of block device: %s" reason
       | Ok boot ->
         match Fat_boot_sector.detect_format boot with
         | Error reason ->
-          Fmt.kstrf Lwt.fail_with
+          Fmt.kstr Lwt.fail_with
             "error detecting the format of block device: %s" reason
         | Ok format -> Lwt.return (boot, format)
     in
@@ -200,7 +200,7 @@ module Make (B: Mirage_block.S) = struct
     >>= function
     | Ok t    -> Lwt.return t
     | Error e ->
-      Fmt.kstrf Lwt.fail_with "error reading essential sectors: %a" pp_error e
+      Fmt.kstr Lwt.fail_with "error reading essential sectors: %a" pp_error e
 
   let disconnect _ = Lwt.return ()
 
@@ -496,7 +496,7 @@ module Make (B: Mirage_block.S) = struct
       | Ok buffer    ->
         let buffer =
           Cstruct.sub buffer preceeding
-            (Cstruct.len buffer - preceeding - succeeding)
+            (Cstruct.length buffer - preceeding - succeeding)
         in
         Ok [ buffer ]
 
diff --git a/src/fat.mli b/src/fat.mli
index cb5df23..0be4dfa 100644
--- a/src/fat.mli
+++ b/src/fat.mli
@@ -48,7 +48,12 @@ module Make (B: Mirage_block.S): sig
   val pp_write_error: write_error Fmt.t
   (** [pp_write_error] is the pretty-printer for write errors. *)
 
-  include Mirage_device.S
+  type t
+  (** The type representing the internal state of the device *)
+
+  val disconnect: t -> unit Lwt.t
+  (** Disconnect from the device. While this might take some time to
+      complete, it can never result in an error. *)
 
   val connect : B.t -> t Lwt.t
   val format : B.t -> int64 -> (t, write_error) result Lwt.t
diff --git a/src/fat_boot_sector.ml b/src/fat_boot_sector.ml
index 73451b1..7d75a22 100644
--- a/src/fat_boot_sector.ml
+++ b/src/fat_boot_sector.ml
@@ -60,7 +60,7 @@ let _ = assert(sizeof = 512)
 let fat_id = 0xf8 (* fixed disk *)
 
 let marshal (buf: Cstruct.t) t =
-  for i = 0 to Cstruct.len buf - 1 do
+  for i = 0 to Cstruct.length buf - 1 do
     Cstruct.set_uint8 buf i 0
   done;
   set_t_oem_name t.oem_name 0 buf;
@@ -81,8 +81,8 @@ let marshal (buf: Cstruct.t) t =
 
 let unmarshal (buf: Cstruct.t) : (t, string) result =
   let open Rresult in
-  ( if Cstruct.len buf < sizeof
-    then Error (Printf.sprintf "boot sector too small: %d < %d" (Cstruct.len buf) sizeof)
+  ( if Cstruct.length buf < sizeof
+    then Error (Printf.sprintf "boot sector too small: %d < %d" (Cstruct.length buf) sizeof)
     else Ok () ) >>= fun () ->
   let signature = get_t_signature buf in
   ( if signature <> 0xaa55
diff --git a/src/fat_entry.ml b/src/fat_entry.ml
index 9530474..7911fb4 100644
--- a/src/fat_entry.ml
+++ b/src/fat_entry.ml
@@ -29,7 +29,7 @@ let to_string = function
 type fat = Cstruct.t
 
 let of_fat16 n fat =
-  if Cstruct.len fat < (2 * n + 2)
+  if Cstruct.length fat < (2 * n + 2)
   then Bad
   else
     let x = Cstruct.LE.get_uint16 fat (2 * n) in
@@ -44,7 +44,7 @@ let to_fat16 n fat x =
   Cstruct.LE.set_uint16 fat (2 * n) x'
 
 let of_fat32 n fat =
-  if Cstruct.len fat < (4 * n + 4)
+  if Cstruct.length fat < (4 * n + 4)
   then Bad
   else
     let x = Cstruct.LE.get_uint32 fat (4 * n) in
diff --git a/src/fat_name.ml b/src/fat_name.ml
index b73e2ff..7700cc7 100644
--- a/src/fat_name.ml
+++ b/src/fat_name.ml
@@ -331,7 +331,7 @@ let sizeof = sizeof_name
 let _ = assert(sizeof_lfn = sizeof_name)
 
 let unmarshal buf =
-  if Cstruct.len buf = 0
+  if Cstruct.length buf = 0
   then End
   else if get_lfn_v_0f buf = 0x0f then begin
     let seq = get_lfn_seq buf in
@@ -392,7 +392,7 @@ let unmarshal buf =
   end
 
 let marshal (buf: Cstruct.t) t =
-  for i = 0 to Cstruct.len buf - 1 do
+  for i = 0 to Cstruct.length buf - 1 do
     Cstruct.set_uint8 buf i 0
   done;
   match t with
@@ -450,7 +450,7 @@ let marshal (buf: Cstruct.t) t =
 
 let blocks buf =
   let rec loop acc offset remaining =
-    if Cstruct.len remaining < sizeof (* ignore extra space at the end *)
+    if Cstruct.length remaining < sizeof (* ignore extra space at the end *)
     then List.rev acc
     else
       let this = offset, (Cstruct.sub remaining 0 sizeof) in
@@ -504,7 +504,7 @@ let next bits =
     Note the update may be beyond the end of [block] indicating more space needs
     to be allocated. *)
 let add block r =
-  let after_block = Cstruct.len block in
+  let after_block = Cstruct.length block in
   let next_byte = match next block with
     | Some b -> b
     | None -> after_block in
diff --git a/src/fat_sector_map.ml b/src/fat_sector_map.ml
index c1d944c..1576f0e 100644
--- a/src/fat_sector_map.ml
+++ b/src/fat_sector_map.ml
@@ -40,13 +40,13 @@ let byte_range bps start len =
 
 let clip (preceeding, _, succeeding) = function
   | [] -> []
-  | [c] -> [Cstruct.sub c preceeding (Cstruct.len c - succeeding - preceeding)]
+  | [c] -> [Cstruct.sub c preceeding (Cstruct.length c - succeeding - preceeding)]
   | c :: cs ->
     let cs' = List.rev cs in
     let last = List.hd cs' in
     let middle' = List.tl cs' in
-    let last = Cstruct.sub last 0 (Cstruct.len last - succeeding) in
-    let head = Cstruct.sub c preceeding (Cstruct.len c - preceeding) in
+    let last = Cstruct.sub last 0 (Cstruct.length last - succeeding) in
+    let head = Cstruct.sub c preceeding (Cstruct.length c - preceeding) in
     head :: (List.rev middle') @ [ last ]
 
 let compose a b =
diff --git a/src/fat_update.ml b/src/fat_update.ml
index 46e1f98..6e1a2a6 100644
--- a/src/fat_update.ml
+++ b/src/fat_update.ml
@@ -20,11 +20,11 @@ type data =
 
 let bytes = function
   | String x -> String.length x
-  | Cstruct x -> Cstruct.len x
+  | Cstruct x -> Cstruct.length x
 
 let string_of_data = function
   | String x -> Printf.sprintf "(string of length %d)" (String.length x)
-  | Cstruct x -> Printf.sprintf "(Cstruct.t of length %d)" (Cstruct.len x)
+  | Cstruct x -> Printf.sprintf "(Cstruct.t of length %d)" (Cstruct.length x)
 
 type t = {
   offset: int64;
@@ -48,7 +48,7 @@ let apply buf { offset; data } =
   let offset = Int64.to_int offset in
   match data with
   | String x -> Cstruct.blit_from_string x 0 buf offset (String.length x)
-  | Cstruct x -> Cstruct.blit x 0 buf offset (Cstruct.len x)
+  | Cstruct x -> Cstruct.blit x 0 buf offset (Cstruct.length x)
 
 let sub offset length =
   let offset = Int64.to_int offset in
diff --git a/test/dune b/test/dune
index 9bf6a80..f1f3b21 100644
--- a/test/dune
+++ b/test/dune
@@ -1,5 +1,5 @@
 (test
  (libraries fat-filesystem lwt lwt.unix cstruct alcotest
-   mirage-block-unix io-page.unix mirage-block mirage-block-combinators)
+   mirage-block-unix mirage-block mirage-block-combinators)
  (name test)
  (deps (glob_files *.dat)))
diff --git a/test/test.ml b/test/test.ml
index 51c6695..e26849e 100644
--- a/test/test.ml
+++ b/test/test.ml
@@ -19,7 +19,7 @@ open Block
 
 module MemFS = Fat.Make(Mirage_block_combinators.Mem)
 
-let fail fmt = Fmt.kstrf Lwt.fail_with fmt
+let fail fmt = Fmt.kstr Lwt.fail_with fmt
 
 let (>>*=) m f = m >>= function
   | Error e -> fail "%a" MemFS.pp_write_error (e :> MemFS.write_error)
@@ -213,7 +213,7 @@ let make_pattern tag length =
   assert (String.length tag <= length);
   let buffer = alloc length in
   Cstruct.blit_from_string tag 0 buffer 0 (String.length tag);
-  for i = String.length tag to Cstruct.len buffer - 1 do
+  for i = String.length tag to Cstruct.length buffer - 1 do
     Cstruct.set_char buffer i tag.[i mod (String.length tag)]
   done;
   buffer
@@ -285,7 +285,7 @@ let test_read () =
     let buffer = make_pattern "basic writing test " length in
     MemFS.write fs filename 0 buffer >>*= fun () ->
     MemFS.read fs filename 0 length >>*= fun buffers ->
-    let count buffers = List.fold_left (+) 0 (List.map Cstruct.len buffers) in
+    let count buffers = List.fold_left (+) 0 (List.map Cstruct.length buffers) in
     Alcotest.(check int) __LOC__ length (count buffers);
     MemFS.read fs filename 0 (length * 2) >>*= fun buffers ->
     Alcotest.(check int) __LOC__ length (count buffers);
@@ -324,7 +324,7 @@ let test_write ((filename: string), (_offset, length)) () =
     let cstruct =
       Alcotest.testable
         (fun ppf x ->
-           Fmt.pf ppf "\"%s\"(%d)" (Cstruct.to_string x) (Cstruct.len x)
+           Fmt.pf ppf "\"%s\"(%d)" (Cstruct.to_string x) (Cstruct.length x)
         ) Cstruct.equal
     in
     Alcotest.(check cstruct) __LOC__ buffer (List.hd buffers);