diff --git a/lib_eio/file.ml b/lib_eio/file.ml
index 3353a714d..1b8e34179 100644
--- a/lib_eio/file.ml
+++ b/lib_eio/file.ml
@@ -72,6 +72,7 @@ module Pi = struct
 
     val pread : t -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
     val stat : t -> Stat.t
+    val seek : t -> Optint.Int63.t -> [`Set | `Cur | `End] -> Optint.Int63.t
     val close : t -> unit
   end
 
@@ -80,6 +81,8 @@ module Pi = struct
     include READ with type t := t
 
     val pwrite : t -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
+    val sync : t -> unit
+    val truncate : t -> Optint.Int63.t -> unit
   end
 
   type (_, _, _) Resource.pi +=
@@ -140,3 +143,15 @@ let pwrite_all (Resource.T (t, ops)) ~file_offset bufs =
     )
   in
   aux ~file_offset bufs
+
+let seek (Resource.T (t, ops)) off cmd =
+  let module X = (val (Resource.get ops Pi.Read)) in
+  X.seek t off cmd
+
+let sync (Resource.T (t, ops)) =
+  let module X = (val (Resource.get ops Pi.Write)) in
+  X.sync t
+
+let truncate (Resource.T (t, ops)) len =
+  let module X = (val (Resource.get ops Pi.Write)) in
+  X.truncate t len
diff --git a/lib_eio/file.mli b/lib_eio/file.mli
index 532c79f07..01bce61ae 100644
--- a/lib_eio/file.mli
+++ b/lib_eio/file.mli
@@ -1,5 +1,7 @@
 open Std
 
+(** {2 Types} *)
+
 (** Traditional Unix permissions. *)
 module Unix_perm : sig
   type t = int
@@ -54,30 +56,7 @@ type rw_ty = [ro_ty | Flow.sink_ty]
 type 'a rw = ([> rw_ty] as 'a) r
 (** A file opened for reading and writing. *)
 
-module Pi : sig
-  module type READ = sig
-    include Flow.Pi.SOURCE
-
-    val pread : t -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
-    val stat : t -> Stat.t
-    val close : t -> unit
-  end
-
-  module type WRITE = sig
-    include Flow.Pi.SINK
-    include READ with type t := t
-
-    val pwrite : t -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
-  end
-
-  type (_, _, _) Resource.pi +=
-    | Read : ('t, (module READ with type t = 't), [> ro_ty]) Resource.pi
-    | Write : ('t, (module WRITE with type t = 't), [> rw_ty]) Resource.pi
-
-  val ro : (module READ with type t = 't) -> ('t, ro_ty) Resource.handler
-
-  val rw : (module WRITE with type t = 't) -> ('t, rw_ty) Resource.handler
-end
+(** {2 Metadata} *)
 
 val stat : _ ro -> Stat.t
 (** [stat t] returns the {!Stat.t} record associated with [t]. *)
@@ -85,6 +64,8 @@ val stat : _ ro -> Stat.t
 val size : _ ro -> Optint.Int63.t
 (** [size t] returns the size of [t]. *)
 
+(** {2 Reading and writing} *)
+
 val pread : _ ro -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
 (** [pread t ~file_offset bufs] performs a single read of [t] at [file_offset] into [bufs].
 
@@ -108,3 +89,48 @@ val pwrite_single : _ rw -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
 
 val pwrite_all : _ rw -> file_offset:Optint.Int63.t -> Cstruct.t list -> unit
 (** [pwrite_all t ~file_offset bufs] writes all the data in [bufs] to location [file_offset] in [t]. *)
+
+val seek : _ ro -> Optint.Int63.t -> [`Set | `Cur | `End] -> Optint.Int63.t
+(** Set and/or get the current file position.
+
+    Like {!Unix.lseek}. *)
+
+val sync : _ rw -> unit
+(** Flush file buffers to disk.
+
+    Like {!Unix.fsync}. *)
+
+val truncate : _ rw -> Optint.Int63.t -> unit
+(** Set the length of a file.
+
+    Like {!Unix.ftruncate}. *)
+
+(** {2 Provider Interface} *)
+
+module Pi : sig
+  module type READ = sig
+    include Flow.Pi.SOURCE
+
+    val pread : t -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
+    val stat : t -> Stat.t
+    val seek : t -> Optint.Int63.t -> [`Set | `Cur | `End] -> Optint.Int63.t
+    val close : t -> unit
+  end
+
+  module type WRITE = sig
+    include Flow.Pi.SINK
+    include READ with type t := t
+
+    val pwrite : t -> file_offset:Optint.Int63.t -> Cstruct.t list -> int
+    val sync : t -> unit
+    val truncate : t -> Optint.Int63.t -> unit
+  end
+
+  type (_, _, _) Resource.pi +=
+    | Read : ('t, (module READ with type t = 't), [> ro_ty]) Resource.pi
+    | Write : ('t, (module WRITE with type t = 't), [> rw_ty]) Resource.pi
+
+  val ro : (module READ with type t = 't) -> ('t, ro_ty) Resource.handler
+
+  val rw : (module WRITE with type t = 't) -> ('t, rw_ty) Resource.handler
+end
diff --git a/lib_eio_linux/eio_linux.ml b/lib_eio_linux/eio_linux.ml
index aeb936bd9..93abbf218 100644
--- a/lib_eio_linux/eio_linux.ml
+++ b/lib_eio_linux/eio_linux.ml
@@ -192,6 +192,10 @@ module Flow = struct
   let recv_msg_with_fds t ~sw ~max_fds data =
     let _addr, n, fds = Low_level.recv_msg_with_fds t ~sw ~max_fds data in
     n, fds
+
+  let seek = Low_level.lseek
+  let sync = Low_level.fsync
+  let truncate = Low_level.ftruncate
 end
 
 let flow_handler = Eio_unix.Pi.flow_handler (module Flow)
diff --git a/lib_eio_linux/eio_linux.mli b/lib_eio_linux/eio_linux.mli
index 5c5202642..aa8cfa3d1 100644
--- a/lib_eio_linux/eio_linux.mli
+++ b/lib_eio_linux/eio_linux.mli
@@ -167,6 +167,21 @@ module Low_level : sig
       The entries are not returned in any particular order
       (not even necessarily the order in which Linux returns them). *)
 
+  val lseek : fd -> Optint.Int63.t -> [`Set | `Cur | `End] -> Optint.Int63.t
+  (** Set and/or get the current file position.
+
+      Like {!Unix.lseek}. *)
+
+  val fsync : fd -> unit
+  (** Flush file buffers to disk.
+
+      Like {!Unix.fsync}. *)
+
+  val ftruncate : fd -> Optint.Int63.t -> unit
+  (** Set the length of a file.
+
+      Like {!Unix.ftruncate}. *)
+
   (** {1 Sockets} *)
 
   val accept : sw:Switch.t -> fd -> (fd * Unix.sockaddr)
diff --git a/lib_eio_linux/low_level.ml b/lib_eio_linux/low_level.ml
index 613f23618..c1ed4f873 100644
--- a/lib_eio_linux/low_level.ml
+++ b/lib_eio_linux/low_level.ml
@@ -341,6 +341,27 @@ external eio_getrandom : Cstruct.buffer -> int -> int -> int = "caml_eio_getrand
 
 external eio_getdents : Unix.file_descr -> string list = "caml_eio_getdents"
 
+let lseek fd off cmd =
+  Fd.use_exn "lseek" fd @@ fun fd ->
+  let cmd =
+    match cmd with
+    | `Set -> Unix.SEEK_SET
+    | `Cur -> Unix.SEEK_CUR
+    | `End -> Unix.SEEK_END
+  in
+  Unix.LargeFile.lseek fd (Optint.Int63.to_int64 off) cmd
+  |> Optint.Int63.of_int64
+
+let fsync fd =
+  (* todo: https://github.com/ocaml-multicore/ocaml-uring/pull/103 *)
+  Eio_unix.run_in_systhread @@ fun () ->
+  Fd.use_exn "fsync" fd Unix.fsync
+
+let ftruncate fd len =
+  Eio_unix.run_in_systhread @@ fun () ->
+  Fd.use_exn "ftruncate" fd @@ fun fd ->
+  Unix.LargeFile.ftruncate fd (Optint.Int63.to_int64 len)
+
 let getrandom { Cstruct.buffer; off; len } =
   let rec loop n =
     if n = len then
diff --git a/lib_eio_posix/flow.ml b/lib_eio_posix/flow.ml
index d84c8796c..2588bf113 100644
--- a/lib_eio_posix/flow.ml
+++ b/lib_eio_posix/flow.ml
@@ -79,6 +79,10 @@ module Impl = struct
     let _addr, n, fds = Low_level.recv_msg_with_fds t ~sw ~max_fds (Array.of_list data) in
     n, fds
 
+  let seek = Low_level.lseek
+  let sync = Low_level.fsync
+  let truncate = Low_level.ftruncate
+
   let fd t = t
 
   let close = Eio_unix.Fd.close
diff --git a/lib_eio_posix/low_level.ml b/lib_eio_posix/low_level.ml
index 4138cabb9..fe490a0eb 100644
--- a/lib_eio_posix/low_level.ml
+++ b/lib_eio_posix/low_level.ml
@@ -256,6 +256,26 @@ external atime_nsec : stat -> int = "ocaml_eio_posix_stat_atime_nsec" [@@noalloc
 external ctime_nsec : stat -> int = "ocaml_eio_posix_stat_ctime_nsec" [@@noalloc]
 external mtime_nsec : stat -> int = "ocaml_eio_posix_stat_mtime_nsec" [@@noalloc]
 
+let lseek fd off cmd =
+  Fd.use_exn "lseek" fd @@ fun fd ->
+  let cmd =
+    match cmd with
+    | `Set -> Unix.SEEK_SET
+    | `Cur -> Unix.SEEK_CUR
+    | `End -> Unix.SEEK_END
+  in
+  Unix.LargeFile.lseek fd (Optint.Int63.to_int64 off) cmd
+  |> Optint.Int63.of_int64
+
+let fsync fd =
+  Eio_unix.run_in_systhread @@ fun () ->
+  Fd.use_exn "fsync" fd Unix.fsync
+
+let ftruncate fd len =
+  Eio_unix.run_in_systhread @@ fun () ->
+  Fd.use_exn "ftruncate" fd @@ fun fd ->
+  Unix.LargeFile.ftruncate fd (Optint.Int63.to_int64 len)
+
 let pipe ~sw =
   let unix_r, unix_w = Unix.pipe ~cloexec:true () in
   let r = Fd.of_unix ~sw ~blocking:false ~close_unix:true unix_r in
diff --git a/lib_eio_posix/low_level.mli b/lib_eio_posix/low_level.mli
index b93acf425..aea817966 100644
--- a/lib_eio_posix/low_level.mli
+++ b/lib_eio_posix/low_level.mli
@@ -35,6 +35,10 @@ val send_msg : fd -> ?fds:fd list -> ?dst:Unix.sockaddr -> Cstruct.t array -> in
 
 val getrandom : Cstruct.t -> unit
 
+val lseek : fd -> Optint.Int63.t -> [`Set | `Cur | `End] -> Optint.Int63.t
+val fsync : fd -> unit
+val ftruncate : fd -> Optint.Int63.t -> unit
+
 type stat
 
 val create_stat : unit -> stat
diff --git a/lib_eio_windows/flow.ml b/lib_eio_windows/flow.ml
index b56580342..5eacd68da 100755
--- a/lib_eio_windows/flow.ml
+++ b/lib_eio_windows/flow.ml
@@ -76,6 +76,10 @@ module Impl = struct
 
   let recv_msg_with_fds _t ~sw:_ ~max_fds:_ _data = failwith "Not implemented on Windows"
 
+  let seek = Low_level.lseek
+  let sync = Low_level.fsync
+  let truncate = Low_level.ftruncate
+
   let fd t = t
 
   let close = Eio_unix.Fd.close
diff --git a/lib_eio_windows/low_level.ml b/lib_eio_windows/low_level.ml
index d9fd69ffa..df68f9aad 100755
--- a/lib_eio_windows/low_level.ml
+++ b/lib_eio_windows/low_level.ml
@@ -231,6 +231,26 @@ let rename ?old_dir old_path ?new_dir new_path =
   in_worker_thread @@ fun () ->
   eio_renameat old_dir old_path new_dir new_path
 
+let lseek fd off cmd =
+  Fd.use_exn "lseek" fd @@ fun fd ->
+  let cmd =
+    match cmd with
+    | `Set -> Unix.SEEK_SET
+    | `Cur -> Unix.SEEK_CUR
+    | `End -> Unix.SEEK_END
+  in
+  Unix.LargeFile.lseek fd (Optint.Int63.to_int64 off) cmd
+  |> Optint.Int63.of_int64
+
+let fsync fd =
+  Eio_unix.run_in_systhread @@ fun () ->
+  Fd.use_exn "fsync" fd Unix.fsync
+
+let ftruncate fd len =
+  Eio_unix.run_in_systhread @@ fun () ->
+  Fd.use_exn "ftruncate" fd @@ fun fd ->
+  Unix.LargeFile.ftruncate fd (Optint.Int63.to_int64 len)
+
 let pipe ~sw =
   let unix_r, unix_w = Unix.pipe ~cloexec:true () in
   let r = Fd.of_unix ~sw ~blocking:false ~close_unix:true unix_r in
diff --git a/lib_eio_windows/low_level.mli b/lib_eio_windows/low_level.mli
index 189aed7bf..22c843bb8 100755
--- a/lib_eio_windows/low_level.mli
+++ b/lib_eio_windows/low_level.mli
@@ -34,6 +34,10 @@ val send_msg : fd -> ?dst:Unix.sockaddr -> bytes -> int
 
 val getrandom : Cstruct.t -> unit
 
+val lseek : fd -> Optint.Int63.t -> [`Set | `Cur | `End] -> Optint.Int63.t
+val fsync : fd -> unit
+val ftruncate : fd -> Optint.Int63.t -> unit
+
 val fstat : fd -> Unix.LargeFile.stats
 val lstat : string -> Unix.LargeFile.stats
 
diff --git a/tests/fs.md b/tests/fs.md
index d56dc74d0..6b46e985a 100644
--- a/tests/fs.md
+++ b/tests/fs.md
@@ -675,3 +675,23 @@ Exception: Failure "Simulated error".
 +<native-sub:/etc/passwd> -> Some /etc/passwd
 - : unit = ()
 ```
+
+# Seek, truncate and sync
+
+```ocaml
+# Eio_main.run @@ fun env ->
+  Eio.Path.with_open_out (env#cwd / "seek-test") ~create:(`If_missing 0o700) @@ fun file ->
+  Eio.File.truncate file (Int63.of_int 10);
+  assert ((Eio.File.stat file).size = (Int63.of_int 10));
+  let pos = Eio.File.seek file (Int63.of_int 3) `Set in
+  traceln "seek from start: %a" Int63.pp pos;
+  let pos = Eio.File.seek file (Int63.of_int 2) `Cur in
+  traceln "relative seek: %a" Int63.pp pos;
+  let pos = Eio.File.seek file (Int63.of_int (-1)) `End in
+  traceln "seek from end: %a" Int63.pp pos;
+  Eio.File.sync file;    (* (no way to check if this actually worked, but ensure it runs) *)
++seek from start: 3
++relative seek: 5
++seek from end: 9
+- : unit = ()
+```