diff --git a/mirage/config.ml b/mirage/config.ml
index 097394e..bf0aa96 100644
--- a/mirage/config.ml
+++ b/mirage/config.ml
@@ -47,5 +47,13 @@ let git_client =
   merge_git_clients (git_tcp tcp git)
     (git_http ~authenticator:tls_authenticator tcp git)
 
+let program_block_size =
+  let doc = Key.Arg.info [ "program-block-size" ] in
+  Key.(create "program_block_size" Arg.(opt int 512 doc))
+
+let kv_rw =
+  let block = block_of_file "db" in
+  chamelon ~program_block_size block
+
 let () = register "mirror"
-    [ mirror $ kv_rw_mem () $ default_time $ default_posix_clock $ stack $ dns $ paf default_time stack $ git_client ]
+    [ mirror $ kv_rw $ default_time $ default_posix_clock $ stack $ dns $ paf default_time stack $ git_client ]
diff --git a/mirage/unikernel.ml b/mirage/unikernel.ml
index 5e3a3f2..20891c0 100644
--- a/mirage/unikernel.ml
+++ b/mirage/unikernel.ml
@@ -343,9 +343,11 @@ module Make
                            (hash_to_string h) (hex_to_string v) (hex_to_string v'));
               false
             end) hm
-      then
+      then begin
+        Logs.info (fun m -> m "KV.set (%d)" (String.length data));
         KV.set t.dev (Mirage_kv.Key.v sha256) data >|= function
         | Ok () ->
+          Logs.info (fun m -> m "KV.set done");
           t.md5s <- SM.add md5 sha256 t.md5s;
           t.sha512s <- SM.add sha512 sha256 t.sha512s;
           Logs.info (fun m -> m "wrote %s (%d bytes)" (hex_to_string sha256)
@@ -353,7 +355,7 @@ module Make
         | Error e ->
           Logs.err (fun m -> m "error %a while writing %s"
                        KV.pp_write_error e (hex_to_string sha256))
-      else
+      end else
         Lwt.return_unit
 
     let find_key t h v =
@@ -468,7 +470,7 @@ module Make
       Lwt_list.iter_p (fun (url, csums) ->
           HM.fold (fun h v r ->
               r >>= function
-              | true -> Disk.exists disk h v
+              | true -> Disk.exists disk h (hex_to_string v)
               | false -> Lwt.return false)
             csums (Lwt.return true) >>= function
           | true ->
@@ -476,7 +478,9 @@ module Make
             Lwt.return_unit
           | false ->
             follow 20 url >>= function
-            | Some str -> Disk.write disk str csums
+            | Some str ->
+              Logs.info (fun m -> m "writing (%d)" (String.length str));
+              Disk.write disk str csums
             | None -> Lwt.return_unit)
         (SM.bindings urls) >|= fun () ->
       Logs.info (fun m -> m "done")