diff --git a/cista/api.py b/cista/api.py
index d08705f..55c760c 100644
--- a/cista/api.py
+++ b/cista/api.py
@@ -104,11 +104,11 @@ async def watch(req, ws):
     )
     uuid = token_bytes(16)
     try:
-        with watching.tree_lock:
+        with watching.state.lock:
             q = watching.pubsub[uuid] = asyncio.Queue()
             # Init with disk usage and full tree
-            await ws.send(watching.format_du())
-            await ws.send(watching.format_tree())
+            await ws.send(watching.format_du(watching.state.space))
+            await ws.send(watching.format_tree(watching.state.root))
         # Send updates
         while True:
             await ws.send(await q.get())
diff --git a/cista/app.py b/cista/app.py
index 5672a17..2bf7e93 100644
--- a/cista/app.py
+++ b/cista/app.py
@@ -1,10 +1,8 @@
 import asyncio
 import datetime
 import mimetypes
-from collections import deque
 from concurrent.futures import ThreadPoolExecutor
-from importlib.resources import files
-from pathlib import Path, PurePath
+from pathlib import Path, PurePath, PurePosixPath
 from stat import S_IFDIR, S_IFREG
 from urllib.parse import unquote
 from wsgiref.handlers import format_date_time
@@ -12,15 +10,13 @@ from wsgiref.handlers import format_date_time
 import brotli
 import sanic.helpers
 from blake3 import blake3
-from natsort import natsorted, ns
 from sanic import Blueprint, Sanic, empty, raw
-from sanic.exceptions import Forbidden, NotFound, ServerError, ServiceUnavailable
+from sanic.exceptions import Forbidden, NotFound, ServerError
 from sanic.log import logging
 from stream_zip import ZIP_AUTO, stream_zip
 
 from cista import auth, config, session, watching
 from cista.api import bp
-from cista.protocol import DirEntry, DirList
 from cista.util.apphelpers import handle_sanic_exception
 
 # Workaround until Sanic PR #2824 is merged
@@ -186,28 +182,26 @@ async def wwwroot(req, path=""):
     return raw(data, headers=headers)
 
 
-def get_files(wanted: set) -> list:
-    if not isinstance(watching.tree[""], DirEntry):
-        raise ServiceUnavailable(headers={"retry-after": 1})
-    root = Path(watching.rootpath)
-    with watching.tree_lock:
-        q: deque[tuple[list[str], None | list[str], DirList]] = deque(
-            [([], None, watching.tree[""].dir)]
-        )
-        while q:
-            locpar, relpar, d = q.pop()
-            for name, attr in d.items():
-                loc = [*locpar, name]
-                rel = None
-                if relpar or attr.key in wanted:
-                    rel = [*relpar, name] if relpar else [name]
-                    wanted.discard(attr.key)
-                isdir = isinstance(attr, DirEntry)
-                if isdir:
-                    q.append((loc, rel, attr.dir))
-                if rel:
-                    files.append(("/".join(rel), root.joinpath(*loc)))
-    return natsorted(files, key=lambda f: "/".join(f[0]), alg=ns.IGNORECASE)
+def get_files(wanted: set) -> list[tuple[PurePosixPath, Path]]:
+    loc = PurePosixPath()
+    idx = 0
+    ret = []
+    level: int | None = None
+    parent: PurePosixPath | None = None
+    with watching.state.lock:
+        root = watching.state.root
+        while idx < len(root):
+            f = root[idx]
+            loc = PurePosixPath(*loc.parts[: f.level - 1]) / f.name
+            if parent is not None and f.level <= level:
+                level = parent = None
+            if f.key in wanted:
+                level, parent = f.level, loc.parent
+            if parent is not None:
+                wanted.discard(f.key)
+                ret.append((loc.relative_to(parent), watching.rootpath / loc))
+            idx += 1
+    return ret
 
 
 @app.get("/zip/<keys>/<zipfile:ext=zip>")
@@ -220,7 +214,7 @@ async def zip_download(req, keys, zipfile, ext):
     if not files:
         raise NotFound(
             "No files found",
-            context={"keys": keys, "zipfile": zipfile, "wanted": wanted},
+            context={"keys": keys, "zipfile": f"{zipfile}.{ext}", "wanted": wanted},
         )
     if wanted:
         raise NotFound("Files not found", context={"missing": wanted})
@@ -230,20 +224,25 @@ async def zip_download(req, keys, zipfile, ext):
             s = p.stat()
             size = s.st_size
             modified = datetime.datetime.fromtimestamp(s.st_mtime, datetime.UTC)
+            name = rel.as_posix()
             if p.is_dir():
-                yield rel, modified, S_IFDIR | 0o755, ZIP_AUTO(size), b""
+                yield f"{name}/", modified, S_IFDIR | 0o755, ZIP_AUTO(size), iter(b"")
             else:
-                yield rel, modified, S_IFREG | 0o644, ZIP_AUTO(size), contents(p)
+                yield name, modified, S_IFREG | 0o644, ZIP_AUTO(size), contents(p, size)
 
-    def contents(name):
+    def contents(name, size):
         with name.open("rb") as f:
-            while chunk := f.read(65536):
+            while size > 0 and (chunk := f.read(min(size, 1 << 20))):
+                size -= len(chunk)
                 yield chunk
+        assert size == 0
 
     def worker():
         try:
-            for chunk in stream_zip(local_files(files)):
-                asyncio.run_coroutine_threadsafe(queue.put(chunk), loop)
+            with open("test2.zip", "wb") as f:
+                for chunk in stream_zip(local_files(files)):
+                    f.write(chunk)
+                    asyncio.run_coroutine_threadsafe(queue.put(chunk), loop).result()
         except Exception:
             logging.exception("Error streaming ZIP")
             raise
@@ -256,7 +255,10 @@ async def zip_download(req, keys, zipfile, ext):
     thread = loop.run_in_executor(app.ctx.threadexec, worker)
 
     # Stream the response
-    res = await req.respond(content_type="application/zip")
+    res = await req.respond(
+        content_type="application/zip",
+        headers={"cache-control": "no-store"},
+    )
     while chunk := await queue.get():
         await res.send(chunk)
 
diff --git a/cista/protocol.py b/cista/protocol.py
index a949d67..86617f6 100644
--- a/cista/protocol.py
+++ b/cista/protocol.py
@@ -112,47 +112,36 @@ class ErrorMsg(msgspec.Struct):
 ## Directory listings
 
 
-class FileEntry(msgspec.Struct):
-    key: str
-    size: int
-    mtime: int
-
-
-class DirEntry(msgspec.Struct):
-    key: str
-    size: int
-    mtime: int
-    dir: DirList
-
-    def __getitem__(self, name):
-        return self.dir[name]
-
-    def __setitem__(self, name, value):
-        self.dir[name] = value
-
-    def __contains__(self, name):
-        return name in self.dir
-
-    def __delitem__(self, name):
-        del self.dir[name]
-
-    @property
-    def props(self):
-        return {k: v for k, v in self.__struct_fields__ if k != "dir"}
-
-
-DirList = dict[str, FileEntry | DirEntry]
-
-
-class UpdateEntry(msgspec.Struct, omit_defaults=True):
-    """Updates the named entry in the tree. Fields that are set replace old values. A list of entries recurses directories."""
-
+class FileEntry(msgspec.Struct, array_like=True):
+    level: int
     name: str
     key: str
-    deleted: bool = False
-    size: int | None = None
-    mtime: int | None = None
-    dir: DirList | None = None
+    mtime: int
+    size: int
+    isfile: int
+
+
+class Update(msgspec.Struct, array_like=True):
+    ...
+
+
+class UpdKeep(Update, tag="k"):
+    count: int
+
+
+class UpdDel(Update, tag="d"):
+    count: int
+
+
+class UpdIns(Update, tag="i"):
+    items: list[FileEntry]
+
+
+class Space(msgspec.Struct):
+    disk: int
+    free: int
+    usage: int
+    storage: int
 
 
 def make_dir_data(root):
diff --git a/cista/watching.py b/cista/watching.py
index 978a056..aa704e5 100644
--- a/cista/watching.py
+++ b/cista/watching.py
@@ -1,20 +1,133 @@
 import asyncio
 import shutil
+import stat
 import sys
 import threading
 import time
+from os import stat_result
 from pathlib import Path, PurePosixPath
 
 import msgspec
+from natsort import humansorted, natsort_keygen, ns
 from sanic.log import logging
 
 from cista import config
 from cista.fileio import fuid
-from cista.protocol import DirEntry, FileEntry, UpdateEntry
+from cista.protocol import FileEntry, Space, UpdDel, UpdIns, UpdKeep
 
 pubsub = {}
-tree = {"": None}
-tree_lock = threading.Lock()
+sortkey = natsort_keygen(alg=ns.LOCALE)
+
+
+class State:
+    def __init__(self):
+        self.lock = threading.RLock()
+        self._space = Space(0, 0, 0, 0)
+        self._listing: list[FileEntry] = []
+
+    @property
+    def space(self):
+        with self.lock:
+            return self._space
+
+    @space.setter
+    def space(self, space):
+        with self.lock:
+            self._space = space
+
+    @property
+    def root(self) -> list[FileEntry]:
+        with self.lock:
+            return self._listing[:]
+
+    @root.setter
+    def root(self, listing: list[FileEntry]):
+        with self.lock:
+            self._listing = listing
+
+    def _slice(self, idx: PurePosixPath | tuple[PurePosixPath, int]):
+        relpath, relfile = idx if isinstance(idx, tuple) else (idx, 0)
+        begin, end = 0, len(self._listing)
+        level = 0
+        isfile = 0
+        while level < len(relpath.parts):
+            # Enter a subdirectory
+            level += 1
+            begin += 1
+            if level == len(relpath.parts):
+                isfile = relfile
+            name = relpath.parts[level - 1]
+            namesort = sortkey(name)
+            r = self._listing[begin]
+            assert r.level == level
+            # Iterate over items at this level
+            while (
+                begin < end
+                and r.name != name
+                and r.isfile <= isfile
+                and sortkey(r.name) < namesort
+            ):
+                # Skip contents
+                begin += 1
+                while begin < end and self._listing[begin].level > level:
+                    begin += 1
+                # Not found?
+                if begin == end or self._listing[begin].level < level:
+                    return slice(begin, begin)
+                r = self._listing[begin]
+            # Not found?
+            if begin == end or r.name != name:
+                return slice(begin, begin)
+        # Found an item, now find its end
+        for end in range(begin + 1, len(self._listing)):
+            if self._listing[end].level <= level:
+                break
+        return slice(begin, end)
+
+    def __getitem__(self, index: PurePosixPath | tuple[PurePosixPath, int]):
+        with self.lock:
+            print(self._slice(index))
+            return self._listing[self._slice(index)]
+
+    def __setitem__(
+        self, index: tuple[PurePosixPath, int], value: list[FileEntry]
+    ) -> None:
+        rel, isfile = index
+        with self.lock:
+            if rel.parts:
+                parent = self._slice(rel.parent)
+                if parent.start == parent.stop:
+                    raise ValueError(
+                        f"Parent folder {rel.as_posix()} is missing for {rel.name}"
+                    )
+            self._listing[self._slice(index)] = value
+
+    def __delitem__(self, relpath: PurePosixPath):
+        with self.lock:
+            del self._listing[self._slice(relpath)]
+
+    def _index(self, rel: PurePosixPath):
+        idx = 0
+        ret = []
+
+    def _dir(self, idx: int):
+        level = self._listing[idx].level + 1
+        end = len(self._listing)
+        idx += 1
+        ret = []
+        while idx < end and (r := self._listing[idx]).level >= level:
+            if r.level == level:
+                ret.append(idx)
+        return ret, idx
+
+    def update(self, rel: PurePosixPath, value: FileEntry):
+        begin = 0
+        parents = []
+        while self._listing[begin].level < len(rel.parts):
+            parents.append(begin)
+
+
+state = State()
 rootpath: Path = None  # type: ignore
 quit = False
 modified_flags = (
@@ -26,23 +139,22 @@ modified_flags = (
     "IN_MOVED_FROM",
     "IN_MOVED_TO",
 )
-disk_usage = None
 
 
 def watcher_thread(loop):
-    global disk_usage, rootpath
+    global rootpath
     import inotify.adapters
 
     while True:
         rootpath = config.config.path
         i = inotify.adapters.InotifyTree(rootpath.as_posix())
-        old = format_tree() if tree[""] else None
-        with tree_lock:
-            # Initialize the tree from filesystem
-            tree[""] = walk(rootpath)
-        msg = format_tree()
-        if msg != old:
-            asyncio.run_coroutine_threadsafe(broadcast(msg), loop)
+        # Initialize the tree from filesystem
+        old, new = state.root, walk()
+        if old != new:
+            with state.lock:
+                state.root = new
+                msg = format_tree(new)
+            asyncio.run_coroutine_threadsafe(broadcast(msg), loop).result()
 
         # The watching is not entirely reliable, so do a full refresh every minute
         refreshdl = time.monotonic() + 60.0
@@ -52,9 +164,12 @@ def watcher_thread(loop):
                 return
             # Disk usage update
             du = shutil.disk_usage(rootpath)
-            if du != disk_usage:
-                disk_usage = du
-                asyncio.run_coroutine_threadsafe(broadcast(format_du()), loop)
+            space = Space(*du, storage=state.root[0].size)
+            if space != state.space:
+                state.space = space
+                asyncio.run_coroutine_threadsafe(
+                    broadcast(format_du(space)), loop
+                ).result()
                 break
             # Do a full refresh?
             if time.monotonic() > refreshdl:
@@ -75,141 +190,136 @@ def watcher_thread(loop):
 
 
 def watcher_thread_poll(loop):
-    global disk_usage, rootpath
+    global rootpath
 
     while not quit:
         rootpath = config.config.path
-        old = format_tree() if tree[""] else None
-        with tree_lock:
-            # Initialize the tree from filesystem
-            tree[""] = walk(rootpath)
-        msg = format_tree()
-        if msg != old:
-            asyncio.run_coroutine_threadsafe(broadcast(msg), loop)
+        old = state.root
+        new = walk()
+        if old != new:
+            state.root = new
+            asyncio.run_coroutine_threadsafe(broadcast(format_tree(new)), loop).result()
 
         # Disk usage update
         du = shutil.disk_usage(rootpath)
-        if du != disk_usage:
-            disk_usage = du
-            asyncio.run_coroutine_threadsafe(broadcast(format_du()), loop)
+        space = Space(*du, storage=state.root[0].size)
+        if space != state.space:
+            state.space = space
+            asyncio.run_coroutine_threadsafe(broadcast(format_du(space)), loop).result()
 
-        time.sleep(1.0)
+        time.sleep(2.0)
 
 
-def format_du():
-    return msgspec.json.encode(
-        {
-            "space": {
-                "disk": disk_usage.total,
-                "used": disk_usage.used,
-                "free": disk_usage.free,
-                "storage": tree[""].size,
-            },
-        },
-    ).decode()
+def format_du(usage):
+    return msgspec.json.encode({"space": usage}).decode()
 
 
-def format_tree():
-    root = tree[""]
+def format_tree(root):
     return msgspec.json.encode({"root": root}).decode()
 
 
-def walk(path: Path) -> DirEntry | FileEntry | None:
+def walk(rel=PurePosixPath()) -> list[FileEntry]:  # noqa: B008
+    path = rootpath / rel
     try:
-        s = path.stat()
-        key = fuid(s)
-        assert key, repr(key)
-        mtime = int(s.st_mtime)
-        if path.is_file():
-            return FileEntry(key, s.st_size, mtime)
+        st = path.stat()
+    except OSError:
+        return []
+    return _walk(rel, int(not stat.S_ISDIR(st.st_mode)), st)
 
-        tree = {
-            p.name: v
-            for p in path.iterdir()
-            if not p.name.startswith(".")
-            if (v := walk(p)) is not None
-        }
-        if tree:
-            size = sum(v.size for v in tree.values())
-            mtime = max(mtime, *(v.mtime for v in tree.values()))
-        else:
-            size = 0
-        return DirEntry(key, size, mtime, tree)
+
+def _walk(rel: PurePosixPath, isfile: int, st: stat_result) -> list[FileEntry]:
+    entry = FileEntry(
+        level=len(rel.parts),
+        name=rel.name,
+        key=fuid(st),
+        mtime=int(st.st_mtime),
+        size=st.st_size if isfile else 0,
+        isfile=isfile,
+    )
+    if isfile:
+        return [entry]
+    ret = [entry]
+    path = rootpath / rel
+    try:
+        li = []
+        for f in path.iterdir():
+            if f.name.startswith("."):
+                continue  # No dotfiles
+            s = f.stat()
+            li.append((int(not stat.S_ISDIR(s.st_mode)), f.name, s))
+        for [isfile, name, s] in humansorted(li):
+            subtree = _walk(rel / name, isfile, s)
+            child = subtree[0]
+            entry.mtime = max(entry.mtime, child.mtime)
+            entry.size += child.size
+            ret.extend(subtree)
     except FileNotFoundError:
-        return None
+        pass  # Things may be rapidly in motion
     except OSError as e:
         print("OS error walking path", path, e)
-        return None
+    return ret
 
 
 def update(relpath: PurePosixPath, loop):
     """Called by inotify updates, check the filesystem and broadcast any changes."""
     if rootpath is None or relpath is None:
         print("ERROR", rootpath, relpath)
-    new = walk(rootpath / relpath)
-    with tree_lock:
-        update = update_internal(relpath, new)
-        if not update:
-            return  # No changes
-        msg = msgspec.json.encode({"update": update}).decode()
-        asyncio.run_coroutine_threadsafe(broadcast(msg), loop)
-
-
-def update_internal(
-    relpath: PurePosixPath,
-    new: DirEntry | FileEntry | None,
-) -> list[UpdateEntry]:
-    path = "", *relpath.parts
-    old = tree
-    elems = []
-    for name in path:
-        if name not in old:
-            # File or folder created
-            old = None
-            elems.append((name, None))
-            if len(elems) < len(path):
-                # We got a notify for an item whose parent is not in tree
-                print("Tree out of sync DEBUG", relpath)
-                print(elems)
-                print("Current tree:")
-                print(tree[""])
-                print("Walking all:")
-                print(walk(rootpath))
-                raise ValueError("Tree out of sync")
-            break
-        old = old[name]
-        elems.append((name, old))
+    new = walk(relpath)
+    old = state[relpath]
     if old == new:
-        return []
-    mt = new.mtime if new else 0
-    szdiff = (new.size if new else 0) - (old.size if old else 0)
-    # Update parents
-    update = []
-    for name, entry in elems[:-1]:
-        u = UpdateEntry(name, entry.key)
-        if szdiff:
-            entry.size += szdiff
-            u.size = entry.size
-        if mt > entry.mtime:
-            u.mtime = entry.mtime = mt
-        update.append(u)
-    # The last element is the one that changed
-    name, entry = elems[-1]
-    parent = elems[-2][1] if len(elems) > 1 else tree
-    u = UpdateEntry(name, new.key if new else entry.key)
+        return
+    old = state.root
     if new:
-        parent[name] = new
-        if u.size != new.size:
-            u.size = new.size
-        if u.mtime != new.mtime:
-            u.mtime = new.mtime
-        if isinstance(new, DirEntry) and u.dir != new.dir:
-            u.dir = new.dir
+        state[relpath, new[0].isfile] = new
     else:
-        del parent[name]
-        u.deleted = True
-    update.append(u)
-    return update
+        del state[relpath]
+    # FIXME: broadcast format_update()
+    msg = format_update(old, state.root)
+    asyncio.run_coroutine_threadsafe(broadcast(msg), loop).result()
+
+
+def format_update(old, new):
+    # Make keep/del/insert diff until one of the lists ends
+    oidx, nidx = 0, 0
+    update = []
+    keep_count = 0
+    while oidx < len(old) and nidx < len(new):
+        if old[oidx] == new[nidx]:
+            keep_count += 1
+            oidx += 1
+            nidx += 1
+            continue
+        if keep_count > 0:
+            update.append(UpdKeep(keep_count))
+            keep_count = 0
+
+        del_count = 0
+        rest = new[nidx:]
+        while old[oidx] not in rest:
+            del_count += 1
+            oidx += 1
+
+        if del_count:
+            update.append(UpdDel(del_count))
+            oidx += 1
+            continue
+
+        insert_items = []
+        rest = old[oidx:]
+        while nidx < len(new) and new[nidx] not in rest:
+            insert_items.append(new[nidx])
+            nidx += 1
+        update.append(UpdIns(insert_items))
+
+    # Diff any remaining
+    if keep_count > 0:
+        update.append(UpdKeep(keep_count))
+    if oidx < len(old):
+        update.append(UpdDel(len(old) - oidx))
+    elif nidx < len(new):
+        update.append(UpdIns(new[nidx:]))
+
+    return msgspec.json.encode({"update": update}).decode()
 
 
 async def broadcast(msg):