diff --git a/piker/data/_sharedmem.py b/piker/data/_sharedmem.py
index c830d051..f1e89fca 100644
--- a/piker/data/_sharedmem.py
+++ b/piker/data/_sharedmem.py
@@ -40,10 +40,11 @@ log = get_logger(__name__)
 
 # how  much is probably dependent on lifestyle
 _secs_in_day = int(60 * 60 * 24)
-# we try for 3 times but only on a run-every-other-day kinda week.
-_default_size = 20 * _secs_in_day
+# we try for a buncha times, but only on a run-every-other-day kinda week.
+_days_worth = 16
+_default_size = _days_worth * _secs_in_day
 # where to start the new data append index
-_rt_buffer_start = int(9*_secs_in_day)
+_rt_buffer_start = int((_days_worth - 1) * _secs_in_day)
 
 
 def cuckoff_mantracker():
@@ -61,13 +62,12 @@ def cuckoff_mantracker():
         def ensure_running(self):
             pass
 
-
     # "know your land and know your prey"
     # https://www.dailymotion.com/video/x6ozzco
     mantracker._resource_tracker = ManTracker()
     mantracker.register = mantracker._resource_tracker.register
     mantracker.ensure_running = mantracker._resource_tracker.ensure_running
-    ensure_running = mantracker._resource_tracker.ensure_running
+    # ensure_running = mantracker._resource_tracker.ensure_running
     mantracker.unregister = mantracker._resource_tracker.unregister
     mantracker.getfd = mantracker._resource_tracker.getfd
 
@@ -262,10 +262,10 @@ class ShmArray:
 
         if fields:
             selection = array[fields]
-            fcount = len(fields)
+            # fcount = len(fields)
         else:
             selection = array
-            fcount = len(array.dtype.fields)
+            # fcount = len(array.dtype.fields)
 
         # XXX: manual ``.view()`` attempt that also doesn't work.
         # uview = selection.view(
@@ -308,6 +308,7 @@ class ShmArray:
 
         field_map: Optional[dict[str, str]] = None,
         prepend: bool = False,
+        update_first: bool = True,
         start: Optional[int] = None,
 
     ) -> int:
@@ -320,10 +321,9 @@ class ShmArray:
 
         '''
         length = len(data)
-        index = start if start is not None else self._last.value
 
         if prepend:
-            index = self._first.value - length
+            index = (start or self._first.value) - length
 
             if index < 0:
                 raise ValueError(
@@ -331,6 +331,9 @@ class ShmArray:
                     f'You have passed {abs(index)} too many datums.'
                 )
 
+        else:
+            index = start if start is not None else self._last.value
+
         end = index + length
 
         if field_map:
@@ -348,12 +351,17 @@ class ShmArray:
             # tries to access ``.array`` (which due to the index
             # overlap will be empty). Pretty sure we've fixed it now
             # but leaving this here as a reminder.
-            if prepend:
+            if prepend and update_first:
                 assert index < self._first.value
 
-            if index < self._first.value:
+            if (
+                index < self._first.value
+                and update_first
+            ):
+                assert prepend, 'prepend=True not passed but index decreased?'
                 self._first.value = index
-            else:
+
+            elif not prepend:
                 self._last.value = end
 
             self._post_init = True
@@ -389,6 +397,7 @@ class ShmArray:
                 f"Input array has unknown field(s): {only_in_theirs}"
             )
 
+    # TODO: support "silent" prepends that don't update ._first.value?
     def prepend(
         self,
         data: np.ndarray,